How the brain works

THE PHYSICAL BRAIN 50 51
The Aging Brain
Subarachnoid space
enlarges, reflecting loss Loss of gray and
white matter enlarges
in brain volume size of ventricles

Decay of white matter Iron accumulates in CAN WE TREAT
leads to inefficient basal ganglia, possibly ALZHEIMER’S?
causing abnormalities Medication can slow down
transmission of signals the progression of the disease
and manage some of the
Old brain symptoms, but a cure for
As we age, brain cells die and spaces within Alzheimer’s has not yet
and around the brain enlarge. The cortex
been found.
thins, and areas like the hippocampus
shrink, often causing memory problems. SUPER-AGERS’ BRAINS
STAY LOOKING YOUNG
Both gray matter (neuron bodies) FOR THEIR WHOLE LIVES
and white matter (densely
packed axons) are lost.

A slow decline? 60 Rapid response Vocabulary keeps

As we get older, our attention to stimuli is first skill increasing until
suffers, and our brains become less
plastic. This makes learning harder, 55 to decline old age
although not impossible. In fact,
learning new things throughout Average test scores 50
life boosts brain health and may
stave off cognitive decline by 45 Numerical At middle age,
strengthening neural synapses. ability skills like spatial
And with age come some benefits: KEY orientation stop
on average, older adults are better 40 Inductive Verbal improving
at extracting the big picture from ability
a situation and using their life reasoning Numerical ability
experience to solve problems. Verbal requires working
Spatial memory memory, which often
Skills and abilities 35 orientation declines with age
The Seattle Longitudinal Study followed
adults for 50 years. It found that skills like Perceptual 46 53 60 67 74 81 88
vocabulary and general knowledge keep 30 speed Age
improving for most of our lives.
25 32 39

As we get older, most of us notice Preventing decline Keep your mind stimulated. Any
a slight reduction in thinking speed mental challenge that involves
as well as a reduction in our We can take a variety of steps to learning—from home repairs to
working memory (see p.135). Some safeguard our brain’s health. A diet cooking to crossword puzzles—can
people experience severe decline or high in vegetables, fruit, “good” stretch cognitive skills. Consider
even dementia (see p.200), but this fats, and nutrients (see pp.54–55) learning a new language, as people
is by no means inevitable. In fact, keeps both brain and body healthy, who speak two or more languages
some cognitive capacities, such as as does moderate but regular have stronger cognitive ability
our overall understanding of life, physical activity. Jogging or other than those who speak just one.
may even improve as we age. aerobic exercise can help delay
age-related declines both in To sum up, you can slow the
We inherit our basic level of memory and thinking speed. cognitive aging process by:
cognitive function from our parents, • Keeping your brain well supplied
but this genetic blueprint is also You can also protect your brain with oxygen and nutrients.
affected by our environment and health by avoiding toxins, such as • Avoiding exposure to toxins
life experiences, including nutrition, alcohol and tobacco. Smoking has such as alcohol and nicotine.
health, education, stress levels, and been linked with damage to the • Exercising your body by building
relationships. Physically, socially, brain’s cortex. If you do drink activity into daily life.
and intellectually stimulating alcohol, keep within healthy • Exercising your mind by
activities also play a key role. drinking limits and have at least learning new skills.
two alcohol-free days per week.

How to Slow the
Effects of Aging

As we age, our thinking and short-term memory may
become less efficient. Nevertheless, we continue to
learn until we die, and we can take active measures
to keep our brain working well at any age.

52 53

Brain Food

Like any other organ, the human brain needs

a constant supply of water and nutrients to

maintain its health and to supply energy for

efficient functioning. fattOy aIcLiYdsF; vIiStaHmins B6, B12, and D

Feeding the brain mega-3 SARDINES CABBAGE
CAULIFLOWER
A healthy diet benefits both the O SALMON
mind and the body. Complex AND
carbohydrates provide a steady BROCCOLI
flow of fuel; these are found in BRUSSELS
SPROUTS
whole grain bread, brown rice,

legumes, potatoes, and sweet

potatoes. Healthy fats are essential

for maintaining brain cells, and

these fats come from oily fish,

vegetable oils, and plant foods such ANCHOVIES

as avocados and flaxseeds. Proteins MACKEREL
supply amino acids. Fruits and

vegetables supply water, RASPBERRIES BERRIES
vitamins, and fiber.

MUL

HYDRATION BLUEBERRIES SWEET OLIVE OIL
POTATOES
Brain cells need adequate BLAC
hydration (water supply) in order STRAWBERRIES GOJI BERR KBERRIES
to function effectively. Studies have
shown that dehydration can impair IES QUINOA
our ability to concentrate and to
perform mental tasks and AntioxiBdaEnRtsR, fIiEbSer, gluco CRANBERRIES
negatively affect memory. Some
of our water intake comes from the LEGUMES
food we consume, but it is helpful
to drink several glasses of water
each day to maintain a healthy
level of hydration.

se WHOLE GRAINS
Sources of nutrients PULSES
Fresh fruits and vegetables, beans
and lentils, whole grains, healthy WHOCLoEmGpleRxAcaINrbSoh&ydSraTtAesR, BCvHitaYmVinEs,GfiEbTerABLES
fats such as olive oil, and oily fish
such as salmon all supply vital
nutrients for the brain.

THE BRAIN IS THE PHYSICAL BRAIN 54 55
60 PERCENT FAT Brain Food
AND NEEDS A
STEADY SUPPLYUSEVAnEtFsG,YfEibGTeRAr, EnBuELtNrEieSSnts Essential nutrients
OF ENERGY
Certain nutrients from food have been found to improve or
A&CnRtDioUAxiCdRaIKFELRO maintain particular brain functions. These substances include
vitamins and minerals, omega-3 and omega-6 fatty acids,
KALE antioxidants, and water. These essential nutrients help keep
SPINACH brain cells healthy, enable the cells to transmit signals quickly
CHARD and effectively, reduce damage from inflammation and free
radicals (atoms that can damage cells, proteins, and DNA), and
help the cells form new connections. They can also promote
the production and function of neurotransmitters. As a result,
regularly eating foods that contain these nutrients can benefit
memory, cognitive functions, concentration, and mood.

NUTRIENT BENEFIT SOURCE

Omega-3 and Help maintain blood flow and cell Oily fish (such as salmon, sardines,
omega-6 fatty membranes in brain; support herring, mackerel)
acids memory and reduce risk of Flaxseed oil, rapeseed oil
B vitamins depression, mood disorders, Walnuts, pine nuts, Brazil nuts
stroke, and dementia
Eggs
Vitamins B6 and B12 and folic acid Whole grains such as oatmeal,
support nervous-system function; brown rice, whole grain bread
choline helps production of Cruciferous vegetables (cabbage,
neurotransmitters broccoli, cauliflower, kale)
Kidney beans, soy beans

Amino acids Support production of Organic meat
neurotransmitters and aid Free-range poultry
memory and concentration Fish
Eggs
OLIVES Dairy products
VEGETABLE OIL Nuts and seeds

Monounsaturated Help keep blood vessels healthy Olive oil
fats and support functions such as Peanuts, almonds, cashews,
memory hazelnuts, pecans, pistachios
Avocados
ED / OIL Antioxidants Protect the brain cells from
inflammation damage due to the Dark chocolate (at least 70
me&gaV-6E,GmEoTnoAuBnLsaEtuOrIatLeSd fats presence of free radicals; improve percent cocoa)
cognitive functions and memory Berries
in older people Pomegranates and juice
Ground coffee
FLAX SE o Tea (especially green tea)
Cruciferous vegetables
OmeOgLa-I3VaEnd Dark leafy greens
Soy beans and products
Nuts and seeds
Nut and seed butters, such
as peanut butter and tahini

Water Keeps brain hydrated to enable Tap water (especially “hard” water)
efficient chemical reactions Fruits and vegetables

CELL NUCLEUS Nonidentical
sex chromosomes
Most (X and Y) indicating
chromosomes a male
occur in
matched pairs Chromosomes ARE GENES
We have around 20,000 ALWAYS ACTIVE?
genes, which are grouped Every DNA-bearing cell has
into chromosomes. Each a full set of genes, but many
cell nucleus has 22 matched genes are normally active in
pairs of chromosomes only one part of the body, such
(known as autosomes), plus as the brain, or at one stage of
a pair of sex chromosomes life, such as babyhood.
(identical XX chromosomes
in females, or a nonidentical DNA and genes
pair, XY, in males). The DNA molecule is a long, twisted
strand formed from pairs of chemicals
What is a gene? called bases—the “letters” of the genetic
code—with a sugar-phosphate backbone
Genes are sections of a long at each edge. When cells divide, half of
molecule called deoxyribonucleic the DNA goes into each new cell. In
acid (DNA), which contains the addition, we inherit one chromosome in
code that governs how our bodies each pair from our mother and one from
develop and function. We inherit a our father, so each parent contributes half
mixture of genes from our parents. of our genes.
These genes produce proteins that
shape physical traits, such as eye Bases on one side of
color, or regulate processes such as strand are paired with
chemical reactions. Their action a complementary base
turns these traits “on” or “off” or on other side
makes them more or less intense.
DNA helix is itself
tightly coiled

Genetics Outer edge of each
and the Brain strand is made of sugar
and phosphate molecules

Genes govern the way our bodies, Four bases—adenine, Adenine (red)
including the brain, develop and function. thymine, guanine, and always bonds with
They work together with our environment cytosine—are arranged in a thymine (yellow)
to shape us throughout our life, from particular sequence that
conception to old age.
encodes our genetic
information

THE PHYSICAL BRAIN 56 57
Genetics and the Brain

MUTATION How faulty genes affect the brain

When cells divide, the double-stranded DNA splits into Genes do not directly control behavior; instead, they
single strands, and each base is matched with a new govern the number and characteristics of nerve cells
complementary base to form two new copies of the whose actions combine to produce our mental
DNA. However, sometimes copying produces changes functions. For example, some genes influence the
in the sequence. These may cause a gene to produce an levels of neurotransmitters (see p.24), which in turn
altered protein or stop it from working at all. Mutations regulate functions such as memory, mood, behavior,
may arise during life or may be inherited from parents. and cognitive skills. A faulty gene may fail to produce
a protein needed for healthy brain function or may
Base Backbone Mutation occurs when increase the risk of a disorder such as Alzheimer’s
pair of DNA base pairs are changed disease. Some faults can be inherited from parents;
molecule two inheritance patterns are shown here.
during copying

Autosomal dominant AFFECTED UNAFFECTED
In an autosomal dominant PARENT PARENT
disorder, such as Huntington’s
New DNA strand made ERROR disease, only one parent has Normal
during cell copying to pass on the faulty gene for gene only
it to cause the disease.

AT LEAST ONE-THIRD Faulty gene
OF ALL OUR GENES present
ARE ACTIVE PRIMARILY
IN THE BRAIN

AFFECTED UNAFFECTED
CHILDREN CHILDREN

Guanine (blue) always bonds Autosomal recessive CARRIER CARRIER
with cytosine (green) In an autosomal recessive PARENT PARENT
disorder, such as Tay-Sachs
disease, the disorder occurs Parent has
only if both parents pass on one faulty
a faulty copy of the gene. and one
Carriers have no disease healthy gene
themselves but can pass on
the faulty gene.

Affected child
has two copies
of faulty gene

Unaffected

Carrier children child

have one faulty and

one healthy gene

WHEN IS THE SEX LARGER IN FEMALE BRA N LARGER IN MALE BRAI
OF A FETUS FIXED? Corpus IN Thalamus
callosum
Chromosomal sex is This area, the “relay station”
determined at the point of The corpus callosum, which between the cortex and deeper
fertilization. Physical sexual links the brain’s left and right brain structures, is larger in men
differentiation occurs seven hemispheres, has been found to than in women. The two sides of
be larger in females. It has been the thalamus are more likely to
to 12 weeks after associated with greater cognitive
fertilization. skills in females, possibly because be connected in females, but
the significance of this
Physical differences brain functions are shared feature is not known.
between hemispheres,
Differences between males and females begin but not in males. ARGER IN MALE BRAIN
with the sex chromosomes at the moment of Hippocampus
conception: XX for females and XY for males. L
In the uterus, the release of testosterone from Males have a larger anterior
the mother during gestation “masculinizes” (front) hippocampus, which
a male fetus, triggering the growth of structural governs acquiring and encoding
sex differences in both the brain and body. As new spatio-visual information,
we grow and develop, these differences will while females have a larger
arise in many different brain structures (see right). posterior hippocampus, which
Cognitive and skill differences between the sexes governs retrieval of existing
are present from childhood. Adult male brains are
8 to 13 percent larger, on average, than adult spatio-visual knowledge.
female brains. In addition, adult male brains also
tend to vary more, in volume and cortical
thickness, than female brains.

Male and ALL HUMAN
Female Brains EMBRYOS START
LIFE WITH FEMALE
Scientists have found that male and female BRAINS—EXTRA
brains show distinct physical differences. However, HORMONES
it is not always clear how these variations affect ARE NEEDED TO
our attitudes, activities, and responses to our CREATE A MALE
environment. Differences may arise from the way a
brain is used in life as well as from its physical form.

58 59

ARGER IN MALE BRAINL Differences in function
Hypothalamus L
Male and female brains differ in
Certain areas governing function as well as structure. Male
male-typical sexual behavior brains seem to be more “lateralized”
and responses to stress in the (with a greater difference in function
hypothalamus are larger in between the left and right hemispheres).
heterosexual males than in Males also vary more than females in
females or homosexual males. cognitive ability. These variations are partly
due to the structure of the “connectome”—
ARGER IN MALE BRAIN the network of neural connections between
Amygdala parts of the brain (see below). They also
result from the action of hormones, and
The amygdala, involved in external influences, throughout our life.
emotional responses, making In particular, our social environment and
decisions, and forming emotional experiences continually shape our neural
memories, is slightly larger in pathways, helping us perform male- or
males. However, differences in female-typical tasks.
functions such as responses to
Few connections Greater connectivity
negative versus positive cross hemispheres within hemispheres
emotional stimuli, are
MALE
more significant.

Brain structures
There are several areas in which
quantifiable physical differences have
been identified between male and female
adult brains. The main regions are shown
here. How these differences can affect
cognition and psychology are currently
the matter of ongoing scientific research.

Many connections Less connectivity
between hemispheres within hemispheres

NONBINARY BRAINS NONBINARY FEMALE
SYMBOL The connectome
Homosexual and transgender people One study, in which more than 900 brains were imaged,
have been found to have certain found that male brains have greater connectivity
distinctive brain structures. For example, within hemispheres, while female brains have denser
some parts of the hypothalamus (see connections between hemispheres. The males were
above) differ in homosexual and found to be better at spatial processing, while the
heterosexual men, and the putamen females scored higher on attention and memory for
(involved in learning and regulation of words and faces.
movement) has more gray matter in
trans women than in cisgendered men.

MUSICAL BRAINS CHROMOSOMES NATURE

Playing music involves multiple parts We inherit
of the brain. Studies comparing the our chromosomes, which
brains of professional musicians and contain our DNA, from our
amateurs revealed that professional parents (see pp.56–57). It’s the
musicians had a greater volume of chromosomes that, at the point of
gray matter in brain areas related fertilization, determine the
to motor, auditory, and visual-spatial chromosomal sex of an embryo
reasoning. The study’s findings show (XX for female and XY for male).
how the brain undergoes structural Chromosomal abnormalities
adaptations in response to the can also cause disease or
environment (dedicating hours
to repetitive rehearsals with developmental
an instrument). problems.

DNA Some psychological
traits, such as the tendency to
THE HIPPOCAMPUS IN develop depression, have been
AN ADULT BRAIN MAKES linked to particular genes—but they
AN ESTIMATED 700 NEW usually involve dozens or even
NEURONS EVERY DAY hundreds of the genes acting
together. The more of those genes

a person inherits, the more
likely they are to develop

that trait.

Genes versus environment WHEN DO
EPIGENETIC CHANGES
People are born with a DNA “template” inherited from
their parents (see pp.56–57): this is the “nature” HAPPEN?
element influencing the brain’s activities, such as
cognitive ability and behavior. Throughout a person’s Epigenetic changes can be
life, though, their networks of neurons (see pp.26–27) induced by environmental
can adapt and change in response to physical and
social experiences (“nurture”). Environmental factors at any point in
influences, if strong and sustained, can alter brain a person’s life, from
structures and also influence the way that genes work—
a process known as epigenetic change (see opposite). development in
utero to old age.
Nature
and Nurture

The two fundamental influences on the brain, “nature”
and “nurture,” are sometimes seen as opposing forces.
However, there is a dynamic interplay between them
that goes on throughout a person’s life.

NURTURE PHYSICAL S THE PHYSICAL BRAIN 60 61
Nature and Nurture

Studies on children have URROUNDINGS Epigenetic changes
found that growing up poor or
Changes in the way genes are used (or expressed)
deprived can impair the that occur during a person’s lifetime are called
development of areas related to epigenetic changes. They affect gene function,
memory, language processing, rather than gene structure, and can be passed on
decision-making, and self-control. to a person’s children, although they may last for only
However, a safe, happy home, a few generations. In the brain, they can influence
functions such as learning, memory, reward-seeking,
with interesting things to do, and response to stress. There are two main forms:
seems to reduce the harm. methylation, in which a compound joins on to the DNA;
and histone modification, which alters how tightly the
STRESS LEVEL S DNA is coiled.
Chronic emotional
stress in children can impair Methyl compound
development of the amygdala, attached to DNA base
hippocampus, and frontal lobes,
leading to problems with memory, DNA methylation
emotion, and learning. It restricts the In this process, a molecule of a methyl
action of genes regulating the compound attaches to one of the bases in
growth of networks of neurons. a gene’s DNA sequence. The effect is to stop
However, moderate “positive” or restrict the activity of that gene.
stress (fun) can aid

learning.

DIET Base pairs in most of
sequence unchanged

A healthy diet (see pp.54–55) STUDYING TWINS
rich in omega-3 fatty acids, B
vitamins, and antioxidants keeps Studies of twins reveal how much of a specific trait, such
blood vessels healthy, improving as intelligence quotient (IQ), is due to inheritance and
blood flow to the brain. These how much is due to environment. Most twins grow up
nutrients have also been linked to in the same home; however, identical twins share 100
percent of their genes, while nonidentical (fraternal)
improving memory and twins share only 50 percent. If a trait is more evident
maintaining cognitive in identical twins than in fraternal ones, or appears in
functions in older people. identical twins who were separated at birth, it suggests
SOCIAL that genetics has a stronger influence than environment.
Loneliness has been
found to alter the production of NETWORKS BIOLOGICAL PARENTS ADOPTIVE PARENTS
neurotransmitters, so people
perceive less reward from social NON-ADOPTED TWIN ADOPTED TWIN
contact and are more likely to
misinterpret others’ attitudes as
threatening. However, maintaining
close social ties can support
memory and cognitive skills.



BRAIN FUNCTIONS
AND THE SENSES

Sensing Touch VISUAL CORTEX
the World Thought to be the
first sense to develop
To survive in our environment, in the womb, touch
we must be able to react to, and neurons respond to pressure,
interact with, stimuli produced by temperature, vibration, pain, and
physical, chemical, and biological light touch. Touch is how
phenomena—sights, sounds, smells, humans make physical contact
tastes, and touches. Sensors in the with the environment and
body pick up these signals and send with each other.
them to the brain for deciphering. Hearing
Sound waves in the
Senses air are collected by the
ear and transmitted into
Each sense has its own set of detectors. Most are the skull, where they are
localized in a specific area of the body, except for turned into electrical impulses
touch, which is spread all over the skin, as well as by the cochlea. Hearing is the
inside the body. Although the neurons and receptors most developed of the senses
for each sense are largely dedicated to that sense at birth but is only
alone, they can sometimes overlap. Sensory information complete by the end
continuously bombards the brain, but only a fraction of the first year.
of the input reaches consciousness. Even so, the
“unnoticed” information can still guide our actions, Sight
particularly in the case of our sixth sense, Sight involves
proprioception, which relays information sensors at the back of
about the body’s position in space. the eye that turn light into
electrical signals. These are
YOUR SENSE OF SMELL transported to the back of the
IMPROVES WHEN YOU ARE HUNGRY brain, where they are converted
into colors, fine details, and
motion. We perceive objects
in as little as half a

second.

SYNESTHESIA Each note is associated
with a different color
Synesthesia is a condition where a
stimulus may be interpreted by two
or more senses at the same time. In
its most common form, a person sees
a number or word as a color. Each
synesthete will have its own color
associations. Almost any combination
of senses can be affected. Combinations
of three or more senses are rare.

BRAIN FUNCTIONS AND THE SENSES 64 65
Sensing the World

OREX ORYSOMCOATROTESXECNOSMROT T Proprioception
The brain is constantly
PRIMAARREYATASTE SECONADRAERAY TASTE processing information
ACUODRITTOERXY from the joints and muscles that
tell it where the body is in space.
OCLOFARTCETXORY It keeps us upright and allows us
to make movements without
Smell conscious effort, such as
Despite having only
400 smell receptors, walking up stairs.
humans can detect up to a
trillion different odors. Smell Taste
is important for survival as it Taste is important
warns us of hazardous in determining what
substances or events, such is safe and nutritious
as something burning. It also to eat. Taste receptors pick up
plays a key role in taste. only five basic tastes: sweet, salty,
bitter, sour, and umami
(savory). We need our sense
of smell to help identify

a taste.

Sense areas of the cortex
Inputs from the sense receptors map to different
areas of the brain’s cortex. Although these areas
are separate, they can often react to inputs from
another sense. For example, visual neurons will
respond better in low-light situations if they are
accompanied by sound.

HOW MANY
SENSES ARE THERE?
Including the six senses
described here, scientists
think there may be as many
as 20 senses, based on
the number of different

receptor types in
the body.

Seeing WHY DO MY EYES
CLOSE WHEN I SNEEZE?
The eye provides us with probably the most
important of our five senses. It gathers the When a nasal irritant triggers
light reflected by an object and delivers this the brain stem control center,
information to the brain via the optic nerve. it causes widespread muscle
contractions, including those
The structure of the eye
in the eyelids. This makes
The eyeball is roughly 1 in (2.5 cm) in diameter. At the back of the you blink momentarily.
eye is the retina, which contains light-sensitive cells that connect
via neurons to the optic nerve. The space inside the eyeball is filled Eyeball is encased Crossed-over rays
with a jellylike substance. The front of the eye contains a hole (the by sclera produce an inverted
pupil), which has a clear lens behind it. Surrounding the pupil is a image on retina
circle of colored muscle, the iris, which controls how much light
enters the eye. The cornea, a clear membrane, covers them and
merges into the white outer membrane called the sclera.

Light rays start to refract
(bend) as they pass from

air and into cornea

LIGHT RETINALens is like a bag of
jelly that changes
LENS shape to help
IRIS focusing
PUPIL
CORNEA

Iris is a ring
of muscle

Seeing things Cornea is a SCLERA CHOROID
The eye is capable of providing the transparent layer
brain with an enormous amount covering front of eye Choroid is a
of detail about what it is looking blood-rich layer
at. However, the image the brain that surrounds
receives is inverted, so it has to be retina
flipped before we can understand it.

1 Light enters the eye 2 Lens and focusing
Light passes through the cornea and Behind the iris is the lens, where the
into the eye through the pupil. The pupil is light rays are bent so the image forms on the
surrounded by a ring of colored muscle, the retina. The lens is connected to muscles that
iris, which can make the pupil contract or allow it to change shape—it flattens for
dilate to vary the amount of light entering. distant objects and thickens for close objects.

BRAIN FUNCTIONS AND THE SENSES 66 67
Seeing

KEY RETINA Light ray travels
The purple arrows show to back of retina
the direction of light
rays. Black and blue Rods work in grayscale,
arrows are nerve signals responding to intensity
going to the optic nerve. of light; they enable us to
see in dim conditions
Light rays
Black and NERVE CELLS LIGHTCRELELCSEPTOR Cones send nerve
white signals in response
Color to green, red, or
blue light; they
Signal for black need bright light to
and white passes produce a signal

from retina to OPTIC NERVE
optic nerve

Ganglion
cell

Signal for CHOROID
color passes
from retina to Wall of pigment
optic nerve cells forming
back of retina
Bipolar cell

3 The retina 4 Nerve signals to brain
The retina is made up The nerve signals trigger
of three layers. Light rays travel
through the first two layers, ganglion
and bipolar cells, and reach the third impulses in the ganglion and
bipolar cells, which connect
layer, which contains light-sensitive directly to the optic nerve. The
rod and cone cells. These convert
light rays into nerve signals. Optic nerve nerve signals travel along the
carries signals optic nerve to the brain.
from light sensors
to brain

OPTIC NERVE

YOUR EYEBALLS THE BLIND SPOT Rods and cones
REMAIN THE SAME Blind spot
SIZE THROUGHOUT To connect to the brain, the nerve fibers
YOUR LIFE of the retina must pass through the where nerve
back of the eye to form the optic nerve. fibers leave eye
This creates a “blind spot” that has no
photoreceptors. We don’t notice this HUMAN EYE
because each eye provides data about a
scene and the brain uses information from
the other eye to complete the picture.

The Visual 3 Recognizing faces
Cortex Features that suggest a face are
sent to the face-recognition area and
amygdala, where they are searched
for details that prompt recognition.

Nerve signals from the eye have to

travel all the way through the brain

before they reach the area dedicated FRONTAL LOBE Lateral geniculate
to decoding this information. This CORTEXnucleus forwards
area is called the visual cortex. Frontal lobe THALAMUS signals from retina
provides conscious to visual cortex
The structure of the cortex recognition of faces
VISUAL
The visual cortex occurs in both Amygdala
brain hemispheres and is further processes facial
divided into eight main areas, each
of which has a different function expressions
(see table, opposite). Signals travel
AMYGDALA

from the retina (see pp.66–67) via

the thalamus and lateral geniculate OPTIC NERVE FACE
nucleus to the primary visual cortex RECOGNITION
(V1). The raw data then passes Rods and cones in
through various vision areas, retina convert light AREA
contributing different details about into nerve signals
Optic nerve carries
nerve signals to brain

shape, color, depth, and motion 1 From eyeball to visual cortex KEY
before combining to form an image. Data from the eyeball travels along the Information from the eye
Some areas provide information that optic nerve until it reaches the optic chiasm (see
helps with immediate recognition of below), where some of the data is sent to the Face recognition pathway
familiar objects, others with spatial opposite side of the brain. Signals then travel to
orientation or visual-motor skills. the lateral geniculate nucleus, which forwards
data to the visual cortex for processing.

Stereoscopic vision Lateral LEFT HEMISPHERE Half of signals travel View of object
geniculate to same hemisphere; from left eye
Our ability to see in 3-D—known as stereoscopic other half cross over
vision—is produced by having both of our eyes nucleus
looking straight ahead and moving together.
As the eyes are slightly apart, different views LEFT VISUAL THALAMUS
are received from each, although they overlap CORTEX
to a small extent. The brain computes the spatial
information from each eye to create an overall RIGCHOTRTVEISXUAL
image, using previous experience to speed
up the processing time and fill in any gaps.

Swapping sides Nerve axons split off RIGHT HEMISPHERE Optic nerves View of object
At a crossover point called the optic chiasm, after lateral geniculate converge at from right eye
nerve axons from the left side of each retina nucleus and radiate to optic chiasm
join and continue to the left visual cortex,
and likewise with nerve axons from the right. areas of visual cortex

OR CORTEX BRAIN FUNCTIONS AND THE SENSES 68 69
The Visual Cortex

INTERI V6
V3A
V3D THE VISUAL CORTEX
V2 IS VERY THIN—
V1 JUST 0.08 IN (2 MM)
V2
Some visual- VP V4V AREA AREAS OF THE VISUAL CORTEX
processing areas OF BRAIN FUNCTION
curve around back of V8
brain into groove V1 Responds to visual stimuli.
between hemispheres V7
BACK V3A V2 Passes on information and responds
Visual cortex, V3 V3A, V3D, to complex shapes.
located in V2 VP
V4D V4D, V4V Registers angles and symmetry and
occipital lobe V1 combines motion and direction.

Responds to color, orientation, form,
and movement.

V5 Responds to movement.

2 The visual cortex V6 Detects motion in periphery of
Nerve signals progress visual field.

through the various layers of V7 Involved in perception of symmetry.
the cortex, each adding more
information to the image. It
takes half a second for the
image to be assessed and V8 Probably involved in processing of color.

become a conscious perception.

VISUAL FIELD OF LEFT EYE FIELDS OF VISION
Image formed by brain
after it combines images BINOCULAR Animals such as primates have
from left and right eyes’ VISUAL FIELD a large field of stereoscopic
visual fields vision and can judge distances
better than herbivores or most RABBIT HUMAN
VISUAL FIELD OF RIGHT EYE birds. However, they have a
blind zone behind them that
can be seen only by turning the
head. Animals with eyes on the
sides and top of the head have
a wider field of 2-D vision and
greater all-around awareness.

Visual field of left eye Visual field of right eye Binocular visual field Blind zone

How We See NEWBORN BABIES
CAN SEE ONLY
Seeing is both a conscious and an unconscious BLACK, WHITE,
action. Each type follows its own pathway in the AND RED
brain. The conscious route helps recognize objects,
while the unconscious route guides movement.

Cell area V1 Cell area V2 Cell area V3
Signals from the eyes are first In the secondary visual cortex Visual area 3 (V3) is involved in
received in the primary visual (V2), some neurons improve on the analyzing angles, position, depth,
cortex (V1). Its neurons are sensitive images from V1, sharpening the lines and the orientation of shapes.
to basic visual signals, including the and edges of complex shapes. It also helps process the direction and
orientation and direction of Other neurons refine the initial speed of objects. A few cells are also

movement of objects and interpretation of the sensitive to color.
pattern recognition. color of objects.

VISUAL CORTEX PATHWAY

Following the path Visual pathway Parietal lobes judge
splits after cell location of object in
relation to observer
area V3
Inferior temporal
As visual information is processed lobe involved
in recognizing
by the layers of the visual cortex (see objects

pp.68–69), it splits into two pathways

known as the upper, or dorsal, route

and the lower, or ventral, route. There

is some uncertainty about where the V3

split occurs, but the dorsal route handles V2 V4
V1
our spatial awareness of where we are

and how we move in relation to things V5

around us, while the ventral route helps

us identify, categorize, and recognize

what we see. The dorsal route is

important in assessing significant

situations, particularly if instant action KEY
Dorsal route
is required to avoid danger, such as Ventral route

moving away from a flying object.

When this happens, the ventral route is

relegated to a secondary position since

the information it carries is not critical.

BRAIN FUNCTIONS AND THE SENSES 70 71
How We See

Cell area V5 Parietal lobe
The middle temporal area (V5) The parietal lobe
judges the overall direction of gauges the depth and position
motion of an object rather than that of an object in relation to the
of its component parts. For example, observer. This allows the person
it processes the general direction of to take immediate action, such as
a flock of birds rather than the ducking from an object coming
movement of a single bird. It toward them rapidly.

also analyzes the motion
of our own body.

“WHERE” PATHWAY (DORSAL ROUTE) Unconscious vision
The dorsal route carries visual
Conscious vision information to the parietal lobes,
The ventral route adds more information to passing through areas that calculate an
the object, such as color and shape. The object’s location, timing, and motion
information goes to the temporal lobe, and make a plan in relation to it. All of
where it is matched to visual memories to this happens without conscious thought.
aid recognition. This is where the visual
stimulus becomes a conscious perception.

“WHAT” PATHWAY (VENTRAL ROUTE)

Cell area V4 Inferior WHAT IS
Visual area 4 (V4) is involved in temporal lobe PROSOPAGNOSIA?
the perception of color, texture, Signals are forwarded to the
orientation, form, and movement. fusiform gyrus of the inferior This is the inability to
This region contains the majority of temporal lobe, which is involved in recognize faces, even of close
color-sensing neurons and is recognizing complex shapes, objects, family, usually due to damage
important in interpreting the space and faces. In conjunction with the to the inferior temporal lobe.
hippocampus, it helps with the Those affected have to learn
between objects. formation of new memories.
to recognize people in
other ways.

Perception Brain is so drawn
to faces that even
Given that visual processing happens in microseconds, pictures are studied
it is not surprising that our brain sometimes struggles
to make sense of the information sent by our eyes and
so makes us doubt what we are seeing.

Processing a scene Openings are scanned,
perhaps for possibility
When we look at a scene, we of intruders
are not really taking it all in.
Instead, the eyes repeatedly scan a Pointing draws
sequence of thumbnail-sized areas attention to an object
that the brain considers points of
interest. The rest of the scene blurs and makes it worthy
until attention falls onto a new area. of a look
Faces tend to be the main focus of a
scene—the brain is programmed to Eye passes straight
look for faces, hence the tendency across floor, pausing
to see them in the unlikeliest of briefly at a potential
places, such as the scorch marks on
a slice of toast. While details of the obstacle, but not
target objects are being scrutinized, stopping long
the conscious brain puts together
the story of the scene, complete enough to see it
with the context of each object.

Scanning for details Brain looks for clues about
Looking at a complex picture, such as relationships by looking at
this café scene, activates processes that individual faces and interplay
distinguish target objects, such as people,
from the background and then selects between characters
which bits of the target to focus on.

WHY DO WE
SEE FACES IN
INANIMATE OBJECTS?

Pareidolia (seeing faces where
there are none) may be a

survival instinct that ensures
we are vigilant for the

unfriendly features of an
enemy or predator.

BRAIN FUNCTIONS AND THE SENSES 72 73
Perception

Illusions

An illusion occurs when what the eye sees is interpreted by the
brain in a way that does not match up with the physical reality
of the actual image. With so many competing signals going to the
brain, it tends to look for familiar patterns. It also tries to predict
what will happen next to compensate for the slight time delay
between stimulus and perception. Both these facts can lead to our
brain misinterpreting visual stimuli. Illusions fall into three main
classes: physiological, cognitive, and physical.

HERMANN GRID KANIZSA’S TRIANGLE

Direction of other Physiological Cognitive
people’s eye gaze Physiological illusions are thought to arise Cognitive illusions happen when the brain
is followed from excessive or competing stimuli, such as makes assumptions about movement or
brightness, color, movement, and position. perspective when viewing an object.
Brain directs eyes to In this grid, gray spots seem to appear at the Sometimes these can lead to the brain
parts of the scene it intersections as your eyes flick over them but switching between two different images
considers significant— vanish when you stare at them. or seeing a shape that is not there.

especially faces Light is refracted as
it leaves water

Apparent
position of fish

Actual
position
of fish

REFRACTION SOME MAMMALS
Physical AND BIRDS ARE
Physical illusions are caused by the optical ALSO FOOLED BY
properties of the physical environment, OPTICAL ILLUSIONS
particularly water. The brain cannot take
account of the way that light bends as it
passes between water and air, so it sees
the fish as further back than it actually is.

How We Hear

The world is full of noise. It travels as sound waves
through the air until it reaches our ears. There, they
are turned into electrical impulses and sent to the
brain for decoding into meaningful sounds.

OUTER EAR Picking up sound
EXTERNAL EAR
Hearing involves the conversion of a sound wave into an
electrical impulse that the brain can interpret. Sound waves
are carried from the outer to the middle ear, where they cause a
series of bones and membranes to vibrate. These vibrations then
reach the cochlea, where they become electrical impulses. These
are passed to the brain stem and thalamus, where direction,
frequency, and intensity are perceived. The data is then sent for
processing by the left and right sides of the auditory cortex. The
left side identifies the sound and gives it meaning, while the
right side assesses the quality of the sound.

Sound waves travel Vibrations make bones MALLEUS
through air rattle against each other (HAMMER) BONE

Sound waves INCUS
vibrate eardrum (ANVIL) BONE

AUDITORY CANAL OSSICLES
(MIDDLE EAR BONES)
EARDRUM
STAPES
(STIRRUP) BONE

1 The outer ear 2 The auditory canal Oval window
Sound waves are caught The sound waves travel along the
by the outer ear, which funnels auditory canal to the eardrum. The auditory
them inside the head via the canal is lined with tiny hairs that filter out M IDDLE
Round window
auditory canal. foreign objects.
E AR

Eustachian tube connects
middle ear to nose and mouth

3 The eardrum 4 Ossicles
The eardrum, or tympanic Vibrations are passed through
membrane, is a thin layer of the eardrum to a set of connected
fibrous tissue that forms a bones called ossicles—the malleus,
barrier between the outer ear incus, and stapes bones. The stapes
and the middle ear. It vibrates bone pushes and pulls on another
when the sound waves traveling membrane, called the oval window.
up the auditory canal hit it. This transmits sound to the inner ear.

BRAIN FUNCTIONS AND THE SENSES 74 75
How We Hear

FILTERING OUT NOISE 9 The primary
auditory cortex
On a busy street, there are lots of After intermediate processing in
conflicting sounds, yet you can still Background the thalamus, the characteristics of
hear someone talking next to you. noise filtered each sound are interpreted by the
This is because the primary auditory out primary auditory cortex, which
cortex can filter out unnecessary
sounds and boost the signals it wants works with other cortical areas
to hear. It does this by dampening to identify the type of sound.
the response to sustained sounds,
such as traffic, while enhancing more
dynamic sounds, such as speech,
and actively listening to them.

Organ of Corti (central spiral Primary auditory cortex THALAMUS
part of the cochlea) rests on a processes sound
basilar membrane and contains
COCHLEAR NERVE
sensitive hair cells
Electrical signals
COCHLEA pass along
cochlear nerve
BRAIN STEM
R CANAL Vestibular canal Specialized cells at
CANAL carries sound top of brain stem
vibrations help determine
direction of sounds

VESTIBULA MPANIC 7 The cochlear nerve 8 The thalamus
ORGAN OF CORTI The electrical signals are Signals are first received in
the brain stem. From here, they travel
transported from each hair cell up to specialized neurons in the
through cochlear nerve endings
TY that join together to form the thalamus for processing. These signals

cochlear nerve. This is responsible are then sent to the primary auditory
cortex, which also feeds information
for transmitting signals to specialized back to the thalamus.
groups of neurons in the brain stem.

INNER EAR Vibrations return
to round window

5 The cochlea 6 The organ of Corti
The cochlea contains three fluid- The movement of
the basilar membrane bends
filled ducts. Vibrations travel along the sensitive hair cells in the organ THE STAPES IS THE
vestibular canal as wavelike movements SMALLEST BONE
that are transferred to the basilar of Corti (see p.76), which is the IN THE BODY
main organ of hearing. The hair
membrane of the organ of Corti. cells convert this movement into
Residual vibrations return along the
tympanic canal to the round window. electrical signals.

Perceiving Sound

Every sound is made up of a number of different This area receives
components. The brain has to take all the details signals from low-
of its frequencies, intensity, and rhythm to frequency sounds
process, identify, and remember the sound.
Corresponds to
The auditory cortex apex of cochlea
The auditory cortex is the main
processing center for sound. It is PSREICMTOEANRRTDYIAARRYY Corresponds to
located in the temporal lobe, just below base of cochlea
the temples on either side of the head.
Receives signals from
Primary auditory cortex high-frequency sounds
identifies frequency and

intensity of sounds

Secondary auditory cortex Hair cells are
interprets complicated disturbed when basilar
membrane vibrates
sounds, such as language
More flexible part
AUDITORY of basilar membrane
CORTEX vibrates more easily

Base of cochlea 500 Hz 1,000 Hz
transmits low-
Tertiary auditory cortex frequency sounds
integrates hearing with
other sensory systems Organ of Corti is main
organ of hearing

Apex of cochlea EMBRANE
transmits high-
frequency sounds 2,000 Hz

Inside the auditory cortex BASILAR M

Signals from the thalamus (see p.75) are sent to LEA 0 Hz

the primary auditory cortex, which is divided into 4,00

sections that respond to a range of frequencies. Row of 16,000 Hz 8,000 Hz COCH
Some of these sections focus on intensity rather hair cells
than frequency, while others pick up more complex

and distinctive sounds, such as whistles, bangs, or

animal noises. Signals then pass to the secondary The cochlea
Areas along the curl of the cochlea respond
auditory cortex, which is thought to focus on to different frequencies of sound, from
high-pitched at the apex to low bass
harmony, rhythm, and melody. The tertiary auditory notes at the base. These are mirrored by
corresponding areas in the auditory cortex.
cortex integrates all the signals to give an overall

impression of the sounds picked up by the ears.

BRAIN FUNCTIONS AND THE SENSES 76 77
Perceiving Sound

Music and the brain Mapping music Processes touch sensations
Scans show that several areas of the brain while dancing or playing
Music engages many areas of the brain. In are active when listening to music, and even an instrument
addition to processing the sounds, listening more are involved when you are playing an
to music triggers the memory and emotion instrument or dancing. Places sounds
centers in the brain, while recalling lyrics in context of
involves the language centers. Performing Coordinates movement memories and
music makes even greater demands: the while dancing or playing experience
visual cortex is stimulated by reading music,
the frontal lobe is involved in planning an instrument
actions, and the motor cortex coordinates
movement. Musicians are known to have a AL
greater ability to use both hands because PRCEOFRRTOEXNT NSROTREYX
music requires coordination of motor control, COSE COMROTETXOR
somatosensory touch, and auditory CCAOLRLOPUSUSM
information. Unlike listeners, who process
music in the right hemisphere, professional AUCDOIRTTOERXY COVRISTUEAXL
musicians use the left. They also have a HIPPOCAMPUS
thicker corpus callosum (the region linking
the two hemispheres) and tend to have
larger auditory and motor cortices.

30,000 Involved in CEREBELLUM
planning and
controlling
expression

THE NUMBER OF FIBERS Connects Amygdala (orange) Activated by
THAT MAKE UP THE hemispheres and nucleus accumbens reading music
AUDITORY NERVE (dark red) are both or watching dance
of brain involved in emotional
reactions to music Involved in movement
and emotional
reaction to music

HIGHS AND LOWS

Humans can detect a good range of 100 kHz ELEPHANT
frequencies, but other animals can hear 5 Hz–12 kHz

things far beyond our limits. Animals such

as bats and dolphins use high frequencies 10 kHz
in echolocation, while elephants and
Human
whales produce low rumbles that can 1 kHz BAT hearing range
travel long distances. Humans are most 2 kHz–120 kHz FREQUENCY
MOUSE
sensitive to frequencies between 2 kHz 1 kHz–100 kHz

and 5 kHz, which do not require great 100 Hz
intensity to be heard. Young people have

the best hearing range, from 20 Hz to

20 kHz, but there is a gradual loss of 10 Hz HUMAN DOG DOLPHIN
20 Hz– 64 Hz–44 kHz 75 Hz–150 kHz
higher frequencies with age, with older 20 kHz

people having a limit of around 15 kHz. 0

TORY EPITHEOLlIfUacMtory bulb 3 Inside the brain
Signals then travel along the olfactory
Nerve tract to the olfactory cortex. The cortex is
axon Olfactory bulb located in the limbic system, which is responsible
processes signals
Supporting before passing to
cell olfactory cortex
OLFAC for emotions and memory. Signals are also sent
Mucus NASAL CAVITY to the amygdala and orbitofrontal cortex.
Orbitofrontal cortex
Dura involved in decision- Olfactory tract, a bundle Olfactory cortex
mater making and emotions as of nerves that carries further processes
Bone well as processing smells signals from olfactory signals sent by
bulb to olfactory cortex olfactory bulb
Mucus ORCBOIRTOTEFXRONTAL
gland OLFACTORY
Receptor OLFACTORY BULB AMYGDALA CORTEX
cell

Cilia

Odor molecule
dissolving in mucus

2 Olfactory receptors Receptor cell nerve Amygdala sends
Each odor molecule activates axons detect odor and warning messages if
a particular combination of olfactory send information to odor is associated
receptors. The activated receptor cells send olfactory bulb with danger
impulses up through nerve axons to the
olfactory bulb for processing.

1 Smell enters the nose
Odor molecules are drawn up
through the nose and warmed to enhance
the scent. The molecules dissolve in mucus
produced by the olfactory epithelium and
stimulate cilia that are connected to Capturing a scent
receptor cells.
When we inhale, odor molecules drift into the nose and activate
Airborne odor receptor cells in the nasal cavity, triggering a reflex to breathe in
molecules enter nostril more deeply. In the nasal cavity, the odors dissolve in the mucus
that covers a sheet of neurons and supporting cells called olfactory
12 MILLION epithelium. The molecules spread through the mucus to hairlike
structures called cilia that are attached to receptor cells. These cells
THE NUMBER OF
OLFACTORY CELLS send signals to the olfactory bulb—a structure located in
IN THE HUMAN BODY the forebrain that forms part of the brain’s limbic
system. Data is then sent to various parts of
the brain, particularly the olfactory cortex.

Smell WHY DO
SMELLS TRIGGER
Identifying a smell out of the many odors in the world
around us involves the olfactory system, which isolates MEMORIES?
different chemicals and then passes signals on to the
brain to determine whether they are “good” or “bad.” Unlike our other senses, smells
bypass the thalamus and go
What makes a smell? straight to the limbic system. BRAIN FUNCTIONS AND THE SENSES
Emotions and memories are Smell
How we identify smells is still a matter of debate. Research suggests processed and stored here,
that most odors fall into 10 groups—or primary odors—each of which especially in the amygdala.
alerts us to something in the environment. Most smells are made up
of a combination of these groups. Smell is a key part of survival, SMELLY OR SWEET?
telling us whether something is safe or dangerous.
Dimethyl sulphide (DMS)
Fragrant Sweet is a very smelly compound. 78 79
Light, natural scents such as Warm, rich, sugary smells with A whiff of the raw chemical
flowers, grasses, and herbs, a touch of creaminess, including can make you wonder
typically used in perfumery. chocolate, malt, and vanilla. whether something is
Fruity Minty rotting or if a pungent
Typically includes warm, ripe fruits Cool, fresh, and invigorating, cheese is in the room.
and other fresh scents that have a epitomized by mint, eucalyptus, However, flavor chemists
sense of smoothness on the nose. and camphor. find it useful in creating
Citrus Toasted and nutty all sorts of tastes. It
Separate from other fruits, citrus Slightly burned and caramelized is used in meat,
has fresh, clean, acidic aromas with warm and fatty overtones, such seafood, milk, egg,
with a touch of sweetness. as popcorn and peanut butter. wine, beer, vegetable,
Woody and resinous Pungent and fruit flavorings,
Earthy, natural smells, such Often unpleasant smells such as usually at
as compost, fungi, spices, manure or sour milk, also onions, minuscule
cedar, pine, and mold. garlic, and pickles. concentrations.
Chemical Decayed
Includes synthetic, medicinal, solvent, Beyond pungent are the odors of
and gasoline odors that are easily rotting food, sewage, household gas,
identifiable. and other “sickening” substances.

Taste The five basic tastes
Taste is an evolutionary adaptation for survival.
Fueling the body requires the intake of nourishing Being able to determine whether something
foods and liquids. Choosing what is safe to eat is is nutritious or potentially poisonous before
largely influenced by our senses of taste and smell. taking it into the body is enormously important.
So far, only five basic tastes have been
Picking up taste discovered, although there may be others.

Taste is actually a limited sense; there are only five basic tastes Sweet
that can be detected (see right). Like smell, taste is a chemosense. Signals the presence of
Chemical substances in food are picked up by the taste buds, carbohydrates, which are
which are mainly found on the tongue. Receptor cells, housed sources of vital sugars.
in structures called microvilli within the taste bud, detect
these chemicals and send signals to the brain for processing. Salty
Detects chemical salts and
1 Tongue minerals that are needed
The tongue is a strong, by the body.
flexible muscle. It is used to push
food around the mouth and for Sour
speech. Its upper surface is Warns against foods that may be
covered in tiny projections called unripe or going bad.
papillae. Most of the papillae are
filiform, or threadlike, structures Bitter
and contain no taste buds. They Poisons and other toxins are often
help grip and wear down food bitter or unpalatable.
while it is being chewed.
Umami
Detects glutamate salts and amino
acids, which are found in meat,
cheese, and other aged or
fermented foods.

Surface of Circumvallate Taste Nerve Microvilli contain receptor proteins,
tongue papilla pore fiber which bind with chemicals in food

Filiform Food
papilla molecule

Taste Neuron
bud

Supporting Gustatory
cell receptor
cell

2 Papillae 3 Taste buds 4 Taste bud cells
In addition to filiform papillae, the A taste bud is a collection of 50–100 When food molecules hit the cells,
tongue has fungiform (mushroomlike), cells that are clustered together like they interact with either receptor proteins or
foliate (leaflike), and circumvallate (wall-like) segments in an orange. They are located porelike proteins called ion channels. This
papillae, which all contain taste buds. Most in the walls of papillae. One end of each causes electrical changes in the cell, which
taste buds are found in the foliate papillae cell protrudes out of the bud, where it gets prompt neurons at the base of the cell to
on the back and sides of the tongue. washed with saliva containing food molecules. send signals to the brain.

Signals travel to secondary Signals sent to Signals travel to OMATOCOSERNTESXORY
taste area, located in primary taste tongue area of
orbitofrontal cortex area, located somatosensory cortex

Taste and smell in insula
ORBCITOROFTERXONTAL
Detecting flavors depends as much on the S
nose as on the taste buds. The nose picks Signals from olfactory
up external odors from food (see pp.78–79), cortex sent to
but this is increased significantly by food- orbitofrontal cortex
particle odors carried up into the nasal cavity
by expired air from the lungs (retronasal OLFACTORY Olfactory cortex THALAMUS
olfaction). Some smell receptors have also BULB
been found in the taste buds. The brain AMYGDALA
combines the information from the nose NASAL CAVITY Amygdala assigns
and taste buds to perceive all the positive or negative
different flavors in the food. These are values to taste and
not the only sensations that contribute smell
to the taste experience—the
somatosensory cortex detects
the texture and temperature of
food, adding context to the flavor.

Smell from food particles MEDULLA
that have been swallowed
are sent for processing by

olfactory bulb

The taste pathway Food particle
Information from the taste buds travels to
the brain via cranial nerves in the jaw and Trigeminal and
throat. Impulses travel up the brain stem glossopharyngeal
to the thalamus and are forwarded to the cranial nerves carry
taste regions of the frontal cortex and signals to medulla
insula, a fold of cortex deep in the brain.
in brain stem
WHY DON’T Expired air from
BABIES LIKE KEY lungs pushes
BITTER FOODS? Taste signals food particles
Retronasal smell
Babies have many more Expired air from mouth into
taste buds than adults so they nasal cavity

taste bitter foods more
intensely. They instinctively

refuse foods that aren’t
as sweet or fatty
as breast milk.

THE AVERAGE ADULT
HAS BETWEEN 2,000
AND 8,000 TASTE BUDS

LIGHT BREEZE TEMPERATURE CHANGE BRUSH OF A FEATHER

EPIDERMIS TOP, DEAD LAYER OF EPIDERMIS
SPINOUS LAYER
DERMIS (DEEP LAYER OF SKIN) HAIR SHAFT Well-defined
BASAL LAYER borders make
Free nerve endings Merkel’s disks
Net of nerve fiber extend into skin’s sensitive to
endings wrapped surface layer shapes and
around base of shaft edges

Hair movement
triggers nerve
impulse

Root hair plexus Free nerve endings Merkel’s disks
Nerves wrapped around the base of a hair Extending up into the spinous layer of Found slightly lower than free nerve
shaft are triggered by things that have not the epidermis, these bare, rootlike nerve endings, Merkel’s disks are particularly
touched the skin, such as air currents or endings are sensitive to cold, heat, light dense in the lips and fingertips. They
objects that brush against the hair. touch, and pain. respond to light touch.

Touch TYPES OF RECEPTORS FUNCTION

The skin is the biggest organ of the body Mechanoreceptors Sensory receptors that respond to
and also the largest sense organ. Packed mechanical pressure or distortion.
with sensors, it enables us to experience This can range from a light touch
a wide variety of sensations, as well as to deep pressure.
an awareness of where we are.
Proprioceptors Receptors that receive stimuli from
Receptors in the skin within the body, particularly in
relation to position and movement.
Skin sensors consist of receptors connected by axons.
Found at various levels in the skin, there are around Nociceptors Sensory neurons that respond
20 types that respond to different sorts of stimuli. The to damaging stimuli by sending
receptors register mechanical, thermal, and, in some cases, “possible threat” signals to the
chemical stimuli and convert them into electrical signals. spinal cord and the brain.
These travel up peripheral nerves to the spinal cord, then
to the brain stem, and finally to the somatosensory cortex, Thermoreceptors Specialized nerve cells that are able
where they are translated into a touch. Chemoreceptors to detect differences in temperature.
They are found all over the skin and
in some internal areas.

Extensions of the peripheral nervous
system that respond to changes in
blood concentrations to maintain
homeostasis (see pp.90–91).

GENTLE TOUCH FIRM MASSAGE VIBRATION

Enlarged,
encapsulated
receptor

Fluid-filled Ruffini endings Large, covered
receptors Also known as bulbous corpuscles, these receptor at
extend into soft, capsulelike cells—located deep in the base of dermis
upper dermis dermis—respond if the skin or joints are
stretched or distorted by pressure. Pacinian corpuscle
Meissner corpuscles The deepest and largest type of
These receptors are rapidly adapting, touch receptor, these rapidly acting
meaning that they respond quickly to mechanoreceptors respond to sustained
stimulation but stop firing if the stimulus pressure as well as vibration.
continues. This gives precise information.

The somatosensory cortex Touch map LEG
Areas of the body rich in touch
All information from touch receptors is processed receptors, such as the hands, require TRUNK
in the somatosensory cortex. This area sits across more processing than others, so they HEAD
the top of the brain like a hair band. Data from the take up a greater proportion of the ARM
right side of the body travels to the left side of the somatosensory cortex.
brain, and vice versa. Each part of the body maps FOOT
to its own area of the cortex. HAND TOES
GENITALS
AXON
LEFT SIDE
Signal travels through Myelinated RIGHT HAND EYE OF BRAIN
nerve bundle sheath SPINAL FACE
CORD
LIPS

TONGUE

PERIPHERAL NERVE

Proprioception Nerve signal from
proprioceptors
The body has its own sense of where it is
and how it is moving in space. This process Stretch receptors
happens almost unconsciously, making it, in skin, muscles,
in essence, the body’s sixth sense. and joints send

information about
position of body parts

Body position sense Knowing your place SPINAL COLUMN Signals travel
Physical self-awareness comes from a along spinal
Inside muscles, tendons, and joints are combination of proprioception with other column to brain
movement receptors called proprioceptors. sensations: a sense of force, a sense of
Every time we move, these receptors measure effort or weight, sight, and information
changes in length, tension, and pressure that from the balance organs in the ears.

relate to that movement and send impulses to

the brain. The information is processed and

a decision is made to stop moving or change

position. Messages are then relayed back to Parietal lobe

the muscles to carry out the decision. All of this

happens without us having to think about it. Inner ear sends
information about
Types of proprioception rotation, acceleration,

and gravity

Most of the information our brain

receives about body position is Parietal lobe Eyes send visual
processed unconsciously, such as information about
how we are constantly adjusting position

the position of our body to maintain

balance. However, proprioceptive

information can become conscious Thalamus Input from
if it requires us to make a decision— pressure and tension
for example, refining muscle
sensors in arms

movement to make a voluntary,

skilled movement.

Proprioception pathways Cerebellum Conscious
Conscious proprioception signals travel up pathway
the brain stem to the thalamus and end at the Unconscious
parietal lobe, which is part of the cerebral pathway
cortex. The unconscious pathway loops back
to the cerebellum, which controls movement.

BRAIN FUNCTIONS AND THE SENSES 84 85
Proprioception

Types of proprioceptors GROWTH SPURTS
CAN CONFUSE
The body contains a variety of proprioceptors, and the combined THE BRAIN
information from these receptors helps the brain construct an overall AS IT CANNOT
picture of the body’s position. There are three main types of KEEP UP WITH
proprioceptors: muscle spindle fibers, which are embedded in our CHANGES IN LIMB
muscles; Golgi tendon organs, which are located at the junction DIMENSIONS
between tendons and muscles; and joint receptors, which attach to our
joints. Special receptors in the skin can also detect stretch (see p.83).

Bone Muscle Muscle
Golgi tendon organ senses
Touch-sensitive changes in muscle tension Muscle spindle
nerves fibers
Ligament receptors Bone
Ligament Signal travels up
Tendon nerve axon

Joint receptors Tendon receptors Muscle receptors
Nerve endings within our joints detect the Golgi tendon organs are found within Muscles have position sensors called spindle
joints’ position. The receptors help prevent the tendons at the ends of muscles. They fibers within them. As they stretch, the
damage through overextension as well as monitor muscle tension to ensure we do spindles send information to the brain
detecting position in normal motion. not overstretch the muscles. about the positions of the muscles.

THE PINOCCHIO ILLUSION

Sometimes proprioception can be confused, making Brain thinks
hand has
the body feel like something is happening when it is not. Hand touching moved away
from face
One such effect is called the Pinocchio illusion. A vibrator nose Vibrator
switched on
is fixed to a person’s bicep. If the person holds her nose Vibrator
while the vibrator is turned on, she will feel as though During stimulation
Vibrations tell the brain that the
her arm is moving away from her nose. It happens arm is moving, creating a sensation
that the nose is growing outward.
because the vibrator stimulates the muscle spindle Before stimulation
fibers in the biceps in the same way as if the muscle At rest, the brain is aware that the
was stretching. Because the fingers are still touching the fingers are touching the nose, but
nose, it feels as if the nose is growing out from the face. there is no movement of the arm.

Feeling Pain WHO FEELS

THE MOST PAIN?

Although unpleasant, pain is a useful warning Women feel pain more
sign that something isn’t right with the body and intensely than men
that we need to act quickly to avoid further injury. because they have

more nerve receptors

Pain signals in their bodies.

Pain receptors are located all over the body and respond to

heat, cold, overstretching, vibration, and chemicals released

by wounds. Electrical signals are sent from the site of injury

to the spinal cord, where they cross over and travel to the SPINAL CORD SIGNAL
opposite side of the brain to the injury. If sudden,
strong pain is experienced, a reflex

reaction occurs (see p.101) within Slow C-fiber
the spinal cord to make the

limb pull away from Nerve bundle
whatever is causing the contains multiple
pain, even before we axons, or nerve fibers

are aware of it. NERVE BUNDLE PAIN SIGNAL

Slow C-fibers Fast A-fiber 2 Pain signals travel up nerve bundles
are widespread Signals from the injury site travel along
nerve bundles toward the spinal cord. The
in skin

A-fiber signals get there within milliseconds
and trigger a withdrawal reflex away from
the source of the pain.

Axon

Nerve cell

1 Pain receptors
activated
Injury prompts the
release of chemicals
called prostaglandins
from damaged cells.
These trigger the nerve
Fast A-fiber axons to send impulses

covered by to the brain. Prostaglandin
molecule
Pain fibers myelin sheath
released by cell
There are two types of nerve fibers, or axons.
Fast A-fibers carry sharp, localized pain from
Damaged cell

an injury such as a cut. Slower C-fibers carry
the more persistent dull feelings from the
area around the injury.

SKIN BRUISE CUT

BRAIN FUNCTIONS AND THE SENSES 86 87
Feeling Pain

Frontal cortex plays Somatosensory cortex NATURAL PAIN RELIEF
role in anticipating identifies intensity, location,
and controlling pain and type of pain The body releases its own chemicals,
called endorphins and enkephalins,
Limbic system to dampen the pain signals. They
is responsible for bind to receptors on the nerve
emotional and endings, preventing further
behavioral transmission of pain signals.
reaction to pain

Reticular Transmission of signal Receiving
formation neuron
modulates Sending
pain signals neuron

Thalamus relays signals Nerve fibers
to different areas of brain descending from
brain intercept and
4 Pain signals processed modify ascending Pain
The signal continues pain signals signal
to the thalamus, which PAIN SIGNAL TRANSMITTED
distributes impulses to the Endorphin blocks
cortex and other areas pain signal reaching
responsible for emotion, receiving neuron
attention, and assessing the 5 Alleviating pain
Descending signals BLOCKED PAIN SIGNAL
significance of the pain. travel back down from the

brain to intercept the pain
signals (see box, right).
Pain signals These trigger the release of
travel up natural painkillers by the
brain stem and spinal cord
spinal cord

that reduce pain signals.

Spinal cord signal
bypasses brain

SPINAL CORD

DORSAL
HORN

3 Pain signals reach
the spinal column
The nerve bundle enters the spinal cord
through the dorsal horn. Pain signals pass
across to the other side of the spinal cord Most nerve bundles
for their onward journey to the brain. enter at back of spine,
known as dorsal horn

How to Use Your Brain
to Manage Pain

When we are in pain, the usual courses of action involve
medical treatment or painkillers. However, we can also
help control pain ourselves by regulating our mental
response—both to the pain and to the stress it causes.

Pain is an emotional as well as effects, including stomach ulcers a big ball of energy outside your
physical response to injury or and liver disease. Your body may body, and “shrink” it in your mind.
disease. Intense fear or anxiety also build up a tolerance to a drug Cognitive behavioral therapy (CBT)
are vital immediate reactions that so that you derive less benefit from uses a similar approach, by training
cause you to avoid sources of pain it as time goes on. you to replace negative thoughts
whenever possible. Sometimes, like “This pain is unbearable,” or “I
however, pain persists even when Mind-body therapies can’t stop this pain,” with positive
the injury or disease is no longer ones such as, “This pain is only
present. A painful sensation can In addition to medication, you can temporary.”
become associated with constant use mind-body techniques such
stress, recurring unpleasant as relaxation and visualization to Practicing mindfulness reduces
memories of what caused the reduce or help control pain—with stress, making you better able to
pain, or the constant fear that it no risk of side effects. Most use cope with pain. In this practice,
will persist or recur. relaxation and deep, controlled adapted from Buddhist teachings,
breathing to reduce the tension you merely acknowledge the pain—
These feelings can be powerful that comes with pain and often instead of allowing it to dominate
and unsettling. Although you makes it worse. Try lying quietly in your thoughts or exhausting
should always seek medical advice a darkened room; breathe in deeply yourself by actively fighting it.
if pain is severe or prolonged, you while counting to 10, hold the
can also use several techniques to breath for a moment, then exhale To sum up, your brain can be a
regulate it by training your mind. slowly for a count of 10. Continue powerful tool for pain control if you:
this for 10–20 minutes. • Practice relaxation and deep
The painkiller problem breathing techniques to reduce
Shifting your attention often stress levels.
Medication is often essential to reduces pain’s severity. Try turning • Employ mental exercises to shift
control pain in the short term, but your attention away from the attention away from pain.
taking painkillers for an extended painful area, focusing instead on • Use CBT techniques to focus on
period can lead to issues such as a nonpainful part of your body. positive thoughts.
addiction or serious physical side Alternatively, imagine the pain as • Practice mindfulness.



The Regulatory GENERAL ANESTHETICS
System
A vital part of modern surgery, how
The human body is a cooperative of 38 trillion general anesthetics work is not fully
cells organized into different systems. Keeping understood. What is known is that they
them functioning at their best is a system of act on the reticular activating system
feedback mechanisms controlled by the brain. (comprising the reticular formation
and its connections) to suppress
Maintaining stability awareness and on the hippocampus to
temporarily suspend memory formation.
The process of maintaining a stable internal environment is called Anesthetics also affect the nuclei of the
homeostasis. Key functions, such as breathing, heart rate, pH, thalamus, preventing the flow of sensory
temperature, and ion balances have to be kept within strict information from the body to the brain.
operating limits to prevent us from becoming ill. As the body works,
its systems are constantly being moved away from their balance or Signals travel to various
set point (the value at which a system works best). When the change areas of cerebral cortex
becomes too great, the body initiates a feedback loop that
returns the system to its ideal level. Many of these
functions are controlled by a part of the
brain stem called the reticular formation.

3 Signals forwarded
Signals are then sent directly
to the thalamus and hypothalamus,
as well as to the appropriate areas
of the cerebral cortex for a decision
and response to the stimulus.
Hypothalamus
Excitatory area of reticular THALAMUS regulates sleep,
formation amplifies hunger, and body
important signals temperature

Thalamus relays
sensory signals
to cerebral cortex

WHAT IS THE SPINAL CORD 2 Signals processed
RETICULAR FORMATION? MEDULLA In the reticular formation,
unwanted signals are suppressed in
the inhibitory area, while others are
amplified in the excitatory area.
The reticular formation
consists of more than 100 Inhibitory area of reticular
nuclei that project to the formation dampens
forebrain, cerebellum, and unwanted signals

brain stem, controlling 1 Signals travel up
many of the body’s the spinal column
vital functions. Incoming sensory signals
Impulses travel from all over the body travel
up spinal cord to the reticular formation.

BRAIN FUNCTIONS AND THE SENSES 90 91
The Regulatory System

RESULT STIMULUS SENSOR
Baby is born. The fetus exerts Stretch receptors
are stimulated and
Positive feedback system pressure on send signals to the
The less common of the two feedback systems, positive the cervix. hypothalamus.
feedback systems are more unstable because they have EFFECTOR
the potential to have a knock-on effect on other systems, Oxytocin CONTROL
creating a “runaway” process. An example of a positive promotes more The hypothalamus
feedback system is the increase in strength and frequency contractions.
of labor contractions, which stop when the baby is born stimulates the
and the cervix is no longer being stretched. posterior pituitary
gland, which releases
Feedback loops
oxytocin.
Biological systems operate on a mechanism
of inputs and outputs, each caused by, and
causing, a certain event. Feedback loops
either amplify the output of a system
(positive feedback) or inhibit the output of
the system (negative feedback). Feedback
loops are important because they allow
living organisms to maintain homeostasis.

RESULT STIMULUS SENSOR
Normal body The body’s Thermoreceptors
temperature is temperature in the skin sense this
changes.
achieved. temperature
change.
Negative feedback system EFFECTOR
Most systems use negative feedback loops, If too hot, the CONTROL
which are very stable and act to reverse the brain induces The hypothalamus
direction of change to restore the system to sweating. If too cold,
normal. They include regulation of blood the brain initiates compares to
glucose and body temperature. temperature set
shivering. point (98.6°F/ 37°C).
95˚F (35˚C)

THE BODY TEMPERATURE AT
WHICH HYPOTHERMIA SETS IN

Nuclei in the hypothalamus DE THE HYPOTHALAMUS Synthesizes oxytocin, Regulates blood pressure
Most of the nuclei have distinct vasopressin, and and heart rate
functions. They secrete hormones somatostatin
that act on the pituitary gland, Initiates intake of
stimulating it to produce hormones DORSAL water and food
that will help achieve homeostasis HYPOTHALAMIC
in the required part of the body. Involved
AREA in memory,
Inhibits eating and MEDINAULCLPERUESOPTIC arousal, sleep,
reduces food intake INSI ANNUTCELREIUOSR PARANVUECNLTERUISCULARDORSOMEDIAL and energy
NUCLEUS balance
Controls thermoregulation LATERAL VENNUTCRLOEUMSEDIAL
PREOPTIC HYPOLATRAPTHENEAAOURLSCTAALELEMRIUISCOR
NUCLEUS

MAMMBOILDLAYRY

Body’s “clock”—controls SUPRACHIASMATIC SUPRAOPTIC LATERAL
circadian rhythms NUCLEUS NUCLEUS TUBERAL
NUCLEI
30 PIGTLUAITNADRY
HORMONES ARE OCNUELROVEMOTOR
PRODUCED BY THE
ENDOCRINE SYSTEM

Neuroendocrine OUT OF BALANCE
System
When homeostasis is disrupted,
Maintaining homeostasis (see p.90) requires the it can lead to disease, as well as to
brain and body to communicate. This is achieved our cells malfunctioning. The body
using chemical messengers called hormones. tries to correct the problem but
may make it worse, depending on
The hypothalamus what is influencing the imbalance.
Genetics, lifestyle, and toxins can
At the center of the brain’s homeostasis system is the hypothalamus all impact homeostasis.
(see p.34). It contains clusters of neurons, called nuclei, that perform
specific functions and has connections to the autonomic nervous system
(see p.13), through which it sends messages to control heart rate, digestion,
and breathing. When the hypothalamus receives a signal from the nervous
system, it secretes neurohormones, which in turn stimulate the pituitary
gland to secrete hormones. These affect organs all over the body and
prompt them to increase or suppress their own hormone production.

BRAIN FUNCTIONS AND THE SENSES 92 93
Neuroendocrine System

Hormone producers Hypothalamus links nervous
Hormones are used for two types of system to endocrine system
communications. The first is between
two endocrine glands, where a Pineal gland releases melatonin
hormone is released to stimulate a in response to light levels—
target gland to alter the amount of melatonin governs body’s
hormone it is secreting. The second is circadian rhythm and regulates
between a gland and a target organ, some reproductive hormones
such as the release of insulin from the
pancreas prompting muscle cells to Controlled by the hypothalamus,
take up glucose. pituitary gland acts as “master gland”;
it secretes its own hormones that
Thyroid gland and control other glands
parathyroid glands
regulate metabolism, PARATHYROID GLAND
blood calcium levels,
THYROID
and heart rate GLAND

Produces cortisol THYMUS Produces white blood
(regulates metabolism, cells that defend against
immune response, and viruses and infections

energy conversion),
aldosterone (controls
blood pressure and salt
balance), and adrenaline
(fight-or-flight hormone)

Secretes renin and STOMACH Releases hunger-
angiotensin, which inducing hormone
ADRENAL ghrelin and hormone
control blood GLAND gastrin, which
pressure, as well as stimulates acid
erythropoietin, which KIDNEY KIDNEY production
stimulates production
Secretes insulin, glucagon, and somatostatin
of red blood cells to control blood sugar; gastrin, which
stimulates stomach cells to produce acid;
Producing hormones PANCREAS and a hormone that controls water secretion
and absorption in intestines
The endocrine system is made up of OVARY
glands that are dedicated specifically to TESTES Produces female reproductive hormones
secreting hormones, as well as organs— estrogen and progesterone, which
such as the stomach—that are not glands prepare uterus for menstruation or
themselves but are able to produce, store, pregnancy
and release hormones. Both types react to
signals from the brain by increasing or Produce testosterone, which is essential
decreasing the production of hormones, in sperm production, maintaining
which then travel, via the bloodstream, muscle mass and strength, libido, and
to a target organ, where they lock onto bone density
specialized receptors on the surfaces of
cells. This triggers a physiological change
that restores homeostasis.

Hunger 5 Feeling full Hypothalamus
Signals that acts as regulator
leptin and insulin US
and Thirst levels are increasing
stimulate the HYPOTHALAM Insulin levels tell
hypothalamus to hypothalamus
produce the hormone whether body
melanocortin, which has enough
makes us feel full. energy
Food and drink are essential
Decreased levels
to human survival. Prompts by Rising levels of of leptin inform
hormones to take in nutrients ghrelin tell hypothalamus of
and water are experienced by low energy stores;
hypothalamus increased leptin
stomach is empty levels help inhibit
appetite
the body as hunger and thirst.

Hunger 4 Signals from
adipose tissue
There are two types of hunger. Hedonic To prevent us from
hunger involves eating food—particularly overeating, adipose tissue
foods high in fat, sugar, and salt—when we cells release a hunger-
are already full, while homeostatic hunger inhibiting hormone called
(see right) is a response to our energy leptin, which travels to
stores depleting. Once food has passed the hypothalamus.
through the stomach and intestines, the
now-empty stomach releases a hormone 3 Signals from pancreas
called ghrelin. This acts on neurons in After we have eaten, the
the hypothalamus to tell us that we are small intestine releases the hormone
hungry, prompting us to eat. A hunger- incretin. This, combined with the
inhibiting hormone called leptin is stomach stretching and increased
then released by adipose (fat-bearing) glucose in the blood, causes the
tissue to stop us from overeating. pancreas to release insulin.

Feeling hungry 2 Urge to eat
The brain, digestive system, and fat stores form Rising levels of ghrelin
an interconnected system that regulates our instruct the hypothalamus to release
feelings of hunger. The sensation of hunger a chemical signal called neuropeptide
can be caused by internal factors, such as our Y, which stimulates our appetite.
stomach being empty or our blood sugar
levels falling, or by external triggers, such as Incretin produced by
seeing or smelling food. intestines triggers
insulin production

DEHYDRATION KEY Stretch receptors
AFFECTS OUR detect expansion
SHORT-TERM Ghrelin
MEMORY, of stomach
CONCENTRATION, Insulin
AND ANXIETY LEVELS STOMACH
Leptin
SMALL INTESTINE PANCREAS Pancreas ADIPOSE
Incretin produces insulin (FAT)
Vagus TISSUE
nerve signal
Movement 1 Empty stomach
of food Once the stomach has been empty
for around two hours, levels of sugar and
insulin in the blood decrease. This causes the
stomach to produce the hormone ghrelin.

BRAIN FUNCTIONS AND THE SENSES 94 95
Hunger and Thirst

Thirst Lamina Organum vasculosum
terminalis (LT) of the lamina terminalis (OVLT)
When water levels in the body drop, salt
levels in the blood increase. Thirst areas Hypothalamus Subfornical organ (SFO)
in the brain detect rising salt levels and
signal to the body to increase water levels Pituitary Thirst areas of the brain
by reducing urine output and taking in gland Two structures, the organum
more fluids. After drinking, it takes around vasculosum of the lamina
15 minutes before salt concentration levels terminalis (OVLT) and the
in the blood return to normal. It is thought subfornical organ (SFO)—both
that the gulping action of the throat when linked to the hypothalamus—help
swallowing liquids sends signals to create the sensation of thirst.
stop drinking. They lack a blood-brain barrier so
are thought to be able to detect
salt levels in the blood.

1 Heart and kidney 2 The SFO and 3 The hypothalamus 4 High levels of
receptors detect OVLT also passes these ADH tell the
decreases in blood receive signals about signals to the pituitary kidneys to retain water
volume and increases in blood volume and salt gland, which then and secrete renin. This in
salt concentration. They concentration. They signal produces antidiuretic turn forms the hormone
alert the brain. to the hypothalamus. hormone (ADH). angiotensin II.

7 Inhibitory neurons 6 The hypothalamus 5 The SFO detects
in the LT are creates the angiotensin II
triggered by gulping sensation of thirst, and stimulates the
movements in the throat. prompting the urge hypothalamus to
These neurons stop to drink so as to restore prompt the formation
further intake of water. water levels. of more ADH.

HOW LONG CAN ARE YOU DEHYDRATED? VERY
YOU SURVIVE WITHOUT HYDRATED
The most obvious symptoms HYDRATED
FOOD OR WATER? of dehydration are a dry mouth
and eyes, and perhaps a slight MODERATELY
Three to four days is the headache. Another good way DEHYDRATED
average without water, but to tell is by the color of your
you can go up to two months urine. It should be pale yellow VERY
at full hydration. A darker DEHYDRATED
without food in certain amber color shows severe DANGEROUSLY
circumstances. dehydration. Adults should DEHYDRATED
take in around 31⁄2–4 pints
(2–2.5 liters) of fluids a day.

READINESS POTENTIAL Putamen feeds stored Posterior parietal cortex receives
information to posterior information from putamen and also
When we prepare for a voluntary action, a buildup of assesses body’s position in relation
electrical activity, called the readiness potential, occurs. parietal cortex
It begins in the SMA and is intensified by activity in the to surroundings
PMA. Activity in the SMA starts up to 2 seconds before DFORROSNOTLAALTECROARLTEX
we become consciously aware of our decision to move— COPAPRORTEISEXTTEARLIOR
which may suggest that we are less in control of our
actions than we might believe (see p.168). BASAL
GANGLIA
Activity in PUTAMEN THALAMUS
SMA Time of actual VISUAL
movement CORTEX
Activity in
PMA
Activity
SPINAL CORD0

–3 –2 –1 0 1 Sensory information is sent
from visual cortex via thalamus
Time (seconds)
to dorsolateral frontal cortex

THE CEREBELLUM CONTAINS 1 Gathering information
MORE THAN 50 PERCENT Sensory areas, such as the visual
OF THE BRAIN’S NEURONS cortex, send signals to the frontal cortex.
The putamen, which stores learned actions,
sends information to the parietal cortex,
which assesses whether these learned
actions could be used in this new situation.

Planning Movement

Conscious movements are those that we deliberately decide
to make. They involve several regions of our brain and include
processes that lie outside our conscious awareness.

The planning process WHY DON’T WE
FORGET HOW TO RIDE
There are several stages involved in carrying out a movement—from
initial perception of the environment, to planning, to adjustments A BICYCLE?
during the movement. These stages involve different areas of the
brain working together to produce a response. The area that prompts Nerve cells in the putamen
the movement is the motor cortex. Different sections of the motor encode the sequence of muscle
cortex send signals to different parts of the body (see p.98). However, movements into our long-term
before an action begins, an action plan is created by the dorsolateral
frontal cortex and the posterior parietal cortex and is passed through memory storage so that
two areas of the motor cortex: the supplementary motor area (SMA) they are easily accessible
and the premotor area (PMA). The cerebellum coordinates the
movement as it is happening. The steps above show the brain areas even years later.
involved and the sequence of signals in a typical movement.

BRAIN FUNCTIONS AND THE SENSES 96 97
Planning Movement

Posterior parietal cortex Brain stem passes Primary motor area has
signals conscious intention to information back to command-feedback links with

move via basal ganglia primary motor cerebellum and basal ganglia
area once it is
SMA BASAL
fine-tuned GANGLIA
DFORROSNOTLAALTECROARLTEX PMA
MOPTROIRMAARREYA
COPAPRORTEISEXTTEARLIOR

THALAMUS BASAL
PUTAMEN GANGLIA

Dorsolateral CEREBELLUM
frontal cortex
sends signals to
basal ganglia

Thalamus relays signals Command for action BRAIN STEM
from basal ganglia to sent via spinal
PMA and SMA
cord to muscles

2 Deciding how to move Basal ganglia 3 Getting ready for action
The dorsolateral frontal cortex strengthen or Signals travel to the primary
weaken signals
and parietal cortex work together to SPINAL CORD motor area, which forwards instructions SPINAL CORD Information exchanged
plan the movement. This information between cerebellum
is sent via the basal ganglia (see pp.32–33) SIDE CROSS SECTto the cerebellum and brain stem to be and brain stem
fine-tuned. Signals from these areas return
to the PMA and SMA, which specify the to the primary motor area, which sends the
sequence of muscle contractions needed.
signal for action to the spinal cord.

CTIONfobreweanrdresOgtuonlcaPetMesdAig, ntahnaaldslaShmMavuAes Putamen receives ION Dentate nucleus
signals from frontal makes subtle
and parietal areas adjustments to
motor plans
FRONT CROSS SE
Substantia PUTAMEN BRAIN STEM DENTATE
nigra controls THALAMUS NUCLEUS
GPLAOLBLUIDSUS CERECBOERLTLEX
strength of
actions

Cerebellar AR
cortex
Globus pallidus
inhibits unwanted coordinates
movements timing

Subthalamic
nucleus involved
in impulse control

Regulating movement Making adjustments KEY
The basal ganglia are a group of nuclei that Signals from the primary motor area are
are linked to the thalamus. Signals from frontal sent to the cerebellum, which plays a role Signals to
and parietal areas are processed by circuits in in measuring time. It also makes real-time cerebellum
the basal ganglia that amplify or inhibit adjustments to movements in response to
movement signals. our environment. Signals from
cerebellum

Making a Move SIMPLE AND COMPLEX MOVEMENTS

Once our brain has planned a movement (see pp.96–97), A motor homunculus shows PRIMA
it sends signals to the appropriate muscles in the body, which areas of the motor MOTOR HOMUNCULUS
via the nervous system, to turn intention into action. cortex control which areas of
the body. Areas for adjacent
From brain to spine body parts—such as the arm
and hand—are generally
Signals from the motor and parietal areas of the cortex are sent along the grouped together. The body
axons of neurons, through the brain stem, to communicate with motor parts are shown in proportion;
neurons in the spinal cord. Most of the axons form part of a bundle called those areas that make complex
the lateral corticospinal tract, which crosses over at the base of the brain movements, such as the face
stem so that axons from one brain hemisphere connect to motor nerves for and the hand, take up more
the opposite side of the body. Other nerve tracts originate in different parts space in the cortex than those
of the midbrain and perform specific movement functions. making simple movements,
such as the foot.
RY MOTOR AREA
TRACTS SPINAL CORD LEFT SICDOPEARRTOIEEXFTABLRAIN

Red PRIMARY
nucleus MOTOR

Reticular AREA
KEY SPINALformation
Lateral corticospinal Lateral corticospinal Most signals
tract THALAMUSAxons cross totract begins inoriginate in
SPINAL CORDopposite sidecortex and runs
Rubrospinal tract through thalamus primary
of body in motor area
Vestibulospinal midbrain
tract Rubrospinal
tract aids fine
Reticulospinal motor control
tract

Motor-nerve axon

PONS

1 Nerve tracts Vestibulospinal tract, Axons cross MIDBRAIN CEREBELLUM
The axons of which originates in over to opposite
the lateral corticospinal brain stem, helps side of body Axons collect in
tract send signals to regulate balance just below midbrain and join
and body brain stem
orientation spinal cord
muscles that connect to Reticulospinal Neurons from brain (upper
the skeleton to produce tract helps
voluntary limb movements. coordinate motor neurons) pass
Other groups of axons are movement signals down spinal cord
responsible for the body’s
involuntary responses, such
as balance, as well as for
fine-tuning movements.

TERMAIXNOANL NEDUirReOctiMonUSCULAR Muscle contracts RADIAL NERVE Lower motor neurons
and moves joint, pass signals from
of signal spinal cord to muscles
causing arm to
bend

Receptor for JUNCTION
acetylcholine
Acetylcholine Upper motor
SYNAPTIC CLEFT Lower motor neurons
neurons
WHITE SPINAL CORD
MUSCLE FIBER MUSCLE MATTER

3 At the neuromuscular junction, the end GRAY
of the axon releases acetylcholine, a MATTER
neurotransmitter (see p.24). The acetylcholine VENTRAL
binds to receptors in the muscle cell HORN

membrane. This triggers chemical
reactions that make the muscle
fiber contract. BRAIN FUNCTIONS AND THE SENSES
Making a Move
2 The upper and lower motor neurons meet
in the ventral horn of the spinal cord.
The outer part of the ventral horn carries
nerves that run to the hands and feet;
the central part carries nerves to
the upper arms and thighs.

RIGHT ARM
Executing movement
Nerve signals make a muscle HOW LONG DOES IT 98 99
contract and pull on the associated TAKE FOR A SIGNAL
joint to move the part of the limb
just beyond it. Muscles used in fine TO TRAVEL FROM
movements have more nerve endings BRAIN TO MUSCLE?
than those used for simple movements.
Signals can travel
From spine to muscle from the brain to our
muscles at a speed of up
Inside the spinal cord, the axons of the corticospinal tract, which are
covered with a myelin sheath, form the white matter. The gray matter at to 395 ft (120 m)
the center of the spinal cord consists of the cell bodies of motor neurons. per second.
The ends of the corticospinal axons (known as upper motor neurons)
synapse on to motor neurons (known as lower motor neurons) in the
ventral horn of the gray matter. The axons of the lower neurons exit the
spine through gaps in the vertebrae (see p.12) and extend to the muscle
fibers. The point where the nerve endings activate the muscle fibers to
complete the movement is called the neuromuscular junction.

Unconscious WHY DOES BEING
Movement TIRED SLOW DOWN
OUR REACTION TIME?
We perform many voluntary actions without having When we are tired, neurons
to think about them because they are so familiar. in our brain slow down,
Another kind of unconscious movement is the affecting our visual
reflex action—an instinctive response to danger. perception and memory.
This means we respond to
Reaction pathways VISUAL UPPER events more slowly.
CORTEX (DORSAL)
Visual information is vital in helping us Visual pathways in the brain
plan our movements. Information from the ROUTE The dorsal route carries information
visual cortex follows two routes in the brain LOWER on the position of the body and other
(see pp.70–71). The upper (or dorsal) route, objects, while the ventral route draws
which leads to the parietal lobe, guides (VENTRAL) on perception and memory for
our actions in real time. Meanwhile, the ROUTE identifying objects. The brain uses this
lower (or ventral) route, which ends at information to judge the strength and
the temporal lobe, triggers stored visual direction required for a movement.
experiences to help us interpret what we
see and respond accordingly.

Coordinated actions Attention focused on Body readies
what the player can see, itself to
Any sequence of actions demands such as opposing player respond
coordination between different
parts of the brain—first to focus Thalamus Frontal lobe Putamen stores
attention on the task, then to focuses inhibits learned actions,
integrate sensory information and distracting such as how to
memory to create a plan, then attention on thoughts return a ball
to engage the motor area to act. opponent
Acquiring a new skill, such as PARIETAL
driving or playing a sport, involves FROLNOTBAEL CORTEX PUTAMEN
learning and practicing movement
sequences so that they become THALAMUS
almost unconscious. When we learn
a skill, our brain cells form new 1 Attention 2 Memory
connections. By the time we have To prepare for action, the Visual cues trigger the parietal
mastered a skill (see box, right), thalamus directs attention to the area cortex to call up memories of action
there is far less cortical activity where the activity will occur (such as the sequences from the putamen. The
associated with performing that opposing player), while the frontal lobes parietal cortex uses this information
task than there was when we were block distracting thoughts so the player to assess the context and create an
a novice. As a result, the actions can concentrate on the visual cues. internal model for the action.
of a skilled person—such as a
professional tennis player—are
more rapid, precise, and subtle.