11.3 the Kidne y And osMoregulAtion
though the interstitial fuid is now hypertonic relative to the ltrate; i.e., from the proximal to the distal
it has a higher solute concentration. convoluted convoluted
tubule tubule
Normal body fuids have a concentration o 300 mOsm. The pump
proteins that transer sodium ions out o the ltrate can create a gradient 300 300 100
o up to 200 mOsm, so an interstitial concentration o 500 mOsm is
clearly achievable. The cells in the wall o the descending limb are H2O Na+
permeable to water, but are impermeable to sodium ions. As ltrate 600 600 Na+ 400
fows down the descending limb, the increased solute concentration
o interstitial fuid in the medulla causes water to be drawn out o the descending limb
ltrate until it reaches the same solute concentration as the interstitial ascending limb
fuid. I this was 500 mOsm, then ltrate entering the ascending limb H2O Na+
would be at this concentration and the sodium pumps could raise the 900 900 Na+ 700
interstitial fuid to 700 mOsm. Fluid passing down the descending
limb would thereore reach 700 mOsm, and the sodium pumps in the H2O Na+
ascending limb could cause a urther 200 mOsm rise. The interstitial 1200 1200 Na+ 1000
fuid concentration can thereore rise urther and urther, until a
maximum is reached, which in humans is 1 ,200 mOsm. 1200
This system or raising solute concentration is an example o a Figure 12 Solute concentrations in
countercurrent multiplier system. It is a countercurrent system because o the loop of Henl (in mOsm)
the fows o fuid in opposite directions. It is a countercurrent multiplier
because it causes a steeper gradient o solute concentration to develop in
the medulla than would be possible with a concurrent system. There is also
a countercurrent system in the vasa recta. This prevents the blood fowing
through this vessel rom diluting the solute concentration o the medulla,
while still allowing the vasa recta to carry away the water removed rom
ltrate in the descending limb, together with some sodium ions.
some animal ave relatively long loop of henl
The length of the loop of Henl is positively correlated with
the need for water conservation in animals.
The longer the loop o Henl, the more water volume will be reclaimed.
Animals adapted to dry habitats will oten have long loops o Henl.
Loops o Henl are ound within the medulla. In order to accommodate
long loops o Henl, the medulla must become relatively thicker.
daa-ba q: Medulla thickness and urine
concentration
Table 4 shows the relative medullary thickness ( RMT) and maximum
solute concentration (MSC) o the urine in mOsm or 1 4 species
o mammal. RMT is a measure o the thickness o the medulla in
relation to the overall size o the kidney. All the species in the table
that are shown with binomials are desert rodents.
1 Discuss the relationship between maximum solute [3]
concentration o urine and the habitat o the mammal.
2 Plot a scattergraph o the data in the table, either by [7]
hand or using computer sotware.
493
11 ANIMAL PHYSIOLOGY ( AHL)
3 a) Using the scattergraph that you have plotted, state the [1 ]
relationship between RMT and the maximum solute
concentration o the urine.
b) Suggest how the thickness o the medulla could aect the
maximum solute concentration o the urine. [4]
speie rMt Msc
(mom)
beaver 1.3 517
pig 1.6 1076
human 3.0 1399
dog 4.3 2465
cat 4.8 3122
rat 5.8 2465
Octomys mimax 6.1 2071
Dipodomys deserti 8.5 5597
Jaculus jaculus 9.3 6459
Tympanoctomys barrerae 9.4 7080
Psammomys obesus 10.7 4952
Eligmodontia typus 11.4 8612
Calomys mus 12.3 8773
Salinomys delicatus 14.0 7440
(a) low ADH (b) high ADH Table 4
interstitial
uid
125 300 300 Function of ADh
150 600 600 ADH controls reabsorption of water in the collecting duct.
175 900 900 When ltrate enters the distal convoluted tubule rom the loop o
Henl, its solute concentration is lower than that o normal body
200 1200 1200 fuids it is hypotonic. This is because proportionately more solutes
than water have passed out o the ltrate as it fows through the loop
renal pelvis o Henl in the medulla.
Figure 13 Solute concentrations in the I the solute concentration o the blood is too low, relatively little
collecting duct water is reabsorbed as the ltrate passes on through the distal
convoluted tubule and the collecting duct. The wall o these parts
o the nephron can have an unusually low permeability to water.
A large volume o urine is thereore produced, with a low solute
concentration, and as a result the solute concentration o the blood is
increased (see gure 1 3a) .
I the solute concentration o the blood is too high, the hypothalamus
o the brain detects this and causes the pituitary gland to secrete a
hormone antidiuretic hormone or ADH. This hormone causes the
walls o the distal convoluted tubule and collecting duct to become
494
11.3 the Kidne y And osMoregulAtion
much more permeable to water, and most o the water in the ltrate is
reabsorbed. This is helped by the solute concentration gradient o the
medulla. As the ltrate passes down the collecting duct, it fows deep
into the medulla, where the solute concentration o the interstitial fuid
is high. Water continues to be reabsorbed along the whole length o the
collecting duct and the kidney produces a small volume o concentrated
urine (gure 1 3b) . As result the solute concentration o the blood is
reduced. The action o the kidney thereore helps to keep the relative
amounts o water and solutes in balance at an appropriate level. This is
called osmoregulation.
daa-ba q: ADH release and feelings of thirst
The plasma solute concentration, plasma b) Compare intensity o thirst and plasma [1 ]
antidiuretic hormone (ADH) concentration ADH concentration.
and eelings o thirst were tested in a group [2]
o volunteers. Figures 1 4 and 1 5 show the c) Outline what would happen to plasma [2]
relationship between intensity o thirst, plasma ADH solute concentration and ADH
concentration and plasma solute concentration. concentration i a person were to
drink water to satisy his/her thirst.
a) Identiy the plasma ADH concentration [1 ]
at a plasma solute concentration o 300 d) State two reasons why a persons plasma
mOsmol kg-1 using the line o best t. solute concentration may increase.
intensity of thirst/arbitrary units10 20
plasma ADH/pmol dm-39 18
8 16
7 14
6 12
5 10
4 8
3 6
2 4
1 2
0 0
280 290 300 310 320 280 290 300 310 320
plasma solute concentration/mOsmol kg-1 plasma solute concentration/mOsmol kg-1
Figure 14 Figure 15
Ama va m f p f
wa pc
The type of nitrogenous waste in animals is correlated
with evolutionary history and habitat.
When animals break down amino acids and nucleic acids, nitrogenous
waste in the orm o ammonia is produced. Ammonia is highly basic
and can alter the pH balance. It is also toxic as it is a highly reactive
chemical. I the organism lives in a marine or reshwater habitat, such as
sh, echinoderms or coelenterates, they can release the waste directly as
ammonia as it can be easily diluted within that environment. Terrestrial
495
11 ANIMAL PHYSIOLOGY ( AHL)
Figure 16 The white paste in bird organisms will expend energy to convert ammonia to the less toxic
droppings is uric acid orms o urea or uric acid depending on their habitats and evolutionary
history. Marine mammals, despite their habitat, release urea because o
their evolutionary history.
Some organisms like amphibians release the waste as ammonia when they
are larva and ater metamorphosis, release the waste as urea. Converting
ammonia to urea requires energy and converting it to uric acid requires
even more energy. The advantage o uric acid is that it is not water-soluble
and thereore does not require water to be released. Birds and insects
release their nitrogenous waste as uric acid. For birds, not having to carry
water or excretion means less energy needs to be expended on fight.
Uric acid is linked to adaptations or reproduction. Nitrogenous wastes
are released by the developing organism within eggs. Uric acid is released
as it is not soluble and crystallizes rather than building up to toxic
concentrations within the egg.
Dehydration and overhydration
Consequences of dehydration and overhydration.
Dehydration is a condition that arises when more to increases in heart rate. Body temperature
water leaves the body than comes in. It can arise regulation may be aected because o an inability
rom a number o actors including exercise, to sweat.
insucient water intake or diarrhoea. It can lead
to the disruption o metabolic processes. Overhydration is less common and occurs when
there is an over-consumption o water. The result
One sign o dehydration is darkened urine due to is a dilution o blood solutes. It might occur when
increased solute concentration. Water is necessary large amounts o water are consumed ater intense
to remove metabolic wastes so dehydration can exercise without replacing the electrolytes lost at
lead to tiredness and lethargy due to decreased the same time. This makes body fuids hypotonic
eciency o muscle unction and increased tissue and could result in the swelling o cells due to
exposure to metabolic wastes. Blood pressure osmosis. I this occurs, the most notable symptoms
can all due to low blood volume. This can lead are headache and nerve unction disruption.
Treatment options for kidney failure
Treatment of kidney failure by hemodialysis or kidney transplant.
Kidney ailure can occur or a number o reasons blood pass through the membrane, but the larger
but most commonly occurs as a complication blood cells and proteins cannot. The puried blood
rom diabetes or chronic high blood pressure is then returned to the patient via a vein. This
(hypertension) as a result o diabetes. procedure takes several hours.
Figure 1 7 shows a patient undergoing renal dialysis An alternative to dialysis is a kidney transplant. In
(hemodialysis) . The dialysis machine (articial this treatment option, a kidney rom one person
kidney) is on the let. Hemodialysis is required is placed in the body o a person whose kidneys
when the kidneys are no longer able to lter arent unctioning. The donor can either be living
waste products rom the blood properly. D uring or deceased. A living donor is possible because a
the procedure, a steady fow o blood passes over person can survive with one unctional kidney.
an articial semi-permeable membrane in the This approach can result in greater independence
dialysis machine. The small waste products in the o movement and reedom to travel as compared
496
vein 11.3 the Kidne y And osMoregulAtion
artery
blood in tubing ows
shunt through dialysis uid
blood pump
used dialysis uid
air detector dialysis machine fresh dialysis compressed
uid air
Figure 17
to dialysis. Dialysis also carries with it the risk o micrograph through a transplanted kidney that
inection and other complications. has been rejected by the recipients immune
system. Numerous lymphocytes (with small dots)
A drawback to a transplant is that the recipients have infltrated the kidney tissue.
body can reject the organ. Figure 1 9 is o a light
Figure 18 Figure 19
Urinalysis
Blood cells, glucose, proteins and drugs are detected in urinary tests.
Urine is a product o osmoregulation, excretion The colours displayed can then be compared
and metabolism. These processes can be disrupted to a results chart on the testing kit. This test
by illness or drug abuse. Urinalysis is a clinical indicates the pH, protein level and glucose
procedure that examines urine or any deviation level in the urine. High levels o glucose and
rom normal composition. protein in the urine can be an indication o
diabetes. High protein levels can indicate damage
Figure 20 shows a urine test strip being to the kidneys as these do not get through
compared to the results chart on the testing ultrafltration in a healthy kidney. The strip in
kit bottle. This strip contains three test areas the picture is a normal negative result or protein
designed to change colour to indicate a positive and glucose.
or negative result ater being dipped in urine.
497
11 ANIMAL PHYSIOLOGY ( AHL)
presence o traces o banned and controlled drugs
in urine. Figure 21 shows a drug test card being
dipped into a sample o urine. The card contains
fve vertical strips that each test or a dierent
drug. Here, the results are negative or all but the
one second rom let. This indicates a positive test
or opiates.
Figure 20 Figure 21
The panel drug test also uses test strips based on Microscopic examination o urine is carried out
monoclonal antibody technology to look or the to determine i cells are present, as under normal
circumstances, these cells should not be present.
Figure 22 shows white blood cells. The presence
o 61 0 neutrophils (white blood cells with a
nucleus visible) can be a sign o urinary tract
inection. Figure 23 indicates the presence o
red blood cells (erythrocytes) in the urine o this
patient. This can be a sign that there is a kidney
stone or a tumour in the urinary tract.
Figure 22 Figure 23
498
11.4 sexuAl reproduction
11.4 sa
Udertadig Applicatio
Spermatogenesis and oogenesis both involve The average 38-week pregnancy in humans
mitosis, cell growth, two divisions o meiosis can be positioned on a graph showing the
and dierentiation. correlation between animal size and the
development o the young at birth or other
Processes in spermatogenesis and oogenesis mammals.
result in dierent numbers o gametes with
dierent amounts o cytoplasm. skill
Fertilization involves mechanisms that prevent Annotation o diagrams o seminierous
polyspermy. tubule and ovary to show the stages o
gametogenesis.
Fertilization in animals can be internal or
external. Annotation o diagrams o mature sperm and
egg to indicate unctions.
Implantation o the blastocyst in the
endometrium is essential or the continuation nature of ciece
o pregnancy.
Assessing risks and benefts associated with
hCG stimulates the ovary to secrete scientifc research: the risks to human male
progesterone during early pregnancy. ertility were not adequately assessed beore
steroids related to progesterone and estrogen
The placenta acilitates the exchange o were released into the environment as a result
materials between the mother and embryo. o the use o the emale contraceptive pill.
Estrogen and progesterone are secreted by the
placenta once it has ormed.
Birth is mediated by positive eedback
involving estrogen and oxytocin.
similaritie betwee oogeei ad
permatogeei
Spermatogenesis and oogenesis both involve mitosis, cell
growth, two divisions o meiosis and dierentiation.
Oogenesis is the production o egg cells in the ovaries. Oogenesis starts
in the ovaries o a emale etus. Germ cells in the etal ovary divide by
mitosis and the cells ormed move to distribute themselves through the
cortex o the ovary. When the etus is our or fve months old, these cells
grow and start to divide by meiosis. By the seventh month, they are still
in the frst division o meiosis and a single layer o cells, called ollicle
cells, has ormed around them. No urther development takes place until
ater puberty. The cell that has started to divide by meiosis, together
with the surrounding ollicle cells, is called a primary follicle. There
are about 400,000 in the ovaries at birth. No more primary ollicles
are produced, but at the start o each menstrual cycle a small batch are
499
11 ANIMAL PHYSIOLOGY ( AHL)
stimulated to develop by FSH. Usually only one goes on to become a
mature follicle, containing a secondary oocyte.
primary ollicle
maturing ollicle
Figure 1 Light micrograph o a section through tissue rom an ovary, showing a primary
ollicle (let) and a maturing ollicle (centre) . Primary ollicles contain a central oocyte
(emale germ cell, egg) surrounded by a single layer o ollicle cells. A mature ovarian
ollicle has many more ollicle cells, outer and inner ollicle cells and cavities, and the
oocyte is now more ully developed compared to the primordial and primary stages
Spermatogenesis is the production o sperm. It happens in the testes,
which are composed o a mass o narrow tubes, called seminiferous
tubules, with small groups o cells lling the gaps between the tubules.
These gaps are called interstices, so the cells in them are interstitial
cells. They are sometimes called Leydig cells. The seminierous tubules
are also made o cells. The outer layer o cells is called the germinal
epithelium. This is where the process o sperm production begins. Cells
in various stages o sperm production are ound inside the germinal
epithelium, with the most mature stages closest to the fuid-lled centre
o the seminierous tubule. Cells that have developed tails are called
spermatozoa, though this is almost always abbreviated to sperm.
Also in the wall o the tubule are large nurse cells, called Sertoli cells.
Figure 3 shows a small area o testis tissue, in which the structures
described above can be seen.
Figure 2 Coloured scanning electron spermatogonium
micrograph (SEM) o ovary tissue, showing agella o spermatozoa
two secondary ollicles. A secondary oocyte lumen o seminierous tubule
(pink) is seen at the centre o one ollicle.
Follicles are surrounded by two types o ollicle Figure 3 Transverse section through a seminierous tubule
cells (coloured blue and green) . Between the
ollicle cells a space develops (at centre right,
coloured brown) , into which ollicular uid is
secreted. The amount o uid will increase
signifcantly as the ollicle matures
500
11.4 sexuAl reproduction
Diagrams of a seminiferous tubule and the ovary
Annotation of diagrams of seminiferous tubule and ovary to show the stages
of gametogenesis.
basement membrane
12 An outer layer called spermatogonium
germinal epithelium cells
(2n) divide endlessly 2 Diploid cells grow
by mitosis to produce larger and are then
more diploid cells called primary
spermatocytes (2n)
primary
spermatocyte 32 Each primary
spermatocyte carries out
secondary the rst division of meiosis
spermatocyte to produce two secondary
spermatocytes (n)
62 Sperm detach from
Sertoli cells and 42 Each secondary
eventually are carried spermatocyte carries
out of the testis by the out the second division
uid in the centre of the of meiosis to produce
seminiferous tubule two spermatids (n)
Figure 4 spermatids
52 Spermatids become associated
with nurse cells, called Sertoli cells
which help the spermatids to develop
into spermatozoa (n) . This is an
example of cell dierentiation
developing
2 In a secondary follicle, the follicle secondary oocyte follicles 1 Primary follicles consist of a central
cells proliferate, a uid-lled cavity fo l l i c l e primary follicles oocyte surrounded by a single layer
develops and the oocyte starts the of follicle cells. Every menstrual cycle,
second division of meiosis a few primary follicles start to develop
and the oocyte completes the rst
division of meiosis
degenerating
corpus luteum
mature follicle
corpus luteum
developing
corpus luteum
ovulated ovum
Figure 5
501
11 ANIMAL PHYSIOLOGY ( AHL)
Diagrams of sperm and egg
Annotation of diagrams of mature sperm and egg to indicate functions.
haploid
nucleus
two centrioles cytoplasm (or yolk)
containing droplets of fat
rst polar cell
Diameter of egg
cell = 110 m
plasma
membrane
cortical granules
layer of follicle cells layer of gel composed
(corona radiata) of glycoproteins
zona ellucida
Figure 6 Structure of the female gamete
head (3 m wide and 4 m long) haploid nucleus
acrosome mid-piece tail (40 m long, two-thirds of
it omitted from this drawing)
(7 m long)
centriole microtubules in a
9+2 arrangement
plasma membrane helical protein bres to
mitochondria strengthen the tail
Figure 7 Structure of the male gamete
502
11.4 sexuAl reproduction
daa-ba q: Sizes of sperm
Sperm tails have a 9 + 2 arrangement o 2 Outline the relationship between tail length
microtubules in the centre, with thicker protein and cross-sectional area o protein fbres. [2]
fbres around. Table 1 shows the structure o 3 Explain reasons or the relationship. [2]
sperm tails o eight animals in transverse section,
with the tail lengths and the cross-sectional area 4 Discuss whether there is a relationship
o the protein fbres. between the size o an animal and the size
1 Draw a graph o tail length and cross-sectional o its sperm. [2]
area o protein fbres in the eight species o
animal. [4]
h a ga ham b m hma a
ham g h
cross-sectional area o 0.22 0.16 0.13 0.11 0.08 0.04 0.02 0
fbrous sheaths / m2
length of sperm / m 258 187 107 187 54 123 58 45
Table 1
Diferences in the outcome o spermatogenesis Figure 8 The micrograph shows a primary
and oogenesis oocyte split into two cells, known as the
secondary oocyte (green) and the frst
Processes in spermatogenesis and oogenesis result in polar body (yellow)
dierent numbers o gametes with dierent amounts
o cytoplasm. 503
While there are similarities in spermatogenesis and oogenesis, there are
dierences that are necessary to prepare the gametes or their dierent
roles. Each mature sperm consists o a haploid nucleus, a system or
movement and a system o enzymes and other proteins that enable the
sperm to enter the egg. Each complete meiotic division results in our
spermatids. The process o sperm dierentiation eliminates most o the
cytoplasm, whereas the egg must increase its cytoplasm.
All o the requirements or beginning the growth and development o the
early embryo must be present in the egg. In emales, the frst division o
meiosis produces one large cell and one very small cell (fgure 8) . The small
cell is the frst polar body which eventually degenerates. The large cell
goes on to the second division o meiosis, completing it ater ertilization.
Again one large cell and one very small cell are produced. The small cell
is the second polar body and it also degenerates and dies. Only the large
cell, which is the emale gamete, survives. The result is that the egg is
much larger than the sperm cell. Figures 6 and 7 show the dierences
in structure. Note that the scale bars indicate that the sperm and egg are
drawn to dierent scale and that the egg is much larger than the sperm.
11 ANIMAL PHYSIOLOGY ( AHL)
sperm try to The process o egg ormation happens once per menstrual cycle in
push through humans and usually only one egg cell per cycle is produced. During
the layers of the years rom puberty to the menopause only a ew hundred emale
follicle cells gametes are likely to be produced.
around the
egg From puberty onwards, the testes produce sperm continuously. At any
time, there are millions o sperm at all stages o development.
fo l l i c l e
cell preventing olysermy
zona Fertilization involves mechanisms that prevent
pellucida polyspermy.
plasma membrane of egg
Fertilization is the union o a sperm and an egg to orm a zygote.
acrosomal
cap The membranes o sperm have receptors that can detect chemicals
released by the egg, allowing directional swimming towards the egg.
Figure 9 illustrates that multiple sperm arrive at the egg. Once the egg
is reached, a number o events take place (see fgure 1 0) . These events
are designed to result in the union o a single sperm with the egg. The
events are also designed to prevent multiple sperm entering, known as
p o ly s p e r m y.
tail and
mitochondria
usually remain
outside
cortical granules
hardened
zona pellucida
exocytosis sperm nucleus
of contents
of cortical
granules
two haploid Figure 9 Micrograph of egg surrounded by sperm
nuclei from the
sperm and the egg 1 The acrosome reaction
Figure 10 Stages in fertilization The zona pellucida is a coat o glycoproteins that surrounds the egg.
The acrosome is a large membrane-bound sac o enzymes in the head
504 o the sperm. In mammals, the sperm binds to the zona pellucida and
the contents o the acrosome are released. The enzymes rom it digest
the zona pellucida.
11.4 sexuAl reproduction
2 Penetration of the egg membrane FPO
< 839 211 _ph 11 .4.11 >
The acrosome reaction exposes an area o membrane on the tip o the
sperm that has proteins that can bind to the egg membrane. The rst Figure 11 Breeding pair o Anomalochromis
sperm that gets through the zona pellucida thereore binds and the thomasi cichlids. The emale (bottom) is laying
membranes o sperm and egg use together. The sperm nucleus enters eggs on a rock with the male in close proximity
the egg cell. This is the moment o ertilization.
Figure 12 Blastocyst
3 The cortical reaction
Figure 13 Implantation o the
Not only does the sperm bring male genes, it also causes the activation blastocyst
o the egg. The rst eect o this is on the cortical granules vesicles
located near the egg membrane. There are thousands o these vesicles Figure 14 Growth and diferentiation
and when activation o the egg has taken place their contents are o the early embryo
released rom the egg by exocytosis. In mammals, the cortical vesicle
enzymes result in the digestion o binding proteins so that no urther
sperm can bind. The enzymes also result in a general hardening o the
zona pellucida.
Internal and external fertilization
Fertilization in animals can be internal or external.
Aquatic animals oten release their gametes directly into water in a
process that will lead to ertilization outside o the emales body. S uch
animals oten have behaviours that bring eggs into proximity with
sperm (see gure 1 1 ) . External ertilization has several risks including
predation and the susceptibility to environmental variation such as
temperature and pH fuctuations and more recently, pollution.
Terrestrial animals are dependent on internal ertilization. O therwise,
gametes would be at risk o drying out. Internal ertilization also ensures
sperm and ova are placed in prolonged close proximity to each other.
Marine mammals which have reinvaded aquatic habitats still use
internal ertilization. Once the eggs are ertilized, the developing embryo
can be protected inside the emale.
Implantation of the blastocyst
Implantation of the blastocyst in the endometrium is
essential for the continuation of pregnancy.
Ater ertilization in humans, the ertilized ovum divides by mitosis
to orm two diploid nuclei and the cytoplasm o the ertilized egg cell
divides equally to orm a two-cell embryo. These two cells replicate their
D NA, carry out mitosis and divide again to orm a our-cell embryo. The
embryo is about 48 hours old at this point. Further cell divisions occur,
but some o the divisions are unequal and there is also migration o cells,
giving the embryo the shape o a hollow ball. It is called a blastocyst
(gure 1 2) . At 7 days old the blastocyst consists o about 1 25 cells and
it has reached the uterus, having been moved down the oviduct by the
cilia o cells in the oviduct wall. At this age the zona pellucida, which
has surrounded and protected the embryo, breaks down. The blastocyst
has used up the reserves o the egg cell and needs an external supply
o ood. It obtains this by sinking into the endometrium or uterus
505
11 ANIMAL PHYSIOLOGY ( AHL)
lining in a process called implantation (gure 1 3) . The outer layer o
the blastocyst develops nger-like proj ections allowing the blastocyst
to penetrate the uterus lining. They also exchange materials with the
mothers blood, including absorbing oods and oxygen. The embryo
grows and develops rapidly and by eight weeks has started to orm bone
tissue. It is then considered to be a etus rather than an embryo. It is
recognizably human and soon visibly either male or emale.
Role of hCG in early pregnancy
hCG stimulates the ovary to secrete progesterone during
early pregnancy.
Pregnancy depends on the maintenance o the endometrium, which
depends on the continued production o progesterone and estrogen.
In part these hormones prevent the degeneration o the uterus lining
which is required to support the developing etus. Early in pregnancy
the embryo produces human chorionic gonadotropin hCG. This
hormone stimulates the corpus luteum in the ovary to continue to
secrete progesterone and estrogen. These hormones stimulate the
continued development o the uterus wall, which supplies the embryo
with everything that it needs.
materials exchange by the placenta
The placenta facilitates the exchange of materials
between the mother and embryo.
Humans are placental mammals. There are two other groups o
mammals: the monotremes lay eggs and the marsupials give birth to
relatively undeveloped ospring that develop inside a pouch. By the
stage when a marsupial would be born, a human etus has developed a
relatively complex placenta and so can remain in the uterus or months
longer. The placenta is needed because the body surace area to volume
ratio becomes smaller as the etus grows larger.
The placenta is made o etal tissues, in intimate contact with maternal
tissues in the uterus wall. The etus also develops membranes that orm
the amniotic sac. This contains amniotic fuid, which supports and
protects the developing etus.
The basic unctional unit o the placenta is a nger-like piece o etal
tissue called a placental villus. These villi increase in number during
pregnancy to cope with the increasing demands o the etus or the
exchange o materials with the mother. Maternal blood fows in the
inter-villous spaces around the villi ( gure 1 5 ) . This is a very unusual
type o circulation as elsewhere blood is almost always conned in blood
vessels. Fetal blood circulates in blood capillaries, close to the surace o
each villus. The distance between etal and maternal blood is thereore
very small as little as 5 m. The cells that separate maternal and etal
blood orm the placental barrier. This must be selectively permeable,
allowing some substances to pass, but not others (gure 1 6) .
506
11.4 sexuAl reproduction
maternal fetal blood placental barrier maternal blood
venule
carbon dioxide d i u s i on
maternal
maternal blood pools arteriole d i u s i on oxygen
fetal capillaries glucose
fa ci l i ta te d
umbilical cord d i u s i on
umbilical vein
urea
umbilical
arteries endocytosis antibodies
water osmosis
water
Figure 16 Exchange processes in the placenta
fetal portion of maternal portion
placenta (chorion) of placenta
Figure 15
Release of hormones by the placenta
Estrogen and progesterone are secreted by the placenta
once it has formed.
B y about the ninth week o pregnancy, the placenta has started to
secrete estrogen and progesterone in large enough quantities to sustain
the pregnancy, and the corpus luteum is no longer needed or this role.
There is a danger o miscarriage at this stage o pregnancy i this switch-
over ails.
daa-ba q: Electron micrograph of placenta
Figure 1 7 shows a small region at the edge o a
placental villus. The magnifcation is 1 7,000.
1 a) Identiy the structures that are visible in
the upper part o the micrograph. [1 ]
b) Explain the unctions o these [3]
structures.
2 In much o the area o the electron micrograph
there are rounded structures, surrounded by a
single membrane. These are parts o a system
o tubules called the smooth endoplasmic
reticulum (sER) . Its unction is the synthesis o
lipids, including steroids. Suggest a unction or
the sER in the placenta. [3]
3 Identiy, with reasons, the structure in the Figure 17 Small region at the edge of a placental
villus
lower let part o the micrograph. [3]
507
11 ANIMAL PHYSIOLOGY ( AHL)
Assessing risks of estrogen pollution
Assessing risks and benefts oscientifc research: the risks to human male ertility were
not adequately assessed beore steroids related to progesterone and estrogen were
released into the environment as a result othe use othe emale contraceptive pill.
High levels o estrogen are present in pregnant 2004 that 86% o male fsh sampled at 51 sites
women and inhibit FSH release. I women around the country were intersex, that is male fsh
consume pills containing estrogen, then showed signs o eminization. However, there
this would mimic pregnancy and inhibit the is limited scientifc consensus that pollution with
development o mature ollicles thus preventing steroids related to estrogen and progesterone is the
pregnancy. Ethinyl estradiol is a synthetic causative agent behind reduced male ertility.
orm o estrogen that was frst introduced as a
contraceptive in 1 943. At the time, little thought In 201 2 the European Commission proposed a
was given to the idea that i a large number o policy which would limit the concentrations in
women used this orm o contraception, then water o a widely used contraceptive drug. This
levels o estrogen in bodies o water might has sparked intense lobbying by the water and
be raised through sewage. It wasnt until the pharmaceutical industries, which say that the
mid-1 980s that the frst reports o elevated science is uncertain and the costs too high.
contraceptive pill hormones present in water were
reported. Since then, a number o problems have Upgrading the technology or wastewater
been attributed to estrogen pollution. treatment could eliminate most o the pollution.
Researchers and policy experts suggest sharing the
In 1 992, a review article summarizing 61 dierent costs among all responsible parties, including the
studies concluded that human male sperm counts water and drug industries, and that some expense
have declined by 50% over the past 50 years. would be passed on to the public. The drugs are
widely used in livestock, so preventing animals
In one o the largest studies o the problem, the rom urinating close to rivers could urther reduce
UK governments Environment Agency ound in the amount o drugs leaking into surace waters.
data-base questions: Estrogen pollution 35
Rivers vary in terms o the quantities o synthetic 30 oocytes in testes
estrogen (E2) ound. A study was conducted to feminized reproductive ducts
investigate the relationship between concentrations
o synthetic estrogen in water and impacts on male percent of sh 25
fsh rom the genus Rutilus (roach) (see fgure 1 8) .
20
15
a) State the relationship between synthetic 10
estrogen (E ) and the appearance o 5
2
[1 ] 0 <1 110 >10
oocytes in testes.
b) Determine the mean percentage o male fsh E2 concentration (ng/L)
with oocytes in their testes at concentrations Figure 18
o estrogen greater than 1 0 ng/L. [2] Source: Jobling et al, Environ Health Perspect.
2006 April; 114(S-1) : 3239.
The role of hormones in parturition
Birth is mediated by positive eedback involving estrogen
and oxytocin.
D uring pregnancy, progesterone inhibits secretion o oxytocin by
the pituitary gland and also inhibits contractions o the muscular
outer wall o the uterus the myometrium. At the end o pregnancy,
508
11.4 sexuAl reproduction
hormones produced by the etus signal to the placenta to stop secreting 1 Baby positions itself before birth so that its head
progesterone, and oxytocin is thereore secreted. rests close to the cervix
Oxytocin stimulates contractions o the muscle bres in the myometrium. bladder mucus plug
These contractions are detected by stretch receptors, which signal to
the pituitary gland to increase oxytocin secretion. Increased oxytocin uterus wall (compressed) (pushed down
makes the contractions more requent and more vigorous, causing more
oxytocin secretion. This is an example o a positive eedback system a front of into vagina)
very unusual control system in human physiology. In this case it has the
advantage o causing a gradual increase in the myometrial contractions, pelvis
allowing the baby to be born with the minimum intensity o contraction.
placenta umbilical spine rectum
Relaxation o muscle bres in the cervix causes it to dilate. Uterine cord
contraction then bursts the amniotic sac and the amniotic fuid passes
out. Further uterine contractions, usually over hours rather than 2 Baby passes into vagina and amniotic
minutes, nally push the baby out through the cervix and vagina. The uid is released
umbilical cord is broken and the baby takes its rst breath and achieves
physiological independence rom its mother.
daa-ba q: Hormone levels during pregnancy
In the graph (gure 20) , the thickness o the arrows indicates
relative quantities.
corpus luteum
3 Baby is pushed out of mothers body
30 days
120 days
harmone levels full term
ESTROGEN 4 Placenta and umbilical cord are expelled
hCG from body
placenta becoming
PRO GE STE RON E detached from uterus wall
0123456789 umbilical cord
conception months of pregnancy delivery Figure 19 Stages in childbirth
Figure 20 509
1 Describe the changes over the course o a pregnancy in relative
amounts and source o:
a) hCG [2]
b) estrogen [2]
c) progesterone [2]
2 Suggest reasons or the drop in hCG concentration ater the
second month o the pregnancy. [2]
3 Predict the consequences o the placenta ailing to secrete [2]
estrogen and progesterone during a pregnancy.
11 ANIMAL PHYSIOLOGY ( AHL)
Gestation times, mass and growth, and development strategies
The average 38-week pregnancy in humans can be positioned on a graph showing
the correlation between animal size and the development of the young at birth for
other mammals.
Mammals differ in their growth and development mammals in which the offspring have open eyes,
strategies. Altricial species give birth to relatively hair, are immediately mobile and are somewhat
helpless, incompletely developed offspring. Their able to defend themselves against predators.
newly-born young are relatively immobile, lack
hair and are unable to obtain food on their own. Mammals with a large body size are more likely
At the opposite end of the spectrum are precocial to be precocial. This is correlated with a long
gestation period.
data-base questions: Gestation length and body mass
Figure 21 shows the relationship between
gestation period and body mass for 429 placental
mammal species subdivided into whether the
species is described as altricial or precocial.
3
log10 gestation period 2
1 Figure 22 Laboratory mice are altricial. They have
012345678 a gestation period of about 19 days
log10 body mass
Figure 23 Elephant calves are born after a 22-month
Figure 21 gestation period and they nurse for around three years.
They are categorized as precocial. The African elephant is
1 The solid dots and open dots are the largest and heaviest land animal alive today
representative of two different growth and
development strategies. Deduce which circles
are used to represent precocial mammals. [2]
2 Outline the relationship between adult body
mass and gestation period. [1 ]
3 Explain the relationship between body mass
and the length of gestation. [3]
4 The mean length of human gestation is 283 days
( lo g 283 = 2.45) The mean body mass of an
10
adult human is 65 kg (log10 65 = 1 .8) .
(i) Determine the approximate location of
humans on the graph. [1 ]
(ii) Suggest reasons for humans being an
outlier on this graph. [3]
510
Questions
Questions b) Suggest reasons or calves that have
1 Figure 24 shows how the surace pH o human endured a long and dicult birth being
skin varies between dierent areas o the
body. It also shows dierences between adults more likely to suer rom inection. [2]
and newborn inants (neonates) . Skin pH
protects the skin rom colonization by certain c) Predict how the concentration o antibodies
microorganisms.
might vary in the cows colostrum over the
soles
rst 24 hours ater birth. [2]
back
d) Deduce the reasons or vaccinating sheep
abdomen
against pulpy kidney and other lie-
palms
threatening diseases three weeks beore
forearm
lambs are due to be born. [2]
forehead
e) Explain which method o transport
across membranes is likely to be used or
absorption o antibodies in the stomach o
newborn mammals. [2]
5 6 78 3 The blood glucose concentration o a person with
pH untreated diabetes oten rises to 300500 mg per
neonates adults 1 00 ml o blood. It can even rise to concentrations
above 1 ,000 mg per 1 00 ml. When the blood
Figure 24 How the surace pH o human skin varies between glucose level rises above 225 mg per 1 00 ml,
diferent areas o the body glucose starts to appear in the urine. The volumes
a) Compare the skin pH o neonates and o urine produced become larger than normal,
making the person dehydrated and thirsty.
adults. [2]
b) Suggest how the adult skin pH might be a) Explain how glucose is completely
established. [1 ] reabsorbed rom the glomerular ltrate o
c) Suggest why the use o soaps (which are people who do not have diabetes. [3]
basic) might have a more irritating eect on b) Explain why glucose is not all reabsorbed
the skin o a neonate. [2] rom the glomerular ltrate o diabetic
d) Deduce how basic soaps might undermine patients. [4]
the skins deensive unction. [2] c) Suggest why untreated diabetics tend to
pass large volumes o urine and oten eel
2 Figure 2 5 shows the ability o a cal ( Bos taurus) thirsty. [3]
to absorb antibodies ater birth.
antibodies absorbed/% 4 Muscles oten increase in mass i the amount
100 that they are used increases. An experiment
75 was perormed to examine the eect o fight
on muscle mass in European starlings ( Sturnus
50 vulgaris) . S tudy birds were randomly assigned
to three groups. Over 6 weeks, each group
25 was subjected to 34 1 -hour study periods. The
0 exercise group was trained to fy or 1 hour by
0 6 12 18 24 30 36 42 receiving ood rewards. Control group 1 was
calfs age at rst feeding/hours allowed to eed reely but placed into cages
Figure 25 The ability o a cal (Bos taurus) to absorb that prevented fying. Control group 2 was
antibodies ed the same ood rewards at the same time
a) Describe how the ability o a cal to absorb as the exercise group, but was also placed into
cages that prevented fying. Body mass was
antibodies changes over the initial hours monitored beore and during the experiment
(see gure 26) . At the end o the experiment,
ater birth. [2]
511
131 AniMAl physiology ( Ahl)
the mean mass o the birds pectoralis muscles (a) 85
was compared (fgure 26).
80
a) Compare the changes in body mass in body mass (g)
control group 2 and the exercise group. [2] 75
b) Evaluate the claim that preventing [3] 70
exercise increases pectoralis muscle control 1
mass.
65 control 2
c) Suggest how the mass o the birds exercise group
pectoralis muscle could be determined. [2]
60
d) One hypothesis that might be generated pectoralis mass (g)before 2 weeks 4 weeks 6 weeks
rom this experiment would be that
reducing motion in birds might lead (b) 7.5
to greater muscle mass per bird. Such 7
knowledge might be used in the arming
o poultry. Greate r meat production per 6.5
bird would result rom the motion o 6
the birds being restricted. Discuss the
ethics o designing and carrying out 5.5
experiments to test this hypothesis. [3] 5
control 1 control 2 exercise
Figure 26 The efect o exercise on body mass and muscle
mass in starlings
512
A NEUROBIOLOGY AND BEHAVIOUR
CELL BIOLOGY
Introduction
Neurobiology is the scientifc study o the starts in the earliest stages o embryogenesis and
nervous system. Living organisms use their continues to the fnal years o lie. The parts
nervous system to detect and respond to o the brain specialize in dierent unctions.
changes in the environment. Communication Behaviour patterns can be inherited or learned.
between neurons can be altered through the Natural selection avours types o behaviour
manipulation o the release and reception o that increase the chance o survival and
chemical messengers. Modifcation o neurons reproduction.
A.1 Neural development
Understanding Applications
The neural tube o embryonic chordates is Incomplete closure o the embryonic neural
ormed by inolding o ectoderm ollowed by tube can cause spina bida.
elongation o the tube.
Events such as strokes may promote
Neurons are initially produced by reorganization o brain unction.
diferentiation in the neural tube.
Skills
Immature neurons migrate to a nal location.
Annotation o a diagram o embryonic tissues
An axon grows rom each immature neuron in in Xenopus, used as an animal model, during
response to chemical stimuli. n e u ru l a ti o n .
Some axons extend beyond the neural tube to Nature of science
reach other parts o the body.
Use models as representations o the real
A developing neuron orms multiple synapses. world: developmental neuroscience uses a
variety o animal models.
Synapses that are not used do not persist.
Neural pruning involves the loss o unused
neurons.
The plasticity o the nervous system allows it to
change with experience.
513
A NEUROBIOLOGY AND BEHAVIOUR
Animal models in neuroscience
Use models as representations o the real world: developmental neuroscience
uses a variety o animal models.
Neuroscience is the branch o biology concerned number o species is used or most o this research
with neurons and nervous systems. The aim and these species are known as animal models:
o research in developmental neuroscience is
to discover how nervous systems are ormed Caenorhabditis elegans ( fatworm) because they
as animals grow rom embryo into adult. The have a low xed number o cells as adults and
aim o many neuroscientists is to understand mature very quickly.
and development treatments or diseases o
the nervous system, but many experiments are Drosophila melanogaster ( ruit fy) because
impossible to perorm in humans or ethical they breed readily, have only 4 pairs o
reasons. Also, research into other animal species chromosomes and mature very quickly.
is usually easier because the nervous system
develops more rapidly, is less complex and is Danio rerio ( zebrash) because the tissues are
easier to observe because the embryo develops almost transparent.
externally rather than in a uterus.
Xenopus laevis ( Arican clawed rog) because
For these reasons, even when researchers are the eggs are large and easily manipulated.
trying to make discoveries about humans, they
work with other species. A relatively small Mus musculus ( house mouse) because ater
millennia living near people and their ood, it
shares many human diseases.
neural plate Development of the neural tube
dorsal surface
The neural tube o embryonic chordates is ormed by
gut cavity inolding o ectoderm ollowed by elongation o the tube.
neural groove All chordates develop a dorsal nerve cord at an early stage in their
development. The process is called neurulation and in humans it occurs
lateral edges of neural plate during the rst month o gestation. An area o ectoderm cells on the
join together forming a tube dorsal surace o the embryo develops into the neural plate. The cells in
the neural plate change shape, causing the plate to old inwards orming
a groove along the back o the embryo and then separate rom the rest
o the ectoderm. This orms the neural tube, which elongates as the
embryo grows. The channel inside the neural tube persists as a narrow
canal in the centre o the spinal cord.
neural tube Development of neurons
ectoderm mesoderm Neurons are initially produced by diferentiation in the
endoderm neural tube.
Figure 1 Stages in neurulation There are billions o neurons in the central nervous system (CNS) , most
o them in the brain. The origins o these neurons can be traced back to
the early stages o embryonic development, when part o the ectoderm
develops into neuro-ectodermal cells in the neural plate. Although not
yet neurons, the developmental ate o these cells is now determined
and it is rom them that the nervous system is ormed.
514
A.1 NeurAl developmeNt
The neural plate develops into the neural tube, with continued
prolieration o cells by mitosis and dierentiation along the pathways
leading to the cells becoming unctioning neurons. The mature CNS
has ar more neurons than are initially present in the embryonic neural
tube, so cell prolieration continues in both the developing spinal cord
and brain. Although cell division ceases beore birth in most parts o the
nervous system, there are many parts o the brain where new neurons
are produced during adulthood.
Neurulation in Xenopus
Annotation o a diagram o embryonic tissues in Xenopus, used as an animal
model, during neurulation. 13 22
The diagrams in igure 2 show ive stages
in the de ve lo p me nt o a Xen opus e mb ryo ,
including the development o the neural
tube. They show the notochord, a supportive
structure that is present in all chordates during
the early stages o embryonic development 18 1 36
but which develops into the vertebral column 2
in vertebrates. The notochord is part o the
mesoderm o the embryo.
Make copies o the diagrams and annotate them 0
to show these structures or stages:
ectoderm, mesoderm and endoderm
development o the neural tube
wall o developing gut and gut cavity
notochord neurulation in xenopus
developing dorsal fn.
Figure 2 Five stages of embryonic development in Xenopus
from day 13 to day 36
Spina bifda
Incomplete closure o the embryonic neural tube can cause spina bifda.
In vertebrates, including all mammals, the spine In some cases the two parts o the arch never
comprises a series o bones called vertebrae. Each become properly used together, leaving a gap.
has a strong centrum that provides support and This condition is called spina bifda. It is probably
a thinner vertebral arch, which encloses and caused by the embryonic neural tube not closing
protects the spinal cord. The centrum develops on up completely when it is ormed rom the neural
the ventral side o the neural tube at an early stage groove. Spina bifda is commonest in the lower
in embryonic development. Tissue migrates rom back. It varies in severity rom very mild with no
both sides o the centrum around the neural tube symptoms, to severe and debilitating.
and normally meets up to orm the vertebral arch.
515
A NEUROBIOLOGY AND BEHAVIOUR
toK Migration of neurons
Can reasn n is wn, Immature neurons migrate to a nal location.
independen f sense percepin,
ever give us knwledge? Neuronal migration is a distinctive eature o the development o the
nervous system. The movement o the unicellular organism Amoeba
In the 16th century, both is easy to observe under a microscope. Neural migration can occur by
Descartes and Harvey believed a similar mechanism. The cytoplasm and organelles in it are moved
that the nerves were hollow rom the trailing end o the neuron to the leading edge by contractile
conducting tubes through which actin flaments.
the Animal spirits do rather
beam than are transported. Migration o neurons is particularly important in brain development.
The analogy o messages being Some neurons that are produced in one part o the developing brain
beamed like light, or alternatively, migrate to another part where they fnd their fnal position. Mature,
fowing like a fuid through tubes unctional neurons do not normally move, though their axons and
is a reasonable hypothesis dendrites can oten regrow i damaged.
explaining how our movements
could be smooth, sudden and Development of axons
coordinated quickly. It also
provided an explanation or An axon grows rom each immature neuron in response to
how the refexive response to chemical stimuli.
a stimulus could work. Despite
Descartes insistence on the An immature neuron consists o a cell body with cytoplasm and a
hollow nerve, contemporaries nucleus. An axon is a long narrow outgrowth rom the cell body that
noted that nerves have no carries signals to other neurons. Only one axon develops on each
perceptible cavity internally, as neuron, but it may be highly branched. Many smaller dendrites that
the veins and arteries have. In bring impulses rom other neurons to the cell body may also develop.
other words, the theory based on Chemical stimuli determine neuron dierentiation when the axon grows
reason was contravened by the out rom the cell body and also the direction in which it grows in the
empirical evidence. developing embryo.
Growth of axons
Some axons extend beyond the neural tube to reach other
parts o the body.
Axons grow at their tips. In some cases they are relatively short
and make connections between neurons within the central nervous
system, but other neurons develop very long axons which can reach
any part o the body. D espite only being outgrowths o a single cell,
axons can be more than a metre long in humans and many metres
long in larger mammals such as blue whales. Axons carry impulses
to other neurons or to cells that act as eectors either muscle or
gland cells.
As long as the cell body o its neuron remains intact, its axon may be
able to regrow i severed or damaged in other ways outside the central
nervous system. Regrowth rates can be as rapid as fve millimetres
per day so sensation or control o muscles can sometimes return over
time ater damage. O course this recovery depends on the correct
connections being re-established between an axon and the cells with
which it should be communicating.
516
A.1 NeurAl developmeNt
Development of synapses cell body of
post-synaptic
A developing neuron forms multiple synapses. neuron
The growth o an axon or dendrite is directed so that it reaches a cell nerve endings of
with which it interacts. A synapse is then developed between the neuron pre-synaptic neurons
and the other cell. The axons o motor neurons develop synapses with orming synapses
striated muscle fbres or gland cells or example. Synapse development
involves special structures being assembled in the membranes on either Figure 3 Drawing based on an electron
side o the synapse and in the synaptic clet between them. micrograph showing multiple synapses
between pre-synaptic neurons and one
The smallest number o synapses that a neuron could theoretically have is post-synaptic neuron. Only the nerve
two one to bring impulses rom another cell and another to pass them on. endings of the pre-synaptic neurons
In practice most neurons develop multiple synapses and some neurons in are shown
the brain develop hundreds, allowing complex patterns o communication.
Aciiy
Elimination ofsynapses
Na ning in h
Synapses that are not used do not persist. isa haas
Newborn babies were
Many synapses are ormed at an early stage o development, but new found to have an estimated
synapses can be ormed at any stage o lie. Synapses oten disappear i they 11.2 million neurons in the
are not used. When transmission occurs at a synapse, chemical markers are mediodorsal nucleus of the
let that cause the synapse to be strengthened. Synapses that are inactive do thalamus, but in adult brains
not have these markers so become weaker and are eventually eliminated. the estimated number was
The maxim use it or lose it thereore describes synapses very well. only 6.43 million. Assuming
that no extra neurons
Neural pruning were produced during
childhood, what percentage
Neural pruning involves the loss of unused neurons. of neurons disappears by
neural pruning?
Measurements o the number o neurons have shown that there are
more neurons in at least some parts o newborn babies brains than in
adults, which indicates that some neurons are lost during childhood.
There is also evidence or the removal o dendrites and axon branches
rom some neurons. Neurons that are not used destroy themselves by
the process o apoptosis. The elimination o part o a neuron or the
whole cell is known as neural pruning.
Plasticity of the nervous system
The plasticity of the nervous system allows it to change
with experience.
Connections between neurons can be changed by growth o axons
and dendrites, by the establishment o new synapses and also by the
elimination o synapses and pruning o dendrites, branches o axons
or even whole neurons. This ability o the nervous system to rewire its
connections is known as plasticity. It continues throughout lie, but there
is a much higher degree o plasticity up to the age o six than later.
The stimulus or a change in the connections between neurons comes
rom the experiences o a person and thus how their nervous system
is used. Plasticity is the basis or orming new memories and also or
certain orms o reasoning. It is also very important in repairing damage
to the brain and spinal cord.
517
A NEUROBIOLOGY AND BEHAVIOUR
Figure 4 Angiogram o the brain o a Strokes
48-year-old patient who had sufered a
massive stroke. A middle cerebral artery Events such as strokes may promote reorganization o
has become blocked by a blood clot brain unction.
An ischemic stroke is a disruption o the supply o blood to a part o
the brain. Most strokes are caused by a blood clot blocking one o the
small vessels in the brain, but bleeding rom a blood vessel is another
cause. During a stroke part o the brain is deprived o sufcient
oxygen and glucose. I cell respiration ceases in neurons, they become
irreparably damaged and die.
Strokes vary greatly in severity. Many are so minor that the patient
hardly notices. About one third o suerers rom major strokes make
a ull recovery and another third survive but are let with disability. In
many cases recovery rom strokes involves parts o the brain taking on
new unctions to supplement the damaged areas. Most recovery happens
over the frst six months ater a major stroke and may involve relearning
aspects o speech and writing, regaining spatial awareness and the ability
to carry out skilled physical activities such as dressing or preparing ood.
A.2 The human brain
Understanding Applications
The anterior part o the neural tube expands to Visual cortex, Brocas area, nucleus accumbens
orm the brain. as areas o the brain with specic unctions.
Diferent parts o the brain have specic roles. Swallowing, breathing and heart rate as examples
o activities coordinated by the medulla.
The autonomic nervous system controls
involuntary processes in the body using Use o the pupil reex to evaluate brain damage.
centres located in the medulla oblongata. Use o animal experiments, autopsy, lesions and
The cerebral cortex orms a larger proportion MRI to identiy the role o diferent brain parts.
o the brain and is more highly developed in
humans than other animals. Skills
The human cerebral cortex has become Identication o parts o the brain in a
enlarged principally by an increase in total area photograph, diagram or scan o the brain.
with extensive olding to accommodate it within
the cranium. Analysis o correlations between body size and
brain size in diferent animals.
The cerebral hemispheres are responsible or
higher order unctions. Nature of science
The let cerebral hemisphere receives sensory Use models as representations o the real
input rom sensory receptors in the right side o world: the sensory homunculus and motor
the body and the right side o the visual eld in homunculus are models o the relative
both eyes and vice versa or the right hemisphere. space human body parts occupy on the
somatosensory cortex and the motor cortex.
The let cerebral hemisphere controls muscle
activity in the right side o the body and vice
versa or the right hemisphere.
518 Brain metabolism requires large energy inputs.
A.2 the humAN brAiN
Development of the brain Structure ofthe brain
The anterior part othe neural tube expands to orm the brain. Identication o parts o
the brain in a photograph,
During the development o vertebrate embryos a neural tube orms diagram or scan o the brain.
along the whole o the dorsal side, above the gut, near the surace. Most
o the neural tube becomes the spinal cord, but the anterior end expands Figure 1 is a diagram showing the
and develops into the brain as part o a process called cephalization, main parts o the human brain. Use
the development o a head. The human brain contains approximately it to identiy the parts o the brain
86 billion neurons (8.6 1 010) . visible in the photo o the brain
and the MRI and C AT scans. These
The brain acts as the central control centre or the whole body, both directly three images are in the electronic
rom cranial nerves and indirectly via the spinal cord and numerous signal resources that accompany this book.
molecules carried by the blood. The advantage o having a brain is that
communication between the billions o neurons involved can be more rapid skull
than i control centres were more dispersed. The major sensory organs are cerebral
located at the anterior end o vertebrates: the eyes, ears, nose and tongue. hemisphere
pineal gland
Roles of the parts of the brain
hypothalamus
Diferent parts o the brain have specic roles. cerebellum
medulla oblongata
The brain has regions that are distinguishable by their shape, colour or
by microscopic structure. These regions have dierent roles, identifed by spinal cord
physiological research in humans and other mammals.
pituitary vertebra
The medulla oblongata is used in autonomic control o gut muscles, gland
breathing, blood vessels and heart muscle.
Figure 1 Diagram of the brain
The cerebellum coordinates unconscious unctions, such as posture,
non-voluntary movement and balance.
The hypothalamus is the interace between the brain and the pituitary
gland, synthesizing the hormones secreted by the posterior pituitary,
and releasing actors that regulate the secretion o hormones by the
anterior pituitary.
The pituitary gland: the posterior lobe stores and releases hormones
produced by the hypothalamus and the anterior lobe produces and
secretes hormones that regulate many body unctions.
The cerebral hemispheres act as the integrating centre or high
complex unctions such as learning, memory and emotions.
Methods of brain research
Use o animal experiments, autopsy, lesions and MRI to identiy the role
o diferent brain parts.
Lesion studies gave the frst useul inormation about that this part o the brain is involved with speech.
brain unctions. For example, in the 1 9th century, Another amous case was the railway construction
ater the death and autopsy o a patient who could worker Phineas Gage, who suered severe damage
only say the word Tan, the French neurologist to the rontal lobes o his brain in 1 848 when an
Charcot ound a single large tumour damaging the accident with explosives caused a large metal rod
lower let side o the patients brain. He deduced to pass through his orehead. He recovered rom
519
A NEUROBIOLOGY AND BEHAVIOUR
the wound but the brain damage radically and Figure 2 Image of brain lesion
permanently altered his personality and particularly that are activated by specic thought processes
his capacity or social interaction. to be identied. Active parts o the brain receive
increased blood fow, oten made visible by
Many lesions due to tumours, strokes or injecting a harmless dye, which MRI records.
accidental damage have been investigated by The subject is placed in the scanner and a high-
carrying out an autopsy and relating the position resolution scan o the brain is taken. A series
o the lesion to observed changes in behaviour o low-resolution scans is then taken while the
and capacities, but rather than wait or these subject is being given a stimulus. These scans
ortuitous opportunities, some neuroscientists show which parts o the brain are activated
have studied experimental animals. Removal during the response to the stimulus.
o parts o the skull gives access to the brain
and allows experimental procedures to be Figure 3 fMRI scan of endometriosis pain
perormed. The brain itsel does not eel
pain even today some orms o neurosurgery
are perormed on ully conscious patients. The
eects o local stimulation in an animals brain
can be observed, as can long-term changes in
the animals temperament and capacities. There
are widespread objections to such research,
because o the suering they may cause to the
animal and because at the end the animal is
oten sacriced, but the inormation obtained is
useul to understanding, and thereore treating,
conditions such as epilepsy, Parkinsons disease
and multiple sclerosis. Increasingly genetic
mutants and selective inactivation o genes, which
are technically possible only in mice, are used
to achieve similar experimental modication o
brain structure and behaviour.
Magnetic resonance imaging (MRI) is a more
modern and less controversial technique. Basic
MRI is used to investigate the internal structure
o the body, including looking or tumours or
other abnormalities in patients. Figure 2 shows
the results o an MRI scan o the upper part o a
patients body, including the head and brain.
A specialized version o MRI, called unctional
magnetic resonance imaging (MRI) has been
developed, which allows the parts o the brain
Examples of brain functions
Visual cortex, Brocas area, nucleus accumbens as areas o the brain with
specifc unctions.
Each o the two cerebral hemispheres has there is an initial stage in which a map o visual
a visual cortex in which neural signals inormation is projected in a region called V1 ,
originating rom light sensitive rod and cone cells the inormation is then analysed by multiple
in the retina o the eyes are processed. Although pathways in regions V2 to V5 o the visual
520
A.2 the humAN brAiN
cortex. This analysis includes pattern recognition person with a damaged Brocas area knows that it
and judging the speed and direction o moving is a zebra but cannot say the word.
objects.
There is a nucleus accumbens in each o
B rocas area is a part o the let cerebral the cerebral hemispheres. It is the pleasure or
hemisphere that controls the production o reward centre o the brain. A variety o stimuli
speech. I there is damage to this area an including ood and sex cause the release o
individual knows what they want to say and the neurotransmitter dopamine in the nucleus
can produce sounds, but they cannot articulate accumbens, which causes eelings o well-being,
meaningul words and sentences. For example, i pleasure and satisaction. Cocaine, heroin and
we see a horse-like animal with black and white nicotine are addictive because they articially cause
stripes, Brocas area allows us to say zebra, but a release o dopamine in the nucleus accumbens.
The autonomic nervous system
The autonomic nervous system controls involuntary
processes in the body using centres located in the
medulla oblongata.
The peripheral nervous system comprises all o the nerves outside the
central nervous system. It is divided into two parts: the voluntary and
the autonomic nervous systems. Involuntary processes are controlled by
the autonomic nervous system, using centres in the medulla oblongata.
The autonomic nervous system has two parts: sympathetic and
parasympathetic. These oten have contrary eects on an involuntary
process. For example, parasympathetic nerves cause an increase in
blood fow to the gut wall during digestion and absorption o ood.
Sympathetic nerves cause a decrease in blood fow during asting or
when blood is needed elsewhere.
Activities coordinated by the medulla
Swallowing, breathing and heart rate as examples of activities coordinated
by the medulla.
The rst phase o swallowing, in which ood is more than oxygen concentration. I blood pH
passed rom the mouth cavity to the pharynx, alls, indicating an increase in carbon dioxide
is voluntary and so is controlled by the cerebral concentration, breathing becomes deeper and/or
cortex. The remaining phases in which the ood more requent.
passes rom the pharynx to the stomach via the
esophagus, are involuntary and are coordinated by The cardiovascular centre o the medulla regulates
the swallowing centre o the medulla oblongata. the rate at which the heart beats. Blood pH
and pressure are monitored by receptor cells in
Two centres in the medulla control breathing: blood vessels and in the medulla. In response to
one controls the timing o inspiration; the other this inormation, the cardiovascular centre can
controls the orce o inspiration and also active, increase or decrease the heart rate by sending
voluntary expiration. There are chemoreceptors signals to the hearts pacemaker. S ignals carried
in the medulla that monitor blood pH. The rom the sympathetic system speed up the heart
carbon dioxide concentration in the blood is very rate; signals carried by the parasympathetic
important in controlling breathing rate, even system in the vagus nerve slow the rate down.
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A NEUROBIOLOGY AND BEHAVIOUR
The pupil refex and brain damage
Use o the pupil refex to evaluate brain damage.
Muscles in the iris control the size o the pupil o muscle in the iris, constricting the pupil and
the eye. Impulses carried to radial muscle bres reducing the amount o light entering the eye,
by neurons o the sympathetic system cause protecting the delicate retina rom damage.
them to contract and dilate the pupil; impulses
carried to circular muscle bres by neurons o the Doctors sometimes use the pupil refex to test a
parasympathetic system cause the pupil to constrict. patients brain unction. A light is shone into each
eye. I the pupils do not constrict at once, the
The pupil refex occurs when bright light medulla oblongata is probably damaged. I this
suddenly shines into the eye. Photoreceptive and other tests o brain stem unction repeatedly
ganglion cells in the retina perceive the bright ail, the patient is said to have suered brain
light, sending signals through the optic nerve death. It may be possible to sustain other parts o
to the mid-brain, immediately activating the the patients body on a lie support machine, but
parasympathetic system that stimulates circular ull recovery is extremely unlikely.
The cerebral cortex
The cerebral cortex orms a larger proportion o the brain and
is more highly developed in humans than other animals.
The cerebral cortex is the outer layer o the cerebral hemispheres.
Although it is only two to our millimetres thick, up to six distinctively
dierent layers o neurons can be identied in sections studied under
a microscope. It is has a highly complex architecture o neurons and
processes the most complex tasks in the brain.
Only mammals have a cerebral cortex. Birds and reptiles have regions o
the brain that perorm a similar range o unctions but they are structurally
dierent, with cells arranged in clusters rather than layers. Among the
mammals the cerebral cortex varies in size considerably. In humans it
orms a larger proportion o the brain than in any other mammal.
frontal lobe parietal lobe The evolution o the cerebral cortex
occipital The human cerebral cortex has become enlarged
lobe principally by an increase in total area with extensive
olding to accommodate it within the cranium.
temporal medulla
lobe oblongata The cerebral cortex has become greatly enlarged during human
evolution, and now contains more neurons than that o any other
cerebellum animal. There has been a modest increase in thickness, but the cortex
is still only a ew millimetres thick. The increase is due principally to
Figure 4 The folded structure of the cerebral an increase in total area and that necessitates the cortex becoming
cortex, viewed from the left side. The four lobes extensively olded during development. It is hard to measure, but the
are indicated area is estimated to be about 1 80,000 mm2 or 0.1 8 m2. This is so large
that the brain can only be accommodated inside a greatly enlarged
cranium, orming the distinctive shape o the human skull.
Most o the surace area o the cerebral cortex is in the olds rather than
on the outer surace. In contrast, mice and rats have an unolded smooth
cortex, but in cats there are some olds and elephants and dolphins have
522
A.2 the humAN brAiN
more. Among the primates, monkeys and apes show a range o cortex
size and degree o olding, with larger sizes in primates that are more
closely related to humans.
Comparing brain size elephant 4.8 kg
human 1.4 kg
Analysis o correlations between body chimp 0.42 kg
size and brain size in diferent animals.
Scattergraphs show a positive correlation between
body size and brain size in animals, but that the
relationship is not directly proportional. The data-
based questions below can be used to develop
your skill in analysing this type o data.
daa-as qsons: Brain and body size in mammals
104 monotremes elephant 1 State the relationship between brain and
marsupials human
mass of brain/g (log scale) body mass. [1 ]
placentals
103 dolphin h u m p - ba cke d 2 Explain how the points on the scattergraph
whale
chimpanzee
fox would have been arranged i brain mass was
102 echidna cat sheep
grey kangaroo directly proportional to body mass. [2]
squirrel monkey quokka 3 State which mammals have (a) the largest and
101 platypus brush-tailed possum
opossum (b) the smallest brain mass. [2]
100 bandicoot
rat hedgehog
shrew 4 Discuss the evidence provided by the
0
scattergraph or the hypothesis that humans
101 102 103 104 105 106 107 108
have the largest relative brain mass. [2]
mass of body/g (log scale)
Figure 5 5 Evaluate the hypothesis that marsupials
have relatively small brains compared
The scattergraph in fgure 5 shows the with other mammals. [2]
relationship between brain and body mass in
species o placental, marsupial and monotreme 6 Suggest a reason or the researchers not
mammal.
including more data or monotremes in the
scattergraph. [1 ]
Functions of the cerebral hemispheres
The cerebral hemispheres are responsible or higher
order unctions.
The cerebral hemispheres carry out the most complex o the brains
tasks. These are known as higher order unctions and include learning,
memory, speech and emotions. These higher order unctions involve
association o stimuli rom dierent sources including the eye and
ear and also rom memories. They rely on very complex networks o
neurons that are still only partially understood by neurobiologists. The
most sophisticated thought processes such as reasoning, decision-making
and planning occur in the rontal and prerontal lobes o the cerebral
cortex. Using these parts o the brain we can organize our actions in a
523
A NEUROBIOLOGY AND BEHAVIOUR
logical sequence, predict their outcomes, develop a sense o right and
wrong and be aware o our own existence.
Sensory inputs to the cerebral hemispheres
The let cerebral hemisphere receives sensory input rom
sensory receptors in the right side o the body and the
right side o the visual feld in both eyes and vice versa or
the right hemisphere.
The cerebral hemispheres receive sensory inputs rom all the sense organs
o the body. For example, signals rom the ear pass to the auditory area in
the temporal lobe. Signals rom the let ear pass to the let hemisphere and
rom the right ear to the right hemisphere. Inputs rom the skin, muscles
and other internal organs pass via the spinal cord to the somatosensory
area o the parietal lobe. Perhaps surprisingly, the impulses rom each side
cross in the base o the brain so that the let hemisphere receives impulses
rom the right side o the body and vice versa.
Inputs rom the eye pass to the visual area in the occipital lobe, known as the
visual cortex. Impulses rom the right side o the feld o vision in each eye
are passed to the visual cortex in the let hemisphere, while impulses rom
the let side o the feld o vision in each eye pass to the right hemisphere.
This integration o inputs enables the brain to judge distance and perspective.
Motor control by the cerebral hemispheres
The let cerebral hemisphere controls muscle activity in the
right side othe body and vice versa or the right hemisphere.
Regions in each o the cerebral hemispheres control striated (voluntary)
muscles. The main region is in the posterior part o the rontal lobe
and is called the primary motor cortex. In this region there is a series o
overlapping areas that control muscles throughout the body, rom the
mouth at one end o the primary motor cortex to the toes at the other end.
The primary motor cortex in the let hemisphere controls muscles in the
right side o the body and that in the right side controls muscles in the let
side o the body. So a stroke ( or other brain damage) in the let side o the
brain can cause paralysis in the right side o the body and vice versa.
Homunculi
Use models as representations o the real world: the sensory homunculus and
motor homunculus are models o the relative space human body parts occupy on
the somatosensory cortex and the motor cortex.
Neurobiologists have constructed models o the devoted to sensory inputs rom that part. This
body in which the size o each part corresponds type o model is called a sensory homunculus.
to the proportion o the somatosensory cortex Similar models have been constructed to show the
524
A.2 the humAN brAiN
proportion of the motor cortex that is devoted to of the relative importance given to sensory inputs
control of muscles in each part of the body. These from different parts of the body and to control of
models are useful as they give a good impression muscles in different parts.
wrist hand
elbow thumibndmiedxdl reinligttle
shoulder
trunk
hip
knee
leg
hip
trunk
neck
head
shoulder
arm
elbow
forearm
wrist
hand
little
ri nmg idi nddleex
facneoesyeethumb foot ankle
upper lip toes toes eyneebliecdrokyawenbfdaaclle
lips genitals lips
primary
lower lip primary motor cortex jaw
teeth, gums somatosensory swallowtoinnggue
tongue cortex
pharynx
intra abdominal
Figure 6 Sensory homunculus (left) and motor homunculus ( right)
Energy and the brain
Brain metabolism requires large energy inputs.
Energy released by cell respiration is needed to maintain the resting
potential in neurons and to re-establish it after an action potential, as
well as for synthesis of neurotransmitters and other signal molecules.
The brain contains a huge number of neurons so it needs much oxygen
and glucose to generate this energy by aerobic cell respiration. In most
vertebrates the brain uses less than 1 0% of the energy consumed by
basal metabolism but in the adult human brain it is over 20% and an
even higher proportion in infants and small children.
525
A NEUROBIOLOGY AND BEHAVIOUR
A.3 percetion of stimuli
Understanding Applications
Receptors detect changes in the environment. Red-green colour-blindness as a variant o
normal trichromatic vision.
Rods and cones are photoreceptors located in
the retina. Detection o chemicals in the air by the many
diferent olactory receptors.
Rods and cones difer in their sensitivities to
light intensities and wavelengths. Use o cochlear implants in dea patients.
Bipolar cells send the impulses rom rods and Skills
cones to ganglion cells.
Labelling a diagram o the structure o the
Ganglion cells send messages to the brain via human eye.
the optic nerve.
Annotation o a diagram o the retina to show the
The inormation rom the right eld o vision cell types and the direction o the light source.
rom both eyes is sent to the let part o the
visual cortex and vice versa. Labelling a diagram o the structure o the
human ear.
Structures in the middle ear transmit and
ampliy sound. Nature of science
Sensory hairs o the cochlea detect sounds o Understanding o the underlying science is
specic wavelengths. the basis or technological developments: the
discovery that electrical stimulation in the
Impulses caused by sound perception are auditory system can create a perception o
transmitted to the brain via the auditory nerve. sound resulted in the development o electrical
hearing aids and ultimately cochlear implants.
Hair cells in the semicircular canals detect
movement o the head.
Sensory receptors
Receptors detect changes in the environment.
The environment, particularly its changes, stimulate the nervous
system via sensory receptors. The nerve endings of sensory neurons
act as receptors, for example touch receptors. In other cases there are
specialized receptor cells that pass impulses to sensory neurons, as
with the light-sensitive rod and cone cells of the eye. Humans have the
following types of specialized receptor.
Mechanoreceptors respond to mechanical forces and movements.
Chemoreceptors respond to chemical substances.
Thermoreceptors respond to heat.
Photoreceptors respond to light.
526
A.3 perCeptioN of stimuli
Olfactory receptors
Detection o chemicals in the air by the many diferent olactory receptors.
Olfaction is the sense of smell. Olfactory receptor sense of smell is very insensitive and imprecise
cells are located in the epithelium inside the compared to that of other animals.
upper part of the nose. These cells have cilia
which project into the air in the nose. Their
membrane contains odorant receptor molecules,
proteins which detect chemicals in the air. O nly
volatile chemicals can be smelled in air within the
nose. Odorants from food in the mouth can pass
through mouth and nasal cavities to reach the
nasal epithelium.
There are many different odorant receptor Figure 1 Olfactory receptor cell (centre) with two of its cilia
proteins, each encoded by a different gene. In visible and also cilia in adjacent cells in the nasal epithelium
some mammals such as mice there are over a
thousand different odorant receptors, each of
which detects a different chemical or group of
chemicals (though the exact mechanisms are still
unclear in spite of intensive study) . Each olfactory
receptor cell has just one type of odorant receptor
in its membrane, but there are many receptor cells
with each type of odorant receptor, distributed
though the nasal epithelium. Using these receptor
cells most animals, including mammals, can
distinguish a large number of chemicals in the
air, or in water in the case of aquatic animals.
In many cases the chemical can be detected in
extremely low concentrations but the human
Structure of the eye
Labelling a diagram o the structure o the human eye.
lens sclera
choroid
aqueous humour retina
pupil fovea
iris
blind spot
conjunctiva optic nerve
cornea
vitreous humour
Figure 2 A diagram of the human eye in horizontal section
527
A NEUROBIOLOGY AND BEHAVIOUR
toK Photoreceptors
if ur senses can be fled Rods and cones are photoreceptors located in the retina.
by llusns, wha are he
mplcans fr knwledge Light entering the eye is ocused by the cornea and the lens onto
clams based n emprcal the retina, the thin layer o light-sensitive tissue at the back o
evdence? the eye. Figure 5 shows the cell types in the retina. Two main
types o photoreceptor are present in the human retina, rods
Scientists argue that because and cones. Many nocturnal mammals have only rods and cannot
the visual sense is dominant, distinguish colours. Rods and cones are stimulated by light and so
illusions can arise when together detect the image ocused on the retina and convert it into
conficting inormation is received neural signals.
rom visual inormation and the
other senses. Food dyed with Diferences between rods and cones
colouring to make it appear odd
becomes unpalatable. In the Rods and cones dier in their sensitivities to light
McGurk eect, seeing mouth intensities and wavelengths.
movements corresponding to one
sound paired with the auditory Rods are very sensitive to light, so work well in dim light. In very bright
inormation o another sound light the pigment in them is temporarily bleached so or a ew seconds
causes the subject to hear they do not work. Rod cells absorb a wide range o visible wavelengths
the sound corresponding to the o light (see fgure 3) but cannot respond selectively to dierent colours,
mouth movements. In the rubber so they give us black and white vision.
hand illusion, experimenters can
eect a sensation in subjects by There are three types o cone, which absorb dierent ranges o
stroking a rubber hand that they wavelengths o light. They are named according to the colour that
have stroked in the same way as they absorb most strongly: red, blue or green. When light reaches the
their real hand. retina, the red, blue and green cones are selectively stimulated. By
analysing the relative stimulation o each o the three cone types, the
Acvy colour o light can be very precisely determined, though experiments
show that peoples perception o colour diers quite a lot. Cones are
Caarac surgery only stimulated by bright light and thereore colour vision ades in
dim light.
Accumulation o metabolic
wastes in the cells o the 420 498 534 564
eyes lens gradually turns 100
them yellow so blues ade.
The dierence in colour normalized absorbance 50
perception ater a cataract
operation is startling. Talk to S R ML
a person, probably elderly,
who has had cataract surgery
to nd out how it changed
their colour perception.
0
400 500 600 700
violet blue cyan green yellow red
wavelength (nm)
Figure 3 Absorption spectra for blue (short, S) , green (medium, M) and red (long, L)
wavelength-sensitive cones and for rods (dotted line)
528
A.3 perCeptioN of stimuli
Red-green colour-blindness Figure 4 Red and green colours cannot easily be distinguished
by some males and fewer females
Red-green colour-blindness as a variant
of normal trichromatic vision.
Red-green colour-blindness is a common inherited
condition in humans and some other mammals.
It is due to the absence o, or a deect in, the gene
or photoreceptor pigments essential to either red
or green cone cells. Both genes are located on
the human X chromosome so it is a sex-linked
condition. The normal alleles o both genes are
dominant and the alleles that cause red-green
colour-blindness are recessive. Red-green colour-
blindness is thereore much commoner among
males, who have only one X chromosome, than
emales, and males inherit the allele that causes
the condition rom their mother.
Structure of the retina ga n gl i o n direction of light
cell nerve bres
Annotation of a diagram of the retina to of ganglion
show the cell types and the direction of cells
the light source.
bipolar neuron
The arrangement o the layers o cells in the
retina may seem surprising. The light passes rod cell
frst through a layer o transparent nerve axons
that carry impulses rom the retina to the brain cone cell
through the optic nerve, then through a layer o
specialized bipolar neurons that process signals layer of pigmented
beore they reach the optic nerve, and only then cells
does the light reach the rod and cone cells. This is
shown in fgure 5. Figure 5 Arrangement of cell types in the retina
Bipolar cells
Bipolar cells send the impulses from rods and cones to
ganglion cells.
Rod and cone cells synapse with neurons called bipolar cells in the
retina. I rod or cone cells are not stimulated by light they depolarize
and release an inhibitory neurotransmitter onto a bipolar cell, causing
it to become hyperpolarized and not transmit impulses to its associated
retinal ganglion cell. When light is absorbed by a rod or cone cell it
becomes hyperpolarized and stops sending inhibitory neurotransmitter
to the bipolar cell. The bipolar cell can thereore depolarize, activating
the adjacent ganglion cell.
Groups o rod cells send signals to the brain via a single bipolar cell, so
the brain cannot distinguish which rod absorbed the light. The images
529
A NEUROBIOLOGY AND BEHAVIOUR
transmitted to the brain by rods alone are lower resolution, like a grainy
photograph, whereas those based on the cones are sharper because each
cone cell sends signals to the brain via its own bipolar cell.
Ganglion cells
Ganglion cells send messages to the brain via the
optic nerve.
Retinal ganglion cells have cell bodies in the retina with dendrites that
orm synapses with bipolar cells. Ganglion cells also have long axons
along which impulses pass to the brain. Impulses are passed at a low
requency when the ganglion cell is not being stimulated and at an
increased rate in response to stimuli rom bipolar cells.
The axons o ganglion cells pass across the ront o the retina to orm a
central bundle at the blind spot, so called because their presence makes
a gap in the layer o rods and cones. The axons o the ganglion cells pass
via the optic nerve to the optic chiasma in the brain.
visual eld Vision in the right and let felds
right eye The inormation rom the right feld o vision rom both
right optic nerve eyes is sent to the let part othe visual cortex and vice versa.
optic chiasma
thalamus Simple experiments comparing vision with one eye or with both eyes show
visual cortex the distance and relative size o objects can be judged most precisely when
observed by two eyes simultaneously. Stimuli rom both eyes are integrated
Figure 6 The optic chiasma by the axons o some retinal ganglion cells crossing rom one side to the
other between eye and brain while other axons stay on the same side.
The crossing over o axons between let and right sides happens in the optic
chiasma, shown in fgure 6. As a result, the visual cortex in the right cerebral
hemisphere processes visual stimuli rom the let side o the visual feld o
both eyes, and vice versa or stimuli rom the right side o the feld o vision.
Structure o the ear
Labelling a diagram o the structure o the human ear.
pinna incus
malleus stapes semicircular canals
bones of skull
muscle auditory nerve
attached
ear drum oval window
round window cochlea
Figure 7 The structure of the ear
530
A.3 perCeptioN of stimuli
The middle ear i han a nv ny
can ang , wha
Structures in the middle ear transmit and ampliy sound. cnqnc an gh
h hav h acqn
The middle ear is an air-lled chamber between the outer ear and knwdg?
the inner ear. A thin, taut sheet o fexible tissue called the eardrum
separates the middle ear rom the outer ear. Two other thin sheets o Figure 8 shows the requency
tissue called the oval and round windows separate the middle ear rom sensitivity o six land mammals. The
the inner ear. solid area shows where requency
sensitivity is best, while the lines
Three tiny bones are in the middle ear, the malleus ( hammer) , incus indicate how much louder other
(anvil) and stapes (stirrup) , which articulate with each other to orm requencies need to be in order to be
a connection between the eardrum and the oval window. These bones, heard.
also called ossicles, transmit vibrations rom the eardrum to the oval
window, ampliying sound twentyold because the oval window has 1 Does the world sound the same to
a smaller area than the eardrum. During very loud sounds, the any o the animals?
delicate sound-reception components o the ear are protected by
contraction o the muscles attached to the bones in the middle ear, 2 Which is the real world the one we
which weakens the connections between the ossicles and so damps perceive or the world perceived by
the vibrations. the bat?
The cochlea 3 Animals also difer considerably in
their visual perception. Is what each
Sensory hairs o the cochlea detect sounds o animal sees what is really there, is it
specic wavelengths. a construction o reality, or is reality
a alse concept?
The cochlea is the part o the inner ear where vibrations are transduced
into neural signals. It is a tubular, coiled, fuid-lled structure. Within 0 dB +20 dB +40 dB +60 dB
the cochlea are layers o tissue (membranes) to which sensory cells are
attached. Each o these cells has a bundle o hairs, stretching rom one human
membrane to another. When vibrations are transmitted rom the oval
window into the cochlea, they resonate with the hair bundles o particular cat
hair cells, stimulating these cells. Selective activation o dierent hair cells guinea
enables us to distinguish between sounds o dierent pitch. pig
monkey
The round window is another thin sheet o fexible tissue, located
between the middle and inner ear. I it was sti and indeormable, the bat
oval window would not be able to vibrate, because the incompressible
fuid in the cochlea would prevent it rom moving. When vibrations rat
o the oval window push the fuid in the cochlea inwards, the round
window moves outwards, and when the oval window moves outwards, 10 100 1000 10000 100000
the round window moves inwards, enabling the oval window to frequency (Hz)
transmit vibrations through the fuid in the cochlea.
Figure 8 Sensitivity of mammals to
frequencies of sound
The auditory nerve
Impulses caused by sound perception are transmitted to
the brain via the auditory nerve.
When a hair cell in the cochlea is depolarized by the vibrations that
constitute sounds, it releases neurotransmitter across a synapse,
stimulating an adjacent sensory neuron. This triggers an action potential
in the sensory neuron which propagates to the brain along the auditory
nerve. The auditory nerve is one o the cranial nerves that serve the brain.
531
A NEUROBIOLOGY AND BEHAVIOUR
Cochlear implants
Use of cochlear implants in deaf patients.
Deaness has a variety o causes and in many cases these signals into electrical impulses and an
a hearing aid that amplifes sounds can overcome array o electrodes that carry these impulses
the problem. However, i the hair cells in the to the cochlea. The electrodes stimulate the
cochlea are deective, such hearing aids do not help. auditory nerve directly and so bypass the non-
In this case the best option, as long as the auditory unctional hair cells.
nerve is unctioning properly, is a cochlear implant.
More than a quarter o a million people have had transmitter receiver and stimulator
these devices implanted and although they do not microphone
ully restore normal hearing, they improve it and
usually allow recognition o speech.
Cochlear implants consist o external and
internal parts.
The external parts are a microphone to detect electrode
sounds, a speech processor that selects the array
requencies used in speech and flters out
other requencies, and a transmitter that sends
the processed sounds to the internal parts.
The internal parts are implanted in the Figure 8 Cochlear implant with microphone behind the
mastoid bone behind the ear. They consist o ear connected to the transmitter and adjacent to this the
a receiver that picks up sound signals rom internal receiver and stimulator, with electrodes leading to
the transmitter, a stimulator that converts the auditory nerve that arises in the cochlea
The science behind cochlear implants
Understanding of the underlying science is the basis for technological
developments: the discovery that electrical stimulation in the auditory system can
create a perception of sound resulted in the development of electrical hearing aids
and ultimately cochlear implants.
Research into artifcial electrical stimulation During the 1 970s early versions o cochlear
o the cochlea began as early as the 1 950s. implants were ftted to over a thousand patients.
Early attempts showed that it was possible Since then research has led to huge technological
to give some perception o sound to people developments in these devices with greatly
who were severely or prooundly dea due to improved outcomes or the increasing number
non-unctioning hair cells. Experiments with o people that have had them ftted. Further
humans showed that electrical stimulation improvements can be expected and although
could be used to give perception o dierent cochlear implants can never give severely or
requencies o sound, as in music. Research prooundly dea people normal hearing, they
continued and involved electronic engineers, can allow ar better hearing than without
neurophysiologists and clinical audiologists. An this technology.
understanding o which requencies are used to
understand speech was used to develop speech
processors or example.
532
A.4 iN N Ate AN d le ArN ed beh Aviour ( Ah l)
Detecting head movements 1 3
Hair cells in the semicircular canals detect movement 2
o the head.
Figure 9 Inner ear with cochlea ( left) and
There are three fuid-lled semicircular canals in the inner ear. Each has semicircular canals (right) : superior (1) ,
a swelling at one end in which there is a group o sensory hair cells, with lateral (2) and posterior (3)
their hairs embedded in gel to orm a structure called the cupula. When
the head moves in the plane o one o the semicircular canals, the sti
wall o the canal moves with the head, but due to inertia the fuid inside
the canal lags behind. There is thereore a fow o fuid past the cupula.
This is detected by the hair cells, which send impulses to the brain.
The three semicircular canals are at right angles to each other, so each is in
a dierent plane. They can thereore detect movements o the head in any
direction. The brain can deduce the direction o movement by the relative
amount o stimulation o the hair cells in each o the semicircular canals.
A.4 inna an an a (Ahl)
Understanding Applications
Innate behaviour is inherited rom parents Withdrawal refex o the hand rom a painul
and so develops independently o the stimulus.
environment.
Pavlovs experiments into refex conditioning
Autonomic and involuntary responses are in dogs.
reerred to as refexes.
The role o inheritance and learning in the
Refex arcs comprise the neurons that mediate development o birdsong.
refexes.
Skills
Learned behaviour develops as result o
experience. Analysis o data rom invertebrate behaviour
experiments in terms o the eect on chances
Refex conditioning involves orming new o survival and reproduction.
associations.
Drawing and labelling a diagram o a refex arc
Imprinting is learning occurring at a particular or a pain withdrawal refex.
lie stage and is independent o the
consequences o behaviour. Nature of science
Operant conditioning is a orm o learning which Looking or patterns, trends and discrepancies:
consists o trial and error experiences. laboratory experiments and eld investigations
helped in the understanding o dierent types
Learning is the acquisition o skill or knowledge. o behaviour and learning.
Memory is the process o encoding, storing and
accessing inormation.
533
A NEUROBIOLOGY AND BEHAVIOUR
Innate behaviour
Innate behaviour is inherited rom parents and so
develops independently o the environment.
Animal behaviour is divided into two broad categories, innate and
learned. The orm o innate behaviour is unaected by external
infuences that an animal experiences. It develops independently o the
environment. For example, i an object touches the skin in the palm o
a babys hand, the baby grips the object by closing its ngers around it.
This innate behaviour pattern, called the palmar grasp refex, is seen
in babies rom birth until they are about six months old, whatever
experiences the baby has.
Innate behaviour is genetically programmed, so it is inherited. It can
change through evolution i there is genetically determined variation
in behaviour and natural selection avours one behaviour pattern
over others, but the rate o change is much slower than with learned
b e h a v io u r.
Research methods in animal behaviour
Looking or patterns, trends and discrepancies: laboratory experiments and
eld investigations helped in the understanding o diferent types o behaviour
and learning.
The scientic study o animal behaviour The advantage o laboratory experiments is that
became established as a signicant branch o variables can be controlled more eectively
biology in the 1 930s. Beore then naturalists and innate behaviour in particular can be
observed the behaviour o animals in natural investigated rigorously. The disadvantage is that
habitats but had rarely analysed it scientically. animal behaviour is an adaptation to the natural
Two general types o methodology have since environment o the species and animals oten do
been used: laboratory experiments and eld not behave normally when removed rom that
investigations. environment, especially with learned behaviour.
Invertebrate behaviour experiments
Analysis o data rom invertebrate behaviour experiments in terms o the efect
on chances o survival and reproduction.
Many invertebrates have relatively simple and reproduction and thus how it evolved by
behaviour patterns, so they can be studied more natural selection as an innate behaviour pattern.
easily than mammals, birds or other vertebrates.
A stimulus can be given and the response to it Many dierent invertebrates can be used in
observed. Repeating the stimulus with a number experiments. Planarian latworms, woodlice,
o individuals allows quantitative data to be blowly larvae, snails and beetles are oten used.
obtained and tests o statistical signicance to be Some species can be purchased rom suppliers
done. Once the response to a stimulus has been but it is also possible to use invertebrates rom
discovered, it may be possible to deduce how the local habitats. These should be kept or a short
response improves animals chances o survival time only, protected rom suering during the
534
A.4 iN N Ate AN d le ArN ed beh Aviour ( Ah l)
experiments and then returned to their habitat. Stages in designing an investigation:
Endangered species should not be used.
1 Place the animals in conditions that are similar
Two types o behaviour involving movement to the natural habitat.
could be investigated:
2 Observe the behaviour and see what stimuli
Taxis is movement towards or away rom a aect movement.
directional stimulus. An example is movement
o a woodlouse or slater away rom light. 3 Choose one stimulus that appears to cause a
taxis or kinesis.
Kinesis also involves movement as a response,
but the direction o movement is not 4 Devise an experiment to test responses to the
infuenced by the stimulus. Instead, the speed stimulus.
o movement or the number o times the
animal turns is varied. An example is slower 5 Ensure that other actors do not have an eect
movement, with more requent turning, when on the movement.
woodlice are transerred rom drier to more
damp conditions. 6 Decide how to measure the movement o the
invertebrates.
Refexes
Autonomic and involuntary responses are reerred
to as refexes.
A stimulus is a change in the environment, either internal or external,
that is detected by a receptor and elicits a response. A response is a
change in an organism, oten carried out by a muscle or a gland. Some
responses happen without conscious thought and are thereore called
involuntary responses. Many o these are controlled by the autonomic
nervous system. These autonomic and involuntary responses are known
as refexes.
A refex is a rapid unconscious response to a stimulus. The pupil
refex is an example: in response to the stimulus o bright light,
the radial muscles in the iris o the eye contract, constricting the
pupil. This involuntary response is carried out by the autonomic
nervous system.
Refex arcs
Refex arcs comprise the neurons that mediate refexes.
All refexes start with a receptor that perceives the stimulus and ends
with an eector, usually a muscle or gland, which carries out the
response. Linking the receptor to the eector is a sequence o neurons,
with synapses between them. The sequence o neurons is known as a
refex arc. In the simplest refex arcs there are two neurons: a sensory
neuron to carry impulses rom the receptor to a synapse with a motor
neuron in the spinal cord and a motor neuron to carry impulses on to
the eector. Most refex arcs contain more than two neurons, as there
are one or more relay neurons connecting the sensory neuron to the
motor neuron.
535
A NEUROBIOLOGY AND BEHAVIOUR
Activity The withdrawal refex
refex speed Withdrawal refex o the hand rom a painul stimulus.
The withdrawal refex The pain withdrawal refex is an innate response to a pain stimulus.
takes less than a tenth o For example i we touch a hot object with the hand, pain receptors
a second. Reaction times in the skin o the nger detect the heat and activate sensory neurons,
that involve more complex which carry impulses rom the nger to the spinal cord via the dorsal
processing take longer. Use root o a spinal nerve. The impulses travel to the ends o the sensory
online tests i you want to neurons in the grey matter o the spinal cord where there are synapses
assess your reaction time, with relay neurons. The relay neurons have synapses with motor
using the search term refex neurons, which carry impulses out o the spinal cord via the ventral
test to nd them. root and to muscles in the arm. Messages are passed across synapses
rom motor neurons to muscle bres, which contract and pull the arm
away rom the hot object.
Neural pathways in a refex arc
Drawing and labelling a diagram o a refex arc or a pain withdrawal refex.
Figure 1 shows the refex arc or the pain withdrawal refex.
nerve bre of receptor cells or nerve relay neuron
sensory neuron endings sensing pain central canal
cell body of sensory neuron
in the dorsal root ganglion
dorsal root of
spinal nerve
spinal nerve
nerve bre of ventral root of
motor neuron spinal nerve cell body of
eector (muscle that motor neuron white matter grey matter
pulls hand away from
pain when it contracts) spinal cord
Figure 1 Components o a refex arc
Learned behaviour
Learned behaviour develops as result o experience.
Ospring inherit the capacity or propensity to acquire new patterns
o behaviour during their lie, as a result o experience. This is known
as learned behaviour. O spring learn behaviour patterns rom their
parents, rom other individuals and rom their experience o the
536
A.4 iN N Ate AN d le ArN ed beh Aviour ( Ah l)
environment. For example, human ospring inherit the capacity to
learn a language. The language that they learn is usually that o their
biological parents, but not i they are adopted by adults who speak a
dierent language. The ability to make sense o vocal patterns and then
make them onesel is innate but the specifc language spoken is learned.
Development of birdsong
The role of inheritance and learning in the development of birdsong.
Birdsong has been investigated intensively in including all passerines, males learn mating calls
some species and evidence has been ound or rom their ather. The learned aspects introduce
it being partly innate and partly learned. All dierences, allowing males to be recognized
members o a bird species share innate aspects by their song and in some species mates to be
o song, allowing each individual to recognize chosen by the quality o their singing.
other members o the species. In many species,
daa-as qsns: Birdsong innate or learned?
The sonograms in fgure 2 are a visual c) Suggest two reasons why birds rarely
representation o birdsong, with time on the
x-axis and requency or pitch on the y-axis. imitate other species. [2]
d) Discuss whether Morton and Baptistas
1 Compare sonograms I and II, which are rom observation is evidence or innate or
two populations o white-crowned sparrows learned development o birdsong. [2]
(Zonotrichia leucophrys). [2] I
2 Sonogram III is rom a white-crowned
sparrow that was reared in a place where it
could not hear any other birdsong.
a) Compare sonogram III with sonograms
I and II. [2] II
b) Discuss whether the song o white-crowned
sparrows is innate, learned or due to both
innate actors and learning. [3]
3 In 1 981 Martin Morton and Luis Baptista III
published a very unusual discovery a white- IV
crowned sparrow had learned to imitate the V
song o another species. Sonogram IV is rom
a strawberry fnch (Amandava amandava).
Sonogram V is rom a white-crowned sparrow
that had been hand-reared by itsel until it
was 46 days old and then placed in an aviary
with other white-crowned sparrows and a
strawberry fnch.
a) Compare sonogram V with sonogram IV. [2 ]
b) Compare sonogram V with sonograms
I and II. [2]
Figure 2 Sonograms of birdsong
537
A NEUROBIOLOGY AND BEHAVIOUR
Innate and learned behaviour thus both depend on genes, but whereas
the development o learned behaviour develops as a result o experience,
innate behaviour is independent o it.
Figure 3 Monarch butterfy caterpillars ingest Refex conditioning
toxins (cardenolide aglycones) rom the
milkweed plants that they eat, making them Refex conditioning involves orming new associations.
distasteul to birds
Several dierent types o learning have been dened. One o these,
called refex conditioning, was investigated by the Russian physiologist
Ivan Pavlov, using dogs. Refex conditioning involves orming new
associations by establishing new neural pathways in the brain.
Conditioned refexes are used extensively in animal behaviour and
can greatly increase survival chances.
For example, birds have an innate refex to avoid oods with a bitter
tastethis is an unconditioned refex, but they have to learn which
insects are likely to have that taste. I a bird tries to eat an insect with
warning coloration o black and yellow stripes, or example, and nds
that the insect tastes unpleasant, it develops an association between
black and yellow stripes and bitter taste and thereore avoids all insects
with such a colour pattern. In some cases the smell o the distasteul
insect has to be combined with its coloration to cause avoidance.
Pavlovs experiments
Pavlovs experiments into refex conditioning in dogs.
The 1 9th century Russian physiologist Pavlov developed apparatus to
collect saliva rom the mouth o his experimental dogs. He ound that
saliva was secreted in response to the sight or smell o ood. These
types o stimulus, to which all dogs respond without learning, are
called unconditioned stimuli and the secretion o saliva that results is
the unconditioned response.
Pavlov observed that ater a while the dogs were starting to secrete
saliva beore they received the unconditioned stimulus. Something
else had become a stimulus that allowed the dogs to anticipate the
arrival o ood. He ound that the dogs could learn to use a variety
o signals in this way, including the ringing o a bell, the fashing
o a light, a metronome ticking or a musical box playing. These are
examples o conditioned stimuli and the secretion o saliva that these
stimuli elicit is the conditioned response. Pet dogs and children also
quickly learn indicators that they will soon be ed.
Figure 4 Pavlov's dogs
538
A.4 iN N Ate AN d le ArN ed beh Aviour ( Ah l)
Imprinting Figure 5 Young geese imprinted on
their mother
Imprinting is learning occurring at a particular life stage
and is independent of the consequences of behaviour.
The word imprinting was rst used in the 1 930s by Konrad Lorenz to
describe a type o learning. Imprinting can only occur at a particular
stage o lie and is the indelible establishment o a preerence or
stimulus that elicits behaviour patterns, oten but not always, o trust
and recognition. The example that was made amous by Lorenz was in
greylag geese. Eggs are normally incubated by their mother so that she
is the rst large moving object that the hatchlings see. The young birds
then ollow their mother around during the rst ew weeks o lie. She
leads them to ood and protects them.
Lorenz showed that young geese that are hatched in an incubator and
who do not encounter their mother attach themselves to another large
moving object and ollow it around. This can be a bird o another species,
Lorenzs boots or even an inanimate moving object. This attachment
is what Lorenz called imprinting. The critical period in greylag geese
when imprinting occurs is 1 31 6 hours ater hatching. A distinctive
eature o imprinting is that it is independent o the consequences o the
behaviour in experiments animals remain imprinted on something
even i it does not increase their chance o survival.
Operant conditioning
Operant conditioning is a form of learning which consists
of trial and error experiences.
Operant conditioning is sometimes explained in simple terms as
learning by trial and error. It is a dierent orm o learning rom
refex conditioning. Whereas refex conditioning is initiated by the
environment imposing a stimulus on an animal, operant conditioning
is initiated by an animal spontaneously testing out a behaviour
pattern and nding out what its consequences are. Depending on
whether the consequences are positive or negative or the animal
or its environment, the behaviour pattern is either reinorced or
inhibited.
Lambs learn not to touch electric encing by operant conditioning. They
explore their environment and i electric encing is used to enclose their
fock, lambs sooner or later touch it, probably with their nose. They
receive a painul electric shock and through operant conditioning they
avoid touching the ence in the uture.
Learning
Learning is the acquisition of skill or knowledge.
The behaviour o animals changes during their lietime. In a ew
cases behaviour patterns are lost, or example the palmar grasp
relex and other primitive relexes in human babies. Far more
commonly animals acquire types o behaviour pattern during their
539
A NEUROBIOLOGY AND BEHAVIOUR
lives. In some cases these behaviour changes are a natural part o
growth and maturation, such as the behaviour changes that occur
during puberty in humans. In other cases the modiication o
behaviour is acquired by learning the behaviour does not develop
unless it is learned.
Motor skills such as walking, talking or playing the violin are learned.
Knowledge also has to be learned. For example the rainorest tribes learn
the types o tree that can provide ood or other useul materials and
they also learn the location in the orest o individual trees o the useul
types. Learning is a higher order unction o the brain and humans have
a greater capacity to learn than any other species. The degree o learning
during an animals lietime is dependent on their longevity as well
as their neural capacity. S ocial animals are more likely to learn rom
each other.
Figure 6 Learning starts in children but is Memory
a lifelong process due to neural plasticity
Memory is the process ofencoding, storing and accessing
information.
Memory is one o the higher order unctions o the brain. Encoding is
the process o converting inormation into a orm in which it can be
stored by the brain. Short-term memory lasts up to about a minute and
may or may not lead to long- term memory, which can be retained or
indefnite periods o time. Accessing is the recall o inormation so that it
can be used actively in the thought processes o the brain.
Dierent parts o the brain have a role in the encoding, storage and
accessing o memory. The importance o the hippocampus was strikingly
demonstrated in 1 953 when a patient called Henry Molaison had the
amygdala and a section o hippocampus rom both o his cerebral
hemispheres removed in an experimental attempt to cure epilepsy. He
immediately became incapable o making new memories unless they
were procedural and his recall o memories ormed during the eleven
years beore the surgery was also impaired. Recent research into the role
o the hippocampus has shown that experiences cause large numbers o
new synapses to be ormed, which are then gradually pruned to refne
the memory o the experience and allow it to be recalled when it is
relevant and not at other times.
540
A.5 NeurophArmACology (Ahl)
A.5 Naac (Ahl)
Understanding Applications
Some neurotransmitters excite nerve impulses in Efects on the nervous system o two stimulants
post-synaptic neurons and others inhibit them. and two sedatives.
Nerve impulses are initiated or inhibited in post- The efect o anaesthetics on awareness.
synaptic neurons as a result o summation o Endorphins can act as painkillers.
all excitatory and inhibitory neurotransmitters
received rom pre-synaptic neurons. Skills
Many diferent slow-acting neurotransmitters Evaluation o data showing the impact o
modulate ast synaptic transmission in the brain. MDMA (ecstasy) on serotonin and dopamine
metabolism in the brain.
Memory and learning involve changes
in neurons caused by slow-acting Nature of science
neurotransmitters.
Assessing risk associated with scientic
Psychoactive drugs afect the brain by either research: patient advocates will oten press or
increasing or decreasing post-synaptic the speeding up o drug approval processes,
transmission. encouraging more tolerance o risk.
Anaesthetics act by interering with neural
transmission between areas o sensory
perception and the CNS.
Stimulant drugs mimic the stimulation provided
by the sympathetic nervous system.
Addiction can be afected by genetic
predisposition, social environment and
dopamine secretion.
Excitatory and inhibitory neurotransmitters
Some neurotransmitters excite nerve impulses in post-
synaptic neurons and others inhibit them.
The basic principles o synaptic transmission were described in sub-topic 6.5:
neurotransmitter is released into the pre-synaptic neuron when a depolarization
o the pre-synaptic neuron reaches the synapse. The neurotransmitter
depolarizes the post-synaptic neuron by binding to receptors in its
membrane. Excitatory neurotransmitters excite the post-synaptic neuron
or periods ranging rom a ew milliseconds to many seconds, producing
depolarization that may be sufcient to trigger action potentials.
Some neurotransmitters have a dierent eect they inhibit the ormation
o action potentials in the post-synaptic neuron because the membrane
potential becomes more negative when the neurotransmitter binds to the
post-synaptic membrane. This hyperpolarization makes it more difcult or
541
A NEUROBIOLOGY AND BEHAVIOUR
IPSP the post-synaptic neuron to reach the threshold potential so nerve impulses
are inhibited. Inhibitory neurotransmitters are small molecules that are
inactivated by specic enzymes in the membrane o the post-synaptic neuron.
EPSP Summation
EPSP plus IPSP
Nerve impulses are initiated or inhibited in post-synaptic
action potential neurons as a result o summation o all excitatory and
inhibitory neurotransmitters received rom pre-synaptic
EPSPs neurons.
EPSPs action potential More than one pre-synaptic neuron can orm a synapse with the same
IPSP post-synaptic neuron. Especially in the brain, as there are hundreds or even
thousands o pre-synaptic neurons! Usually a single release o excitatory
100 ms neurotransmitter rom one pre-synaptic neuron is insucient to trigger an
action potential. Either one pre-synaptic neuron must repeatedly release
Figure 1 Excitatory post-synaptic neurotransmitter, or several adjacent pre-synaptic neurons must
potentials (EPSP) , inhibitory post- release neurotransmitter more or less simultaneously. The additive eect
synaptic potentials (IPSP) rom multiple releases o excitatory neurotransmitter is called summation.
Some pre-synaptic neurons release an inhibitory rather than an
excitatory neurotransmitter. S ummation involves combining the eects
o excitatory and inhibitory neurotransmitters. Whether or not action
potentials orm in a post-synaptic neuron depends on the balance
between the eects o the synapses that release excitatory and inhibitory
neurotransmitters and thereore whether the threshold potential is
reached. This integration o signals rom many dierent sources is the
basis o decision-making processes in the central nervous system.
Slow and fast neurotransmitters
Many diferent slow-acting neurotransmitters modulate
ast synaptic transmission in the brain.
The neurotransmitters so ar described have all been ast-acting,
with the neurotransmitter crossing the synapse binding to receptors
less than a millisecond ater an action potential has arrived at the
pre-synaptic membrane. The receptors are gated ion-channels, which
open or close in response to the binding o the neurotransmitter,
causing an almost immediate but very brie change in post-synaptic
membrane potential.
Another class o neurotransmitter is slow-acting neurotransmitters or
neuromodulators which take hundreds o milliseconds to have eects on
post-synaptic neurons. Rather than having an eect on a single post-
synaptic neuron they may diuse through the surrounding fuid and
aect groups o neurons. Noradrenalin/norepinephrine, dopamine and
serotonin are slow-acting neurotransmitters.
Slow acting neurotransmitters do not aect ion movement across post-
synaptic membranes directly, but instead cause the release o secondary
messengers inside post-synaptic neurons, which set o sequences o
intracellular processes that regulate ast synaptic transmission. Slow
542
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