How the brain works

TWHOHOWERKBSRAIN

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BRAIN FUNCTIONS
AND THE SENSES

THE PHYSICAL BRAIN Sensing the World 64
Seeing 66
What the Brain Does 10 The Limbic System 38 The Visual Cortex 68
12 40 How We See 70
The Brain in the Body 14 Imaging the Brain 42 Perception 72
44 How We Hear 74
Human and Animal 16 Monitoring the Brain Perceiving Sound 76
Brains 18 46 Smell 78
20 Babies and Young Taste 80
Protecting the Brain 22 Children 48 Touch 82
24 50 Proprioception 84
Fueling the Brain 26 Older Children 52 Feeling Pain 86
28 and Teenagers How to Use Your Brain 88
Brain Cells 30 54 to Manage Pain
32 The Adult Brain 56 The Regulatory System 90
Nerve Signals 34 Neuroendocrine System 92
The Aging Brain 58 Hunger and Thirst 94
Brain Chemicals 36 Planning Movement 96
How to Slow the 60 Making a Move 98
Networks in the Brain Effects of Aging Unconscious 100
Movement
Brain Anatomy Brain Food Mirror Neurons 102

The Cortex Genetics and
the Brain
Nuclei of the Brain
Male and Female
Hypothalamus, Brains
Thalamus, and
Pituitary Gland Nature and Nurture

The Brain Stem
and Cerebellum

COMMUNICATION CONSCIOUSNESS
AND THE SELF
Emotions 106
Fear and Anger What Is Consciousness? 162
Conscious Emotion 108
Reward Centers Attention 164
Sex and Love 110
Expressions How to Focus 166
Body Language 112 MEMORY, Your Attention
How to Tell if 114 LEARNING,
Someone Is Lying 116 AND THINKING Free Will and 168
Morality the Unconscious
Learning a Language 118
The Language Areas 120 What Is Memory? 134 Altered States 170
Having a Conversation
Reading and Writing How a Memory Forms 136 Sleep and Dreams 172
122 Storing Memories
124 Recalling a Memory 138 Time 174
126 How to Improve Your
128 Memory 140 What Is Personality? 176
130 Why We Forget
142 The Self 178
Memory Problems
144
Special Types 146
of Memories 148

Intelligence 150
152
Measuring
Intelligence 154
156
Creativity
158
How to Boost
Your Creativity

Belief

DISORDERS

Headache and Migraine 196 Seasonal Affective 207
Head Injuries 197 Disorder
Epilepsy 197
Meningitis and 198 Anxiety Disorders 208
Encephalitis
THE BRAIN Brain Abscess 198 Phobias 208
OF THE FUTURE TIA 199
Stroke and Hemorrhage 199 Obsessive- 209
Brain Tumors 200 Compulsive Disorder
Dementia 200
Superhuman Senses 182 Parkinson’s Disease 201 Tourette’s Syndrome 209
Huntington’s Disease 201
Wiring the Brain 184 Multiple Sclerosis 202 Somatic Symptom 210
Motor Neuron Disease 202 Disorder
Paralysis 203
The Unexplored 186 Down Syndrome 204 Munchausen 210
Brain Cerebral Palsy 204 Syndrome
Hydrocephalus 205
Artificial Intelligence 188 Narcolepsy 205 Schizophrenia 211
Coma 206
The Expanded Brain 190 Depression 206 Addiction 212
Bipolar Disorder 207
The Global Brain 192 Personality Disorder 213

Eating Disorders 214

Learning Disabilities 215
and Difficulties

Attention Deficit 216

Hyperactivity Disorder

Autism Spectrum 217
Disorders

INDEX 218
ACKNOWLEDGMENTS 224



THE
PHYSICAL
BRAIN

What the DO BRAINS
Brain Does FEEL PAIN?

Despite the fact that it

The brain is the body’s control center. registers pain from around
It coordinates the basic functions required the body, brain tissue has
for survival, controls body movements, and
processes sensory data. However, it also no pain receptors and
cannot feel pain itself.

encodes a lifetime of memories and creates

consciousness, imagination, and our sense of self.

The physical brain Weight Fat
On average, an adult The brain’s dry weight
At the largest scale, the human human brain weighs is 60 percent fat. Much of
brain appears as a firm, pink-gray 2.6–3.1 lb (1.2–1.4 kg), this fat is present as
solid. It is made mostly from fats which is approximately sheaths coating the
(about 60 percent) and has a 2 percent of total body connections between
density just a little greater than weight. neurons.
that of water. However,
neuroscientists, the people who Water Volume
study the form and function of The brain is 73 percent The average volume of a
the brain, see the organ as being water, while the body as a human brain ranges from
constituted from more than 300 whole is closer to 60 69 to 77 cubic in (1,130 to
separate, although highly percent. The average 1,260 cubic cm), although
interconnected, regions. On a much brain contains around the volume decreases
smaller scale, the brain is made 35 fl oz (1 liter) of water. with age.
from approximately 160 billion cells,
half of which are neurons, or nerve Gray matter White matter
cells, and about half are glia, or About 40 percent of Around 60 percent of
support cells of one kind or another the brain’s tissue is gray the brain’s tissue is white
(see pp.20–21). matter, which is tightly matter. This is made
packed nerve-cell from long, wirelike
bodies. extensions of nerve cells
covered in sheaths of fat.

LEFT BRAIN VS. RIGHT BRAIN RIGHT HEMISPHERE
LEFT HEMISPHERE
It is often claimed that one side, or hemisphere, of
the brain dominates the other—and that this has an
impact on someone’s personality. For example, it is
sometimes said that logical people use their left
brain hemisphere, while artistic (and less logical)
people rely on the right side. However, this is an
extreme oversimplification. While it is true that the
hemispheres are not identical in function—for
example, the speech centers are normally on the
left—most healthy mental tasks deploy regions on
both sides of the brain at the same time.

THE PHYSICAL BRAIN 10 11
What the Brain Does

Memory Movement
The brain remembers a bank To contract, muscles rely on
of semantic knowledge, general the same kind of electrical
facts about the world, as well as a impulses that carry nervous signals
personal record of life history. The through the brain and body. All
function of memory is to aid muscle movement is caused by
future survival by encoding nerve signals, but the conscious

useful information from brain has only limited
the past. control over it.

Emotions What does the Control
Most theories of brain do? The basic body systems, such
emotion suggest that they
are preordained modes of The relationship between the as breathing, circulation,
behavior that boost our survival body and brain has long been a digestion, and excretion, are all
chances when we encounter subject of debate for scientists and under the ultimate control of the
confusing or dangerous situations. philosophers. In ancient Egypt, the brain brain, which seeks to modify
Others suggest emotions are was dismissed as a system for shedding their rates to suit the needs of
animal instincts leaking heat, and the heart was the seat of emotion
through into human the body.
consciousness.

and thought. Although our most significant

feelings are still described as heartfelt,

Communication neuroscience shows that the brain
A unique feature of the drives all body activities.
human brain is the speech
centers that control the Thinking
formulation of language and the The brain is where thought
muscular execution of speech. The and imagination take place.
brain also uses a predictive Thinking is a cognitive activity
system to comprehend what that allows us to interpret the
someone else is saying. world around us, while our
imagination helps us consider
Sensory experience possibilities in the mind without
Information arriving from all
over the body is processed in the input from the senses.
brain to create a richly detailed
picture of the body’s surroundings.
The brain filters out a great deal

of sensory data deemed
irrelevant.

SMOOTHING OUT ALL THE WRINKLES OF THE
BRAIN’S OUTER LAYER WOULD COVER AN AREA
OF ABOUT 2½ SQUARE FT (2,300 SQUARE CM)

The Brain Skull provides Permeating the body
in the Body protection The nervous system extends
to brain throughout the entire body.
It is so complex that all of a
The brain is the primary component of Brain body’s nerves joined end to
the human body’s nervous system, which Spinal cord end could circle the world
coordinates the actions of the body with two and a half times.
the sensory information it receives.
Spinal nerves of
The nervous system peripheral system
join spinal cord
The two main parts of the nervous system are the central of central system

nervous system (CNS) and the peripheral nervous system. Spinal cord
runs down
The CNS is made up of the brain and the spinal cord, a back, through
vertebrae of
thick bundle of nerve fibers that runs from the brain in the spinal column

head to the pelvis. Branching out from this is the peripheral Peripheral
nerves extend
system, a network of nerves that permeates the rest of the through torso
and limbs to
body. It is divided according to function: the somatic hands and feet

nervous system handles Sciatic nerve
is largest and
voluntary movements longest nerve
in body
of the body, while the
Sensory and
autonomic nervous SPINAL motor nerves are
CORD often bundled
system (see opposite) together,
separating at
handles involuntary Motor Sensory their ends
functions. nerve nerve
KEY
Spinal nerves SPINAL NERVE VERTEBRA Central nervous
Most peripheral nerves system (CNS)
connect to the CNS at the Peripheral
spinal cord and split as they Bone nervous system
connect. The rear branch carries vertebra
sensory data to the brain; the protects
forward branch carries motor spinal cord

signals back to the body. SPINAL COLUMN (REAR VIEW)

CRANIAL NERVES

Within the peripheral system, Signals along
optic nerve
12 cranial nerves connect
travel directly
directly to the brain rather to brain
than the spinal cord. Most link

to the eyes, ears, nose, and

tongue and are also involved

in facial movements, chewing,

and swallowing, but the vagus

nerve links directly to the heart, Spinal
lungs, and digestive organs. cord

The autonomic EYES EYES
nervous system LUNGS LUNGS
ARTERIES ARTERIES
The involuntary, or autonomic, HEART HEART
system maintains the internal LIVER
conditions of the body by LIVER
controlling the involuntary
muscles in the digestive system
and elsewhere, as well as heart
and breathing rates, body
temperature, and metabolic
processes. The autonomic system
is divided into two parts. The
sympathetic system generally
acts to elevate body activity
and is involved in the so-called
“fight-or-flight” response. The
parasympathetic system works
in opposition to this, reducing
activity to return the body to
a “rest-and-digest” state.

STOMACH STOMACH

INTESTINES BLADDER
INTESTINES
THE TOTAL LENGTH BLADDER Parasympathetic
OF THE SOMATIC Sympathetic Chiefly associated with the cranial
NERVOUS SYSTEM These nerves emerge from the spinal nerves (see far left), this part of the
IS ABOUT 45 MILES cord in the chest and abdominal autonomous system works to
(72 KM) regions and connect to a chain of reduce energy use when the body
ganglia (nerve bundles) that run down is at rest. It is also involved in sexual
either side of the spine. Nerves then arousal, crying, and defecation.
extend out from there to the body.

Human and KEY Pituitary
Animal Brains Cerebellum gland

The human brain is one of the defining features of our Optic lobe Medulla
species. Comparing the human brain with the brains of
other animals reveals connections between brain size and Cerebrum Olfactory
intelligence and between an animal’s brain anatomy and bulb
the way it lives.
Brain mass

Brain mass as a
percentage of body mass

Brain sizes GOLD BULL

The size of a brain indicates FROG
its total processing power. For FISH
example, a honeybee’s tiny brain
contains 1 million neurons, a Nile
crocodile’s has 80 million, while
a human brain has around 80–90
billion neurons. The link with
intelligence is clear. However,
with larger animals, it is important
to compare brain and body size to
give a more nuanced indication
of cognitive power.

Sizing up 0 0.004 oz (0.1g) 0.04 oz 0 0.04 oz (0.2g) 0.04 oz
There are two ways to compare brain sizes, 0 0.16% 1g 0 0.04% 1g
by total weight and as a percentage of body
weight. The largest brain, at 17 lb (7.8 kg), 2 2
belongs to the sperm whale, but that is a
minute fraction of its 44-ton (45-tonne) body.

Brain shapes Nerves branch Esophagus
out into head runs through
All brains are located in the head, and body from middle of
in close proximity to the primary each ganglion brain
sense organs. However, it would be
a mistake to visualize animal brains Leech Doughnut-
as rudimentary variations, in size The 10,000 cells in a leech’s nervous system shaped
and structure, of the human brain. are arranged in chains of cell clusters called brain
All vertebrate brains follow the ganglia. The brain is a big ganglia, with 350
same development plan, but neurons, located at the front of the body. Octopus
anatomies vary widely to match An octopus’s brain contains 500 million
different sensory and behavioral neurons. Only a third are located in the head;
needs. More variety can be seen in the rest are in the arms and skin, where they
the brains of invertebrates, which are devoted to sensory and motor controls.
account for 95 percent of all animals.

THE PHYSICAL BRAIN 14 15
Human and Animal Brains

VARYING PROPORTIONS Cerebrum

All mammal brains contain the same Cerebrum
components, but they grow in different
proportions. A third of the volume of a rat’s RAT BRAIN HUMAN BRAIN
central nervous system (CNS) is made up of
the spinal cord, indicating its reliance on DOMES HU
reflex movements. By contrast, the spinal
cord is a tenth of a human CNS. Instead,
three-quarters is taken up by the cerebrum,
which is used for perception and cognition.

EUROPE

MAN
TIC CAT
AN QUAIL

0.04 oz 1.76 oz 49.4 oz
47.6 oz (1,350g) 1,400 g
0 0.03 oz (0.9g) 1 g 0 1.05 oz (30g) 50 g 0
2% 2
0 0.9% 20 0.9% 20

Olfactory bulbs Cerebral cortex
sit behind is more folded
nares, which than that of
are nostril-like humans
openings that
smell water DO ALL ANIMALS
HAVE A BRAIN?
Shark
The brain of a shark is Y-shaped due to the Dolphin Sponges have no nerve
large olfactory bulbs that extend out on The hearing and vision centers of a dolphin’s cells at all, while jellyfish
either side. The sense of smell is the shark’s brain are larger and closer together than in a and corals have a netlike
primary means of tracking prey. human brain. It is thought that this helps the nervous system but no
dolphin create a mental image using its sonar.
central control point.

Protecting the Brain

The vital organs are safely secured in the body’s core, but Dural sinuses
because the brain sits in the head at the top of the body, collect oxygen-
it requires its own protection system. depleted blood

The cranium ETAL (2) FRON
PARI
The bones of the head are T SUBARACHNOID SPACESPHENOID (1)
collectively known as the skull TAL (1)IPITAL (1) EMPORAL (2)ETHMOID (1)
but are more correctly divided
into the cranium and the mandible, OCC
or jawbone. It is supported by the
highest cervical vertebra and Paired bones Direction of flow
the musculature of the neck. The brain is enclosed by eight large CSF flows from the
The cranium forms a bony case bones, with a pair of parietal and
completely surrounding the brain. temporal bones forming each side of
It is made of 22 bones that steadily the cranium. The remaining 14 cranial
fuse together in the early years of bones make up the facial skeleton.
life to make a single, rigid structure.
Nevertheless, the cranium has
around 64 holes, known as
foramina, through which nerves
and blood vessels pass, and eight
air-filled voids, or sinuses, which
reduce the weight of the skull.

2

ventricles into the subarachnoid
space, where it then moves up
and over the front of the brain.
Cerebrospinal fluid
WHAT IS WATER CSF IS
The brain does not come into direct ON THE BRAIN? CONTINUALLY
contact with the cranium. Instead it PRODUCED,
is suspended in cerebrospinal fluid Also called hydrocephalus, AND ALL OF IT
(CSF). This clear liquid circulating this condition arises when IS REPLACED
inside the cranium creates a there is too much CSF in the EVERY 6–8
cushion around the brain to protect cranium. This puts pressure HOURS
it during impacts to the head. In on the brain and affects
addition, the floating brain does
not deform under its own weight, its function.
which would otherwise restrict
blood flow to the lower internal Meninges and ventricles
regions. The exact quantity of CSF The brain is surrounded by three membranes,
also varies to maintain optimal or meninges: the pia mater, arachnoid mater, and
pressure inside the cranium. dura mater. The CSF fills cavities called ventricles
Reducing the volume of CSF and circulates around the outside of the brain in
lowers the pressure, which in the subarachnoid space, which lies between the
turn increases the ease with which pia and arachnoid mater.
blood moves through the brain.

16 17

Dura mater

Arachnoid mater The blood-brain barrier

Pia mater Infections from the rest of the
body do not ordinarily reach the
1 Site of fluid production brain due to a system called the
CSF is made from plasma, the liquid blood-brain barrier. As a general
part of blood. Most of it is produced by the rule, blood capillaries in the rest of
choroid plexus, a network of blood vessels the body leak fluid easily (and any
that runs throughout the ventricular system. viruses and germs it contains) into
surrounding tissues through gaps
CSF flows into 4 Reabsorption between the cells that form the
ventricles The CSF is reabsorbed blood vessel’s wall. In the brain,
into the circulatory system, these same cells have a much
VENTRICLE where it remixes with the tighter fit, and the flow of materials
blood. CSF is renewed at a rate between the brain is instead
of three to four times a day. controlled by astrocytes that
LATERAL surround the blood vessels.

CHOROID PLEXUS Substances pass Fat-soluble
THIRD VENTRICLE out of vessel substances pass
through pore though cell
membranes

FOURTH CEREBELLUM Water-soluble NORMAL
VENTRICLE substances BLOOD VESSEL

enter via pore Some water-soluble
between cells substances enter brain

Tight junction
between cells

SKULL

3 Circulation CSF travels Fat-soluble Astrocyte
around downward substances cells surround
at back of move freely blood vessels
spinal cord
BRAIN
KEY BLOOD VESSEL
Blood flow
spinal cord SPINCEANLTCROARLDCANAL
As well as the brain, CSF Flow of
surrounds the spinal cerebrospinal fluid Selectively permeable
cord, flowing down Normal blood vessels allow fluid to pass
along the back of the through easily. However, while oxygen,
spinal cord, into the fat-based hormones, and non-water-soluble
central canal, then up materials pass through the blood-brain
along the front. barrier unhindered, water-soluble items are
blocked so they don’t reach the CSF.

Fueling DOES FOCUSED
the Brain CONCENTRATION USE

The brain is an energy-hungry organ. Unlike MORE ENERGY?
other organs in the body, it is fueled solely
on glucose, a simple sugar that is quick and The brain never stops
easy to metabolize. working, and the overall
energy consumption stays

more or less the same
24 hours a day.

Blood supply Carotid artery

The heart supplies blood to the whole body, but around a sixth Vertebral
of its total effort is devoted to sending blood up to the brain. artery
Blood reaches the brain by two main arterial routes. The two
carotid arteries, one running up each side of the neck, deliver FROM THE
blood to the front of the brain (and the eyes, face, and scalp). The HEART
back of the brain is fed by the vertebral arteries, which weave
upward through the spinal column. Deoxygenated blood then
accumulates in the cerebral sinuses, which are spaces created
by enlarged veins running through the brain. The blood there
drains out of the brain and down through the neck via the
internal jugular veins.

The vascular system delivers 26 fl oz (750 ml) of blood to the
brain every minute, which is equivalent to 1.7 fl oz (50 ml) for
every 3.5 oz (100 g) of brain tissue. If that volume drops below
about 0.7 fl oz (20 ml), the brain tissue stops working.

Crossing the blood-brain barrier BLOOD VESSEL Paracellular transport Diffusion
Water and water-soluble Cells are surrounded
The blood-brain barrier is a physical and metabolic materials, such as salts and ions by a fatty membrane, so
barrier between the brain and its blood supply. It offers (charged atoms or molecules), fat-soluble substances,
extra protection against infections, which are hard to can cross through small gaps including oxygen and alcohol,
combat in the brain using the normal immune system, between capillary-wall cells. diffuse through the cell.
and could make the brain malfunction in dangerous
ways. There are six ways that materials can cross the Water-soluble Fat-soluble
barrier. Other than that, nothing gets in or out. substance substance

Cellular wall ASTROCYTE BRAIN BLOOD-BRAIN Tight junction Molecule
The physical blood-brain barrier is created by the BARRIER moves
cells that make up the walls of capillaries in the brain. through cell
Elsewhere in the body, these are loosely connected,
leaving gaps, or loose junctions. In the brain, the
cells connect at tight junctions.

Astrocytes collect material from
blood and pass it to neurons

THE PHYSICAL BRAIN 18 19
Fueling the Brain

Anterior InternalCIRCLE OF WILLISGLUCOSE FUEL
cerebral artery carotid artery
The human brain makes up just 2
supplies front Posterior cerebral percent of the body’s total weight, but it
of brain artery supplies consumes 20 percent of its energy. The
back of brain large human brain is an expensive organ
Median to run, but the benefits of a big, smart
cerebral artery Cerebellar brain make it a good investment.
artery supplies
supplies side Basilar cerebellum
of brain artery

Direction of
blood flow

Arteries encircle
stalk of pituitary

gland, optic
tracts, and basal

hypothalamus

Vertebral BRAIN BRAIN’S ENERGY
artery SIZE: 2% NEEDS: 20%

UNDERSIDE
OF BRAIN

The Circle of Willis THE BODY’S ENTIRE
The carotid and vertebral supplies connect at the SUPPLY OF BLOOD IS
base of the brain, via communicating arteries, to PUMPED THROUGH THE
create a vascular loop called the Circle of Willis. BRAIN EVERY 7 MINUTES
This feature ensures cerebral blood flow is
maintained, even if one of the arteries is blocked.

Protein transporters Receptors Transcytosis Active efflux
Glucose and other Hormones and similar Large proteins, which are When unwanted materials
essential molecules are substances are picked up by too big to pass through channels, diffuse through the blood-brain
actively moved across the receptors. They are enclosed are absorbed by the membrane barrier, they are removed by a
barrier through channels and in a vesicle (sac) of membrane and enclosed in a vesicle for its biochemical pumping system
gates in the membrane. for passage through the cell. journey through the cell. called efflux transporters.

Glucose Hormone reaches Protein molecule
receptor and enters enclosed in vesicle
vesicle
Waste pumped
into blood vessel

Gates made Vesicle merges with Unwanted
from protein membrane to waste
release contents
products

Brain Cells GRAY MATTER AY MATTER

The brain and the rest of the nervous system The brain is divided into gray and white
contains a network of cells called neurons. The matter. Gray matter is made of neuron
role of neurons is to carry nerve signals through cell bodies, common in the surface
the brain and body as electrical pulses. of the brain. White matter is made
of these neurons’ myelinated axons
Neurons bundled into tracts. They run through
the middle of the brain and down the
Most neurons have a distinctive branched shape with dozens of spinal cord.
filaments, only a few hundred thousandths of a foot thick, extending
from the cell body toward nearby cells. Branches called dendrites bring GR
signals into the cell, while a single branch, called the axon, passes the
signal to the next neuron. In most cases, there is no physical WHITE
connection between neurons. Instead, there is a tiny gap, called the MATTER
synapse, where electrical signals stop. Communication between cells
is carried out by the exchange of chemicals, called neurotransmitters
(see pp.22–23). However, some neurons are effectively physically
connected and do not need a neurotransmitter to exchange signals.

Types of neurons Dendrites act like antennae to Axons can
There are several types of neurons, with different collect signals from be several
combinations of axons and dendrites. Two common centimeters long
types, bipolar and multipolar neurons, are each neighboring nerve cells
suited to particular tasks. Another type of neuron, Dendrites are shorter than
the unipolar neuron, appears only in embyros. Electrical pulse jumps axons, usually up to only
from one myelin segment 16 hundred thousandths
Connection to Dendrite receives signal of a foot
from sense organ to the next, speeding
up nerve signal

brain cells Axon

Cell body AXON
Bipolar neuron
This type of neuron has one dendrite and one Axon delivers
axon. It transmits specialized information signal from
from the body’s major sense organs. neighboring cell

Synapse with Axon Cell body
other cell Dendrite

Multipolar neuron THE HUMAN
Most brain cells are multipolar. They BRAIN CONTAINS
have multiple dendrites connecting to APPROXIMATELY
hundreds, even thousands, of other cells. 86 BILLION NEURONS

THE PHYSICAL BRAIN 20 21
Brain Cells

Chemicals crossing MYELIN Some neurons in
from neighboring cell peripheral nervous
create an electrical system have myelin-
pulse in dendrite producing Schwann cells

Neurofibrils

MYELIN MEMABXROANNE
SHEATH

Cell membrane A single combined Myelin sheath is
conveys nerve electrical signal is sent coiled around axon
impulses
out to the next cell Insulation
ERVE CELL BODY An axon may be covered in a
AXON sheath of fat called myelin. This
DNA works like insulation, preventing
CELL NUCLE electrical charges from leaking out
N and thus speeding up the signal.

US

Golgi body Lysosomes destroy Glia Helper cells
packages waste chemicals There are eight main
chemicals The nervous system relies on a team of helper cells types of glia, but
Mitochondria called glia. Astrocytes control what chemicals enter only five are
process glucose the brain from the blood. Oligodendrocytes produce common in the
myelin for brain cells, forming the white matter. brain. They protect
Ependymal cells secrete the cerebrospinal fluid, while the overall health of
microglia work as immune cells, clearing out waste the nervous system.
cells. Radial cells are the progenitors of neurons.

Blood vessel Myelin sheath
supported produced here

ASTROCYTES OLIGODENDROCYTES Developing
neuron
Inside a neuron Cilia help move MICROGLIA
A neuron contains broadly neurotransmitters Long,
the same set of organelles, straight cell
or internal structures, as any Damaged
other cell for releasing energy, neurons provides
making proteins, and managing support
genetic material. detected here
RADIAL GLIA
EPENDYMAL CELLS

Nerve Signals HOW DOES A
NERVE COMMUNICATE
The brain and nervous system work by sending DIFFERENT INFORMATION?
signals through cells as pulses of electrical Receiving cells have different
charge and between cells either by using types of receptors, which respond
chemical messengers called neurotransmitters to different neurotransmitters.
or by electric charge. The “message” differs according
to which neurotransmitters
Action potential are sent and received and

Neurons signal by creating an action potential—a surge of in what quantities.
electricity created by sodium and potassium ions crossing
the cell’s membrane. It travels down the axon and SOME NERVE IMPULSES
stimulates receptors on dendrites of neighboring cells. TRAVEL FASTER THAN
The junction between cells is called a synapse. In many 330 FT (100 M) PER
neurons, the charge is carried over a minute gap between SECOND
axon and dendrite by chemicals, called neurotransmitters,
released from the tip of the axon. These junctions are
known as chemical synapses. The signal may cause the
neighboring neuron to fire, or it may stop it from firing.

Excess of positive ions on
outside of cell membrane

Membrane channels Excess of ions inside
open to let ions in produces a positive charge

CELL’S AXON MEMBRANE

FLUID INSIDE AXON KEY Positive ions
Direction of rush in
nerve impulse

Flow of ions

Direction of nerve
impulse

1 Resting potential 2 Depolarization
When the neuron is at rest, there are more positive Chemical changes from the cell body
allow positive ions to flood into the cell
ions outside the membrane than inside. This causes a through the membrane. That reverses the
difference in polarization, or electrical potential, across
the membrane called the resting potential. The difference polarization of the axon, making the
potential difference +30 millivolts.
is about –70 millivolts, meaning the outside is positive.

THE PHYSICAL BRAIN 22 23
Nerve Signals

NERVE AGENTS Synapses

Chemical weapons, like novichok Some neurons do not share a physical connection. Instead they
and sarin, work by interfering with meet at a cellular structure, called a synapse, where there is a
how neurotransmitters behave at gap of 40 billionths of a meter, known as the synaptic cleft,
the synapse. Nerve agents can be between the axon of one neuron (the presynaptic cell) and the
inhaled or act on contact with skin. dendrite of another (the postsynaptic cell). Any coded signal
They prevent the synapse from carried by electrical pulses is converted into a chemical message
clearing away used acetylcholine, at the tip, or terminal, of the axon. The messages take the form
which is involved in the control of one of several molecules called neurotransmitters (see p.24),
of muscles. As a result, muscles, which pass across the synaptic cleft to be received by the
including those used by the heart dendrite. Other neurons have electrical synapses rather than
and lungs, are paralyzed. chemical synapses. These are effectively physically connected
and do not need a neurotransmitter to carry
electrical charge between them.

1 Chemical store Synaptic AXON TERMINAL
Neurotransmitters are vesicle

manufactured in the cell body of the
neuron. They travel along the axon to
the terminal, where they are parceled
up into membranous sacs, or vesicles. SYTSYNANPATPITCICCCELLLEFT Neurotransmitter
At this stage, the terminal’s membrane POS
carries the same electrical potential as Receptor for
the rest of the axon. neurotransmitter

Action potential

arrives and

depolarizes

2 Signal received membrane
When an action potential
surges down the axon, its final
destination is the terminal, where Calcium
it temporarily depolarizes the ions flow in

membrane. This electrical change
has the effect of opening protein
channels in the membrane, which
Positive ions allow positively charged calcium
pumped out ions to flood into the cell.

Calcium influx causes Depolarization
synaptic vesicles to causes voltage-
gated channels
release neurotransmitters to open

3 Releasing messages
The presence of calcium
within the cell sets off a complex
process that moves the vesicles Neurotransmitters
to the cell membrane. Once
slot into

there, the vesicles release receptor sites

neurotransmitters into the cleft.
Some diffuse across the gap to
3 Repolarization be picked up by receptors on the
The depolarization of a section of the
axon causes the neighboring section to dendrite. The neurotransmitters
may stimulate an action potential Channels open and
undergo the same process. Meanwhile, the to form in that dendrite, or they cause positive ions
cell pumps out positive ions to repolarize the to flow in and
membrane back to the resting potential. may inhibit one from forming. polarize the cell

Brain IS TECHNOLOGY
Chemicals ADDICTION THE SAME
AS DRUG ADDICTION?
While communication in the brain relies on
electric pulses flashing along wirelike nerve No, technology addiction
cells, the activity of these cells—and the mental is more comparable to
and physical states they induce—are heavily overeating. Release of
influenced by chemicals called neurotransmitters.
dopamine can increase by 75
percent when playing video
games and by 350 percent

when using cocaine.

Neurotransmitters Drugs

Neurotransmitters are active at the synapse, the Chemicals that change mental and physical states,
tiny gap between the axon of one cell and a dendrite both legal and illegal, generally act by interacting
of another (see p.23). Some neurotransmitters are with a neurotransmitter. For example, caffeine
excitatory, meaning that they help continue the blocks adenosine receptors, which has the effect of
transmission of an electrical nerve impulse to the increasing wakefulness. Alcohol stimulates GABA
receiving dendrite. Inhibitory neurotransmitters have receptors and inhibits glutamate, both inhibiting
the opposite effect. They create an elevated negative neural activity in general. Nicotine activates the
electrical charge, which stops the transmission of the receptors for acetylcholine, which has several
nerve impulse by preventing depolarization from taking effects, including an increase in attention as well as
place. Other neurotransmitters, called neuromodulators, elevated heart rate and blood pressure. Both alcohol
modulate the activity of other neurons in the brain. and nicotine have been linked to an elevation of
Neuromodulators spend more time at the synapse, dopamine in the brain, which is what leads to their
so they have more time to affect neurons. highly addictive qualities.

TYPES OF NEUROTRANSMITTERS TYPE OF DRUG EFFECTS
Agonist
There are at least 100 neurotransmitters, some of which are listed A brain chemical that stimulates the
below. Whether a neurotransmitter is excitatory or inhibitory receptor associated with a particular
is determined by the presynaptic neuron that released it. neurotransmitter, elevating its effects.

NEUROTRANSMITTER USUAL POSTSYNAPTIC Antagonist A molecule that does the opposite
CHEMICAL NAME EFFECT Reuptake of an agonist, by inhibiting the action
inhibitor of receptors associated with a
Acetylcholine Mostly excitatory neurotransmitter.

Gamma-aminobutyric acid (GABA) Inhibitory A chemical that stops a
neurotransmitter from being
Glutamate Excitatory reabsorbed by the sending neuron,
thus causing an agonistic response.

Dopamine Excitatory and inhibitory BLACK WIDOW SPIDER VENOM
INCREASES LEVELS OF THE
Noradrenaline Mostly excitatory NEUROTRANSMITTER
ACETYLCHOLINE, WHICH
Serotonin Inhibitory CAUSES MUSCLE SPASMS

Histamine Excitatory

THE PHYSICAL BRAIN 24 25
Brain Chemicals

THE LONG-TERM EFFECTS OF ALCOHOL KEY
Dopamine
Drinking large volumes of alcohol over a long Cocaine
period alters mood, arousal, behavior, and
neuropsychological functioning. Alcohol’s Dopamine and cocaine
depressant effect both excites GABA and inhibits The effects of cocaine are a
glutamate, decreasing brain activity. It also product of its effects on the
triggers the brain’s reward centers by releasing neurotransmitter dopamine
dopamine, in some cases leading to addiction. at synapses in the brain.

Dopamine VESICLE Dopamine VESICLE
held in vesicles released
inside sending

neuron

SENDING NEURON SYNAPSE SENDING NEURON
SYNAPSE

RECEPTOR Unused dopamine RECEPTOR Cocaine blocks
sucked back into dopamine’s path
Once released, sending neuron Concentration
some dopamine of dopamine back into
bonds to receptors in synapse sending neuron
increases
on receiving
neuron

RECEIVING NEURON RECEIVING NEURON
With use of cocaine
Normal dopamine levels Cocaine molecules are reuptake inhibitors of dopamine. When
Dopamine is a neurotransmitter associated with feeling dopamine is released, it moves into the synapse and binds to
pleasure. It creates a drive to repeat certain behaviors that receptors on the receiving neuron as normal. However, the
trigger feelings of reward, perhaps leading to addiction. While cocaine has blocked the reuptake pumps that recycle the
some dopamine molecules bind to receptors on the receiving dopamine, so the neurotransmitter accumulates in a higher
neuron, unused dopamine is recycled by being pumped back concentration, increasing its effects on the receiving neuron.
into the sending neuron and parceled up again.

Networks Calcium ions AXON
in the Brain facilitate signaling
between neurons SYNAPTIC CLEFT
The patterns of nerve-cell connections in the
human brain are believed to influence how Calcium unable Axon releases
it processes sensory perceptions, performs to access channel glutamate
cognitive tasks, and stores memories. neurotransmitter

Wiring the brain Glutamate
neurotransmitter
The dominant theory of how the brain remembers binds to receptor,
and learns can be summed up by the phrase “the eventually causing
cells that fire together, wire together.” It suggests channel to unblock
that repeated communication between cells creates
stronger connections between them, and a network DENDRITE
of cells emerges in the brain that is associated with
a specific mental process—such as a movement,
a thought, or even a memory (see pp.136–37).

KEY Synaptic weight Magnesium ion
Little-used connections blocks channel
Magnesium have channels blocked
ion Channel by magnesium ions. As the
strength of a connection
Calcium ion Glutamate between two neurons in 1 Channel blocked
receptor a network increases, the In a weak connection, magnesium ions block
Glutamate channel is unblocked, and
neurotransmitter the number of receptors the passage of calcium ions into the dendrite of a
at the synapse increases. receiving neuron. A glutamate neurotransmitter
received from the axon will open that channel.

Neuroplasticity Strong Weak
synapses synapses
The networks of the brain are not
fixed but seem to change and adapt WHAT IS THE BRAIN’S
in accordance with mental and DEFAULT MODE NETWORK?
physical processes. This means
that old circuits associated with It is a group of brain regions
one memory or a skill that is no that show low activity levels
longer in use fade in strength as when engaged in a task such
the brain devotes attention to as paying attention but high
another and forms a new network activity levels when awake and
with other cells. Neuroscientists
say the brain is plastic, meaning its not engaged in a specific
cells and the connections between mental task.
them can be reformed many times
over as required. Neuroplasticity BRAIN PATHWAYS
allows brains to recover abilities
lost due to brain damage.

THE PHYSICAL BRAIN 26 27
Networks in the Brain

More
neurotransmitters

received

Magnesium Calcium ions
ion removed pass freely
from channel

Extra glutamate
receptors

introduced

2 Channel open 3 More receptors
With the channel open, calcium ions are With more receptors active, the dendrite
now able to move from the synaptic cleft into the is able to pick up more neurotransmitters, and
dendrite. In response, the dendrite adds more so any signal sent from the neighboring axon
glutamate receptors to the surface of the dendrite. is received much more strongly.

Small-world networks Random Small-world Lattice
A random network Small-world networks By connecting every
Brain cells are not connected in a regular is good at making have good local and cell to its neighbors,
pattern, nor are they in a random network. long-distance distance connections. this network has
Instead, many of them exhibit a form of connections but poor Every cell is more reduced scope to
small-world network, where cells are seldom at linking nearby cells. closely linked than in make long-distance
connected to their immediate neighbors but the other two systems. connections.
to nearby ones. This way of networking
allows each cell to, on average, connect to
any other in the smallest number of steps.

IT IS ESTIMATED THAT
THE HUMAN BRAIN
CONTAINS 100 TRILLION
CONNECTIONS BETWEEN
ITS 86 BILLION NEURONS

Brain Anatomy Surface layer of forebrain, Tracts of white
known as gray matter, is made matter—neurons
The brain is a complex mass of soft tissue composed from unsheathed neurons sheathed with
almost entirely of neurons, glial cells (see fatty myelin
p.21), and blood vessels, which are
grouped into an outer layer, the cortex, CORTEX
and other specialized structures.

Divisions of the brain CEREBRUM

The brain is divided into three GREY MATTER ORPUS CALLOSUM
unequal parts: the forebrain, C
midbrain, and hindbrain. These AMPUS
divisions are based on how they HIPPOC
develop in the embryonic brain, AMYGDALA
but they also reflect differences
in function. In the human brain, THALA MUS
the forebrain dominates, making M
up nearly 90 percent of the brain BRAINSSPTINEAML CORDCEREBELLU
by weight. It is associated with
sensory perception and higher Midbrain MEDULLA
executive functions. The midbrain The smallest brain section, this is associated with PONS
and hindbrain below it are more the sleep-wake cycle, thermoregulation (control
involved with the basic bodily of body temperature), and visual reflexes, such MIDBRAIN
functions that determine survival, as the rapid eye movements that scan complex
such as sleep and alertness. scenes automatically. The substantia nigra,
which is a region associated with planning
SPINAL NERVES smooth muscle control, is in the midbrain.

There are 31 pairs of spinal nerves Hindbrain
that branch out from the spinal cord Made up of the cerebellum at the lower
above each vertebral bone, named rear of the brain and the brain stem, which
after the parts of the spine to which connects to the spinal cord, the hindbrain
they connect. They relay signals is the most primitive part of the brain.
between the brain and sensory The genes that control its development
organs, muscles, and glands. evolved around 560 million years ago.

Cervical
nerves

Thoracic Direct connections
nerves to all three sections
of brain are carried
Lumbar
nerves in spinal cord

Sacral
nerves

28 29

Parietal lobe governs perception Occipital lobe is mostly Hemispheres
of body position and other given over to vision
touch sensations The cerebrum forms in two
halves, or hemispheres, which
Brain handles are divided laterally by a gap
short-term memory called the longitudinal fissure.
Nevertheless, the hemispheres
in frontal lobe share an extensive connection
via the corpus callosum. Each
PARIETAL LOBE hemisphere is a mirror image of
the other, although not all functions
FRONTAL LOBE OCCIPITAL LOBE are performed by both sides (see
CORPUS Cp.10). For example, speech centers
tend to be on the left side.

ALLOSUM White-matter
nerve tracts
form corpus
callosum

TEMPORAL LOBE Same layout
of four lobes
on both sides

Forebrain Temporal lobe is Communication
The forebrain is divided in linked to language fibers from each
two. At its base is the thalamus, and emotion hemisphere switch
which, along with the structures
around it, serves as a junction sides at base of
box for sensory signals and brain stem
movement impulses. The rest
of the forebrain is the cerebrum, Left side of body
which is dominated by the cerebral is controlled by
cortex. This is where consciousness,
language, and memory are right hemisphere
processed, along with the brain’s
higher functions. The cortex is Left and right
further divided into four lobes. The brain and the body are connected
contralaterally, meaning that the left brain
18 in hemisphere handles the sensations and
movements of the right side of the body
(46 CM) THE LENGTH and vice versa.
OF THE SPINAL CORD

The Cortex IAL SURFACE

The cortex is the thin outer layer that forms the
brain’s visible surface. It has several important
functions, including handling sensory data and
language processing. It also works to generate
our conscious experience of the world.
FRONTAL L
A functional map Areas related to Inferior temporal gyrus
OCCIPITAL LOBE MEDconscious emotional is involved in face
The cortex is a multilayered coating of neurons, recognition
with their cell bodies at the top. Neuroscientists divide responses and
it into areas where the cells appear to work together to decision-making
perform a particular function. There are different ways
to reveal this information: through the location of brain located in
damage linked to the loss of a brain function; tracking orbitofrontal cortex
the connections between cells; and through scans of
live brain activity. Cingulate gyrus is fused to
limbic area (pp.38–39)
WHAT IS
PHRENOLOGY? KEY Emotion Vision
Memory Body sensation Olfaction
A 19th-century Gustation Cognition
pseudoscience, in which the Audition

shape of the head was Motor
linked to brain structure,

specific abilities, and
personality.

Folds and grooves OBE PARIETAL LOBE Gyrus
TEMPORAL LOBE Sulcus
The cerebral cortex is a feature of Lobe divisions
all mammal brains, but the human The boundaries between the lobes of the
brain is distinctive because of its cerebral cortex are set by deep grooves. The
highly folded appearance. The frontal lobe meets the parietal lobe at the
many folds increase the total central sulcus, while the temporal lobe starts
surface area of the cortex, thereby next to a sulcus called the lateral fissure.
providing more room for larger
cortical areas. The groove in a fold
is called a sulcus, and the ridge is
called a gyrus. Every human brain
has the same pattern of gyri and
sulci, which neuroscientists
employ to describe specific
locations in the cortex.

THE PHYSICAL BRAIN 30 31
The Cortex

LA TERAL SURFACE PRIMARY Somatosensory
MOTOR cortex processes
Vision-related CORTEX sensory
Brodmann areas information
extend from PARIETAL
lateral surface to CORTEX Parietal cortex
medial surface combines
WERNICKE’S information
AREA from senses to
orientate body
ASSOCIATIVE
VISUAL Wernicke’s area is
CORTEX involved in language
comprehension

Occipital lobe
mainly devoted to
visual processing

Broca’s area is associated Premotor cortex THE CORTEX
with learning language plays a role in CONTAINS
(see pp.126–27) planning movement AROUND
28 BILLION
Brodmann areas NEURONS
This functional map of the brain is based on
research carried out by neuroanatomist Korbinian
Brodmann, who linked cells by similarities in their
size, shape, and connections. There are 52 regions
in total, and each one can be associated with one
or more approximate functions.

Cell structure Layer 1 receives Molecular
inputs from thalamus External granular
The cells of the human cortex are Layer 2 contains a mass
arranged in six layers, with a total External pyramidal
thickness of 0.09 in (2.5 mm). of cortical neurons
Each layer contains different Internal granular
types of cortical neurons that Layer 3 receives inputs
receive and send signals to other from other cortical areas Internal pyramidal
areas of the cortex and the rest
of the brain. The constant relaying Layer 4 is linked to To opposite Multiform
of data keeps all parts of the brain corpus callosum, brain hemisphere White matter
aware of what is going on
elsewhere. Some of the more stem, and thalamus To brain stem To thalamus
primitive parts of the human and spinal cord
brain, such as the hippocampal Layer 5 cells extend
fold, have only three layers. beneath cortex

Layer 6 sends signals
back to thalamus

CORTICAL LAYERS

Nuclei of Globus Subthalamic
the Brain pallidus nucleus

Caudate
nucleus

In brain anatomy, a nucleus is a cluster Central location Substantia
of nerve cells that have a discernible set Most of the basal ganglia are nigra
of functions and are connected to each positioned at the base of the forebrain
other by tracts of white matter. around the thalamus. The nuclei sit REAR SL
within a region filled with white-
matter tracts called the striatum.

FThe basal ganglia and other nuclei ICE WHITE
MATTER
An important group of nuclei collectively known PUTAMEN
as the basal ganglia sit within the forebrain and CAUDATE
have strong links with the thalamus and brain GLAOLLBIUDSUS NUCLEUS
stem. They are associated with learning,
motor control, and emotional responses. All GLOBUS
cranial nerves connect to the brain at a PALLIDUS
nucleus (often two: one for sensory inputs CAUDATE
and another for motor outputs). Other NUCLEUS TAIL
brain nuclei include the hypothalamus
(see p.34), hippocampus (see pp.38–39), AMYGDALA
pons, and medulla (see p.36).

RONT SLICE

WHITE
MATTER

SUBTHALAMIC CAUDATE Each nucleus Nuclei of amygdala
NUCLEUS NUCLEUS develops as a have been classified
mirrored pair, as part of basal
SUBSTANTIA P one in each ganglia by some
NIGRA THALAMUS hemisphere scientists

CAUDATE
NUCLEUS TAIL

Substantia nigra in Nuclei structure
midbrain linked with Nuclei are clusters of gray matter (nerve
cell bodies) situated within the brain’s white
fine motor control matter (nerve axons). Most nuclei do not
have a membrane so, to the naked eye,
seem to blend into the surrounding tissues.

THE PHYSICAL BRAIN 32 33
Nuclei of the Brain

WHAT NUCLEI ARE NUCLEUS REGIONS OF THE BASAL GANGLIA
LOCATED IN THE Caudate
BRAIN STEM? nucleus FUNCTION

The brain stem contains 10 of A motor processing center that involves procedural
the 12 pairs of cranial nuclei. learning of movement patterns and conscious
inhibition of reflex actions.
They provide motor and
sensory function to the Putamen A motor control center, associated with complex
Globus learned procedures such as driving, typing,
tongue, larynx, facial pallidus or playing a musical instrument.
muscles, and more.
A voluntary motor control center that manages
THE BRAIN HAS movements at a subconscious level. When damaged,
MORE THAN 30 SETS it can create involuntary tremors.
OF NUCLEI, MOSTLY
PAIRED LEFT AND RIGHT Subthalamic Although its precise function is not clear, this
nucleus structure is thought to be linked to selecting a specific
movement and inhibiting any competing options.

Substantia Plays a role in reward and movement. Symptoms
nigra of Parkinson’s disease (see p.201) are associated
with the death of dopamine neurons found here.

Amygdala May play a part in integrating activity between
basal ganglia and limbic system, thereby considered
by some to be part of the basal ganglia.

Action selection MOTOR LOOP PREFRONTAL LOOP LIMBIC LOOP INPUT
SOURCE
The basal ganglia have an Motor, premotor, Dorsolateral Amygdala,
important role in filtering out the somatosensory prefrontal cortex hippocampus, ENTRY
noise of competing commands temporal cortex POINT
coming from the cortex and cortex Anterior caudate
elsewhere in the forebrain. Ventral striatum EXIT
This process is called action Putamen Globus pallidus; POINT
selection, and it occurs entirely pars reticulata in Ventral pallidum
subconsciously through a series Lateral globus substantia nigra THALAMUS
of pathways through the basal pallidus, internal Mediodorsal REGION
ganglia. Generally, these pathways Mediodorsal and nucleus
block or inhibit a specific action segment ventral anterior
by having the thalamus loop the
signal back to the start point. Ventral lateral and nuclei
However, when the pathway ventral anterior
is silent, the action goes ahead. nuclei

Basal ganglia loops
The route of the pathway depends on the
source of the inputs from the cortex or
elsewhere in the forebrain. There are three
main pathways, and each one is able to
inhibit or select an action. The motor loop
connects to the main movement control
center, the prefrontal loop carries input from
executive regions of the brain, while the
limbic loop is governed by emotional stimuli.

Hypothalamus, Thalamus,
and Pituitary Gland

The thalamus and the structures around it sit at the center
of the brain. They act as relay stations between the forebrain
and the brain stem, also forming a link to the rest of the body.

The hypothalamus WHAT GLANDS KEY
DOES THE PITUITARY Thalamus
This small region under the forward Hypothalamus
region of the thalamus is the main GLAND CONTROL?
interface between the brain and the Pituitary gland
hormone, or endocrine, system. The pituitary gland is a master
It does this by releasing hormones gland that controls the thyroid
directly into the bloodstream, or by gland, adrenal gland, ovaries,
sending commands to the pituitary and testes. However, it receives
gland to release them. The
hypothalamus has a role in growth, its instructions from the
homeostasis (maintaining optimal hypothalamus.
body conditions), and significant
behaviors such as eating and sex.
This makes it responsive to many
different stimuli.

THE EPITHALAMUS STIMULUS RESPONSES OF THE HYPOTHALAMUS
Day length
This small region covers the top Water RESPONSE
of the thalamus. It contains various Eating
nerve tracts that form a connection Lack of food Helps maintain body rhythms after receiving signals about day
between the forebrain and Infection length from the optical system.
midbrain. It is also the location Stress
of the pineal gland—the source of Body activity When the blood’s water levels drop, releases vasopressin, also called
melatonin, a hormone central to the Sexual activity antidiuretic hormone, which reduces the volume of urine.
sleep–wake cycle and body clock.
When the stomach is full, releases leptin to reduce feelings of hunger.

When the stomach is empty, releases ghrelin to boost feelings
of hunger.

Increases body temperature to help the immune system work faster
to fight off pathogens.

Increases the production of cortisol, a hormone associated with
preparing the body for a period of physical activity.

Stimulates the production of thyroid hormones to boost the
metabolism, and somatostatin to reduce it.

Organizes the release of oxytocin, which helps the formation of
interpersonal bonds. The same hormone is released during childbirth.

Lateral nuclei Incoming signals for MELDOIBAEL Nuclei separated by 34 35
(pulvinar) medial dorsal nuclei sheets of white matter
are from prefrontal The thalamus
send signals cortex ALNOTBE
to visual The word “thalamus” is derived from
cortex PITUITARY GLANDthe Greek word for “inner chamber,”
RIOER and this thumb-sized mass of gray
LATERAL LO BE matter sits in the middle of the brain,
HYPOTHALAMUS between the cerebral cortex and
Signals sent from midbrain. It is formed from several
premotor cortex bundles, or tracts, of nerves, which
received in lateral send and receive signals in both
anterior nucleus directions between the upper and
lower regions of the brain, often in
Sense data from mouth feedback loops (see p.91). It is
transmitted to medial associated with the control of sleep,
ventral posterior nucleus alertness, and consciousness. Signals
from every sensory system, except
Thalamic nuclei smell, are directed through the
The thalamus is divided into three main lobes: thalamus to the cortex for processing.
the medial, lateral, and anterior. They are each
further organized into zones, or nuclei,
associated with particular sets of functions.

WEIGHING JUST 0.1 OZ (4 G), Secretory cells in
THE HYPOTHALAMUS IS hypothalamus
NOT MUCH LARGER THAN
THE END SEGMENT release hormone
OF A LITTLE FINGER
1 Stimulation
The pituitary gland The hormones produced
by the hypothalamus travel along
Weighing about 0.01 oz (0.5 g), the axons to the pituitary gland.
tiny pituitary gland produces many
of the body’s most significant Artery Posterior
hormones under the direction of the pituitary
hypothalamus. The hormones are 2 Production Network lobe
released into the blood supply via a The chemicals from of veins
network of tiny capillaries. Pituitary
hormones include those that control the hypothalamus stimulate
growth, urination, the menstrual the pituitary gland to
cycle, childbirth, and skin tanning. release hormones.
Despite having the volume of a pea,
the gland is divided into two main Anterior
lobes, the anterior and posterior, plus pituitary lobe
a small intermediate lobe. Each lobe
is devoted to the production of a 3 Release
particular set of hormones. The netlike
portal system collects
the hormones and releases
them into the bloodstream.

Hormones pass
into bloodstream

HOW BIG IS Thalamus links brain
THE CEREBELLUM? stem with forebrain,

Most of the brain’s cells are relaying and
located in the cerebellum, preprocessing sensory
although it makes up only and other information

around 10 percent of THALAMUS
the volume of the
whole brain. Midbrain is associated with
control of state of arousal
and body temperature

Connecting the brain MIDBRAIN
The stalklike brain stem forms a link between
the thalamus, the base of the forebrain, and Pons is a major
the spinal cord, which connects to the rest communication pathway
of the body. It is involved in many basic that carries cranial nerves
functions, including the sleep-wake cycle, used for breathing, hearing,
eating, and regulating heart rate.
and eye movements

The brain stem BRAIN STEM

The brain stem is made up of three components, all of which have PONS

an essential role in several of the human body’s most fundamental

functions. The midbrain is the start point of the reticular formation,

a series of brain nuclei (see pp.32–33) that run through the brain stem

and are linked to arousal and alertness and play a crucial role in

consciousness. The pons is another series of

nuclei that send and receive signals from

the cranial nerves associated with the 10 pairs of
cranial nerves
face, ears, and eyes. The medulla THALAMUS emerge from MEDULLA
descends and narrows to merge with
brain stem
the uppermost end of the spinal cord. BRAIN STEM CEREBELLUM
It handles many of the autonomous Cranial nerves
start and end
body functions, such as blood-pressure
at nuclei in
regulation, blushing, and vomiting. brain stem

The Brain Stem Medulla is involved in
and Cerebellum important reflexes

such as breathing rate
and swallowing

The lower regions of the brain are the brain stem, Spinal cord consists SPINAL CORD
which connects directly to the spinal cord, and of a bundle of nerve
the cerebellum, located directly behind it. axons that connect

to peripheral
nervous system

THE PHYSICAL BRAIN 36 37
The Brain Stem and Cerebellum

The little brain Vermis controls EW OF CEREBELLUMREAR VI
The cerebellum, a term that most basic motor
means “little brain,” is a highly patterns, such as SPINOCEREBELLUM ANTERIOR
folded region of the hindbrain VERMIS LOBE
that sits behind the brain stem. eye and limb
Like the cerebrum above it, the movements
cerebellum is divided into two
Anterior lobe of lobes. These are divided laterally Outer layer
cerebellum receives into functional zones. composed of
information about body
posture from spinal cord gray matter

ANTERIOR Body movements Located on both LATERAL
LOBE are coordinated sides of cerebellum, ZONE
in posterior lobe
these zones are
involved in planning

sequences of
movements

Spinocerebellum compares
information about actual body
position to intended position of

planned movements and
modifies sequence as needed

VESTIBULOCEREBELLUM POSTERIOR The cerebellum
LOBE
BELLUM Although the cerebellum appears to play a part in maintaining
CERE attention and processing language, it is most associated with its
role in the regulation of body movement. Specifically, its role is
to convert the broad executive motor requests into smooth and
coordinated muscle sequences, error-correcting all the while.
It routes its outputs through the thalamus. At the microscopic
level, the cerebellum’s cells are arranged in layers. The purpose
of these layers is to lay down fixed neural pathways for all kinds
of learned movement patterns, such as walking, talking, and
keeping balance. Damage to the cerebellum does not result
in paralysis, but slow jerky movements.

Vestibulocerebellum is involved in THE CEREBELLUM AND NEURAL NETWORKS
head control, eye movements, and
maintaining balance through Some artificial intelligences (AI)
information from the inner ear use a system inspired by the
anatomy of the cerebellum.
KNOWLEDGE OF THE AI programs itself by machine
CEREBELLUM WAS learning. It does this with a
ADVANCED BY processor called a neural
STUDYING BRAIN- network, where inputs find
INJURED SOLDIERS their way by trial and error
IN WWI through layers of connections,
a setup that mirrors the way
the cerebellum lays down
patterns for learned movements.

The Limbic THE S-SHAPED
System HIPPOCAMPUS IS NAMED
AFTER ITS RESEMBLANCE
Sitting below the cortex and above TO A SEAHORSE
the brain stem, the limbic system is a
collection of structures associated with Fornix is a bundle of nerve
emotion, memory, and basic instincts. tracts that connects

hippocampus to thalamus
and lower brain beneath

Location and function GYRUS
CINGULATE
The limbic system is a cluster of organs OF FORNIX FORNIX
situated in the center of the brain, COLUMN
occupying parts of the medial surfaces
of the cerebral cortex. Its major structures MAMILLARY MIDBRAIN
form a group of modules that pass signals BODIES
between the cortex and the bodies of the
lower brain. Nerve axons link all of its
parts and connect them to other brain
areas. The limbic system mediates
instinctive drives such as aggression,
fear, and appetite, with learning, memory,
and higher mental activities.

System parts HYPOTHALAMUS
The limbic system’s components
extend from the cerebrum inward AMY
and down to the brain stem. OLFACTORY BULB GDALA
It is usually understood to
include the structures PARAHIPPOCAMPAL
shown here. GYRUS

ELL ORIES IONING ON
SENSE OF SM
NEW MEM
FEAR CONDIT
RECOGNITI

Smell, which is The small mamillary bodies The amygdala is most Involved in forming
processed in the act as relay stations for new associated with fear and retrieving memories
olfactory bulbs, is the memories formed in the conditioning, where associated with fresh
only sense handled by hypothalamus. Damage we learn to be afraid data from the senses,
the limbic system and leads to an inability to of something. It is also the parahippocampal
not sent through the sense direction, particularly involved in memory and gyrus helps us recognize
thalamus. with regards to location. emotional responses. and recall things.

THE PHYSICAL BRAIN 38 39
The Limbic System

WHAT DOES Reward and punishment
LIMBIC MEAN?
The limbic system is closely linked to feelings of rage and
The word “limbic” is derived contentment. Both are due to the stimulation of reward or
from the Latin limbus, punishment centers within the limbic system, particularly
in the hypothalamus. Reward and punishment are crucial
meaning “border,” referring aspects of learning, in that they create a basic response to
to the system’s role as a kind experiences. Without this rating system, the brain would
of transition zone between simply ignore old sensory stimuli that it had already
experienced and pay attention only to new stimuli.
the cortex and lower
brain.

Pleasure Disgust Fear
Associated with the release This emotion is linked to the Fear is linked to specific
of dopamine, the brain sense of smell. Its primordial stimuli by the amygdala.
seeks to repeat behaviors role is to protect us from This can lead to a controlled
that create this feeling. infection. rage or fight response.

POCAMPUS Cingulate gyrus helps

form memories Klüver-Bucy syndrome SYMPTOM DESCRIPTION
associated with Amnesia
HIP strong emotion This condition is caused by damage Damage to the
to the limbic system and results in hippocampus leads to
a spectrum of symptoms associated the inability to form
with the loss of fear and impulse long-term memories.

control. First described in humans Docility With little sensation
in 1975, this neural disorder is of reward for actions,
named after the 1930s investigators sufferers lack
Heinrich Klüver and Paul Bucy, who motivation.

EPISODIC performed experiments that Hyperorality An urge to examine
involved removing various brain objects by putting
regions in live monkeys and noting them in the mouth.

MEMORIES the effects. Pica Eating compulsively,
In humans, the syndrome may including inedible
substances like earth.
be caused by Alzheimer’s disease,

complications from herpes, or brain

The hippocampus receives damage. It was first documented in Hypersexuality A high sex drive often
and processes inputs from the people who had undergone surgical Agnosia associated with
cerebrum. It is involved in removal of parts of the brain’s fetishes or atypical
creating episodic memories, temporal lobe. The condition can attractions.
or memories about what you be treated with medication and
have done, and creating assistance with daily tasks. Losing the ability to
spatial awareness. recognize familiar
objects or people.

Imaging the Brain Layer of thermal
insulation keeps
Modern medicine and neuroscience can see through Liquid helium cools liquid helium cold
the skull to observe structures within the living brain. electromagnet to
However, imaging this soft and intricate organ has
required the invention of advanced technology. about –453°F (–270°C)

MRI scanners Superconducting LIQUID HELIUM
electromagnet generates
A magnetic resonance imaging (MRI) extremely strong magnetic field
machine gives the best general view
of the brain’s nervous tissue and is Gradient magnets focus
most often deployed to search for magnetic field around area
tumors. MRI does not expose the
brain to high-energy radiation, unlike to be scanned
other scanning systems, which makes
it safe to use for long periods and Radiofrequency coil
multiples times. Two refinements emits and detects
of MRI, called fMRI and DTI, are radio waves
also useful for monitoring brain
activity (see p.43). Although ideal as Patient lies inside body of
a tool for research and diagnosis, MRI scanner during scanning
is expensive. With its liquid-helium
coolant system and superconducting Motorized MOTORIZED TABLE
electromagnets, one machine also table moves
uses the power of six family homes. patient into

How MRI works scanner
MRI makes use of the way that protons in
hydrogen atoms align to magnetic fields. Protons Additional
Hydrogen is found in water and fats, aligned
which are both common in the brain. randomly south-facing
A scan takes about an hour, then the data
is processed to create detailed images. INACTIVE ACTIVE proton
ELECTROMAGNET
ELECTROMAGNET

Proton faces
south

Magnetic
field line

THE ELECTROMAGNET INACTIVE ACTIVE Proton faces
IN AN MRI SCANNER ELECTROMAGNET ELECTROMAGNET north
CAN GENERATE A
MAGNETIC FIELD 1 Protons unaligned 2 Protons align to magnetic field
40,000 TIMES AS Before the MRI machine is Activating the machine’s powerful magnetic
STRONG AS EARTH’S activated, the protons in the brain’s field forces all the protons to align with each other.
molecules are unaligned—the axes Approximately half face the field’s north pole, and
around which the particles are spinning half face south. However, one pole will always have
point in random directions. slightly more protons facing it than the other.

THE PHYSICAL BRAIN 40 41
Imaging the Brain

MRI SCANNER CT scans Person being X-ray
scanned detectors
RAGDREIAOLEDFCRIETENRQOTUMMENAACGGYNNCEEOTTIL Computer tomography (CT), or
computerized axial tomography X-ray source
Radio-wave pulse (CAT), takes a series of X-ray images Rotating X-ray
RADIOFREQUENCY COIL through the brain from different The X-ray source shines through the
angles. A computer then compares brain, arcing around the patient to vary
the images to create a single cross the angle of each image.
section of the brain. CT scans are
quicker than MRI and are best for
detecting strokes, skull fractures,
and brain hemorrhages.

OTHER TYPES OF SCANNING TECHNOLOGY
Imaging certain brain features require particular scanning techniques,
which may also be used if MRI or CT are dangerous or unsuitable.

TYPE OF SCAN TECHNOLOGY AND USES

PET (positron Used in order to image the blood flow through the brain and highlight
emission active regions. PET scans track the location of radioactive tracers
tomography) injected into the blood.

DOI (diffuse An array of newer techniques that works by detecting how bright
optical imaging) light or infrared rays penetrate into the brain. DOI provides a way
of monitoring blood flow and brain activity.

Cranial A safe imaging technique that is based on the way ultrasonic waves
ultrasound bounce off structures in the brain. Cranial ultrasound is mostly used
on infants. It is used less often in adults because the images lack detail.

Flipped proton Computer Image shows
realigns processes tissues in
signal data cross section
RADIOFREQUENCY COIL

COMPUTER
MONITOR
RADIOFREQUENCY COIL RADIOFREQUENCY COIL
Radiofrequency coil
Additional proton flips into Radio signal detects signal and passes
different orientation emitted it to computer

3 A pulse of radio waves 4 Radio signal emitted 5 Receiver creates image
With the magnetic field on, the MRI Once the pulse is switched off, the All the signal data is then processed by
machine’s radiofrequency coil sends a pulse unaligned protons flip back into alignment with computer to create two-dimensional “slices”
of radio waves through the brain. This input the magnetic field. This causes them to release of the brain. Protons in different body tissues
of extra energy makes the spare protons energy as a radio signal, which is detected by produce different signals, so scans can show
flip out of alignment. the machine. the tissues distinctly and in great detail.

Monitoring the Brain

Being able to collect information from a living brain WHY DOES
at work has revolutionized both our understanding THE BRAIN PRODUCE
of how the brain functions and brain medicine. ELECTROMAGNETIC FIELDS?

EEG Neurons use pulses of electric
charge to transmit messages.
The simplest brain monitor is the electroencephalograph (EEG). It uses
electrodes positioned all over the cranium to pick up an electrical field The activity of billions of
created by the activity of neurons in the cerebral cortex. The varying cells accumulates into
levels may be displayed as waves (“ordinary EEG”) or colored areas a constant field.
(quantitative EEG, or QEEG). EEG can reveal evidence of seizure disorders,
such as epilepsy, and signs of injury, inflammation, and tumors. The
painless procedure is also used to assess brain activity in coma patients.

Types of EEG wave GAMMA WAVES High-frequency waves are
Neighboring cells in the cortex fire in packed tightly together
synchrony, creating wavelike changes in the MORE THAN
intensity of the electrical field. Characteristic Amplitude 32 HZ Low-frequency waves
wave patterns (named after letters of the are widely spaced
Greek alphabet) have been found to be
closely associated with certain brain states. Time DELT
These rhythms are associated with
BETA WAVES Amplitude 0.1–4 HZ A WAVES
learning and complex problem-
Amplitude 14–32 HZ solving tasks. They may originate Time
These waves are typically seen
Time from the binding together of during some stages of sleep but
Originating from both hemispheres groups of neurons into
networks. also when a person is
at the front of the brain, beta engaged in complex
waves are associated with
physical activity and with problem-solving
states of concentration tasks.
and anxiety.

A WAVES THET

8–14 HZ A WAVES
ALPH Amplitude 4–8 HZ

Amplitude

Time Electrodes held close Time
These typically originate from the to skull by cap Usually seen in young children,
back of the brain and are usually
Wire carries signal theta waves are also evident
stronger in the dominant to an amplifier during states of relaxation,
hemisphere. They are seen
creativity, and
during both relaxed meditation.
and alert states.

THE PHYSICAL BRAIN 42 43
Monitoring the Brain

MEG Functional MRI and
diffusion tensor imaging
In addition to making electrical activity, the brain produces a faint
magnetic field. This is detected by a magnetoencephalography MRI (see pp.40–41) can be extended
(MEG) machine and can be used to create a real-time account to collect information about what
of activity in the cerebral cortex. MEG is limited by the weakness the brain is doing. Functional MRI
of the brain’s magnetism, but the technique can detect rapid (fMRI) scanning tracks the flow of
fluctuations in brain activity, which take place over a few blood through the brain, specifically
thousandths of a second, better than other monitoring systems. showing where it is giving oxygen
to neurons and thus indicating
SQUID array in Cerebral Direction of Magnetic field which regions are active in real
form of skull cap cortex nerve pulse around nerve pulse time. Subjects are asked to carry out
mental and physical tasks while
How MEG works monitored by fMRI to create a
MEG uses sensitive functional map of the brain and
detectors called spinal cord that combines anatomy
superconducting with activity levels. Diffuse tensor
quantum interference imaging (DTI) also uses MRI but
devices (SQUIDS) to tracks the natural movement of
pick up fleeting water through brain cells. It is used
magnetic fields made to build up a map of the white-
by the electrical matter connections within the brain.
pulses of neurons.
Area of increased
NEUROFEEDBACK activity

This form of cognitive therapy uses an EEG to create a feedback loop Area of reduced
between a person’s mental state and their brain activity. This makes activity
it easier for people to learn ways to control unwanted mental activity,
such as anxiety. Interpreting an fMRI image
An fMRI scan begins with establishing a
1 baseline of activity in the brain. The scan
then shows up regions that fluctuate from
EEG charts this baseline, allowing researchers to figure
electrical activity out which areas are excited or inhibited
during particular tasks.
in the brain.

4 2 Computer

With practice, turns neural
the brain acquires patterns into
the habit of being in a dynamic display,
the rewarded state.
such as an
interactive

game.

3 Game gives

reward when the
required brain state

is registered (for
example, low
anxiety).

Brain development Cerebrum
The first nerve cells are produced just days after
conception. These cells form into a plate and then
curl to become a liquid-filled structure, called the
neural tube, which develops into the brain and
spinal cord. One end becomes a bulge and then
splits into distinct areas.

KEY Hindbrain Cerebellum
Forebrain Spinal cord Brain stem
Midbrain

Eye bud

Ear bud

Cranial The cerebrum enlarges, and the eyes
nerves and ears mature, moving into
position. Some parts of the
Ear bud fetus’s body may respond
to touch.
Neural Eye At week seven, the forebrain, 11 WEEKS
tube bud midbrain, and hindbrain divide
Neural tube forms
into bulges that will become
Forebrain the cerebrum, brain stem,
prominence and cerebellum.
Nerve cells develop, Around week five, the neural KS 7 WEE
migrating around the embryo tube begins to form into KS
to form the start of the something recognizable as
brain, spinal cord, and a brain. The eye starts
nerve network. to develop.
5 WEE
3 WEEKS

Babies and RECOGNIZING FACES
Young Children
Babies prefer looking at face-like
The human brain begins to develop after conception images and learn about faces
and changes rapidly for the first few years of life, but rapidly. An area of the cortex called
it takes more than 20 years for a brain to fully mature. the face recognition area (see p.68)
becomes specialized in identifying
Before birth faces. Chess champions also use this
area to recognize board layouts,
An embryo’s brain has a lot of development to do, going from just a few suggesting that the most important
nerve cells three weeks after conception to an organ with specialized patterns in a person’s life are
areas that is ready to start learning from birth. Genes control this process, decoded there.
but the environment can affect it as well. Insufficient nutrition can change
brain development, and extreme stress on the mother during pregnancy FACELIKE NOT FACELIKE
can have an impact, too.

THE PHYSICAL BRAIN 44 45
Babies and Young Children

Insula is found Contours Cerebrum
deep inside of cortex
lateral sulcus Prefrontal
cortex
Frontal lobe
Gyri Prefrontal
form cortex

Parietal Amygdala
cortex
Reticular
Hippocampus formation

Sulci Cerebellum Sensory and motor areas of the brain are well
form connected and developed, but large areas,

Brain stem such as the prefrontal cortex, are still
immature. Changes in the hippocampus
In the last couple of months of gestation, the and amygdala allow long-term
cerebral cortex grows and develops rapidly, memories to be retained. 3 YEAR

and characteristic grooves appear.
Babies are born with as many
neurons as adults, but most
are not yet mature. 9 MONTHS /
CerebellumS
BIRTH
The brain stem is mostly S
mature and controls reflexes AT THE PEAK OF BRAIN
such as blinking. Sleep and DEVELOPMENT, ABOUT
wake cycles begin, and 250,000 NEURONS
the fetus responds to FORM EVERY MINUTE
loud noises.
5 MONTH

Children’s brains WHY IS OUR
BRAIN WRINKLY?
After birth, babies’ brains are like sponges; they are incredible
at taking in information from the world around them and trying As human intelligence
to make sense of it. During the first few years, the brain grows evolved, our cortex expanded.
and develops rapidly, with brain volume doubling in the first But bigger heads would mean
year of life. Synapses grow and form new connections quickly
and easily, a process called neuroplasticity. that babies could not fit
through the birth canal.
Building A folded cortex packs
connections
Peak plasticity for more tissue into a
each region of the smaller volume.
brain is different.
Sensory areas build NEWBORN 9 MONTHS 2 YEARS
synapses rapidly four
to eight months after
birth, but prefrontal
areas do not reach
peak plasticity until
an infant is around
15 months old.

Older Children
and Teenagers

Teenage brains undergo dramatic restructuring.
Unused connections are pruned, and insulating
myelin coats the most important connections,
making them more efficient.

Teenage behavior Risk-taking
Pleasure-seeking parts of
Teenagers have a reputation for being teenagers’ brains are well
impulsive, rebellious, self-centered, and connected, but impulse-control
emotional. A lot of this is due to the changes mechanisms are underdeveloped,
happening in adolescent brains. Human brains which can lead to risk-taking.
change and develop in set patterns, leaving
teenagers with a mix of mature and immature Frontal
brain regions as they grow. The last area cortex
to fully develop is the frontal cortex, which
regulates the brain and controls impulses.
This area allows adults to exert self-control
over their emotions and desires, which is
something adolescents can struggle with.

Sleep cycles SYNAPTIC PRUNING

During our teenage years, we need plenty of sleep as our brain Synaptic pruning, which is when
continues to develop. But at this time, our circadian rhythms shift unused neural connections die off,
as melatonin, the hormone that is released in the evening and starts during childhood and
makes us feel sleepy, begins to be released later than usual. This continues through our teen years.
is why teenagers often want to go to bed later than children and Cortical areas are pruned from the
adults and may struggle to get up for school in the morning. back to the front. Pruning makes
each area more efficient, so until it
Onset of KEY is finished, that region cannot be
sleep in Adult sleep time considered fully mature.
Adolescent
adults sleep time

Onset of Teenagers Out of sync IMMATURE MATURE
sleep occurs wake later Waking teenagers early for
Midnight in morning school is like giving them
later in than adults constant jet lag. Studies
teenagers have shown that starting
than in adults Noon school an hour later
improved attendance and
Noon grades. Fights and even car
accidents also decreased.

THE PHYSICAL BRAIN 46 47
Older Children and Teenagers

Clumsiness
During rapid growth
spurts, the brain’s body
maps can’t keep up. Brain and
body get out of sync, causing

clumsiness.

Motor
cortex

Extreme
emotions
The limbic system is
highly reactive in teenagers,
meaning they experience
heightened emotional responses,
feeling things more deeply.

Limbic
system

Peer pressure
Teenagers care deeply about
how their friends see them.
They take more risks with
peers, and being left out can
feel excruciating. Peer pressure
can be a strong influence on
them—for good or bad.

Mental health risks THE BRAIN REACHES ITS
LARGEST PHYSICAL
Some of the brain areas that undergo the most dramatic SIZE BETWEEN
changes during adolescence have been linked with mental AGES 11 AND 14
ill-health. These changes can leave the brain vulnerable to
small issues becoming dysfunctions. This may explain why
so many mental health problems, from schizophrenia to
anxiety disorders, commonly appear during adolescence.

ADHD, conduct disorder Not all mental WHY ARE TEENS
Anxiety disorders illnesses will persist SELF-CONSCIOUS?
into adulthood
When we think about being
Mood disorders embarrassed, a region of our

Disorders in adolescence Schizophrenia prefrontal cortex linked to
Some disorders from early Substance abuse understanding mental states
childhood may disappear during is more active in teenagers
adolescence, while others can
emerge and persist into later life. than adults.

0 5 10 15 20 25
Age (years)

The Adult PARENTHOOD
Brain
A new mother’s brain and body are awash with
Human brains continue to change and hormones such as oxytocin, driving her to care for her
mature throughout early adulthood, as baby. Looking at her infant triggers the brain’s reward
unused connections are pruned away. pathways, and her amygdala becomes more active,
This makes the brain more efficient scanning for danger. Men’s brains are affected
but also less flexible. by parenthood, too, but only if they spend a
lot of time with their baby. The brains of
men who are primary caregivers of an
infant go through similar changes to
women’s, and these changes appear
to be very similar to falling in love.

HEALTH

Last region to Corpus callosum is fully
fully mature is developed to allow
information flow
frontal lobe between hemispheres

Adult life Amygdala is less
A fully developed, mature emotionally reactive
brain is equipped to handle
all the competing demands
and pressures of adult life,
from work and finances
to relationships and health.

FAMILY Hippocampus
continues to produce

new brain cells

FINANC Mature brains

ES Full myelination (the sheathing of axons
in myelin) allows information to flow

freely, but the process is completed only
in a person’s late 20s. The last brain

region to finish maturing is the frontal
lobe, which is responsible for judgment

and inhibition. Compared to children
and teenagers, adults are better able
to regulate their emotions and control

their impulses. They can use their
experiences to better predict the
outcomes of their actions and how
they may make other people feel.

THE VOLUME OF THE PHYSICAL BRAIN 48 49
WHITE MATTER The Adult Brain
IN A PERSON’S
BRAIN PEAKS Neurogenesis
AROUND
AGE 40 Neurogenesis is the development of new neurons by neural stem cells
(cells that can become other cells). In a range of mammals, neurogenesis
ORALITY happens in the hippocampus and olfactory areas and continues
throughout life, with new neurons being produced regularly.
TURE The same is thought to be true in humans, although the evidence is
ORK mixed. Neurogenesis may also play a role in learning and memory.

New neurons Axons and Mature axons
Neurons grow from stem dendrites and dendrites
cells, dividing, specializing, allow neuron
and maturing into develop
functional brain cells. to integrate
with cell
M network
W FU
EARLY LATE MATURE
TYPE 1 TYPE 2A TYPE 2B TYPE 3 IMMATURE IMMATURE NEURON
STEM CELL STEM CELL STEM CELL STEM CELL NEURON NEURON

Disrupting memories Memory storage
Due to the creation
New brain cells help store information, so boosting of new brain cells,
neurogenesis in the brain can improve learning into hippocampal memories
adulthood. However, it also has a role to play in may degrade before
forgetting. Adding in new brain cells with new they can be stored in
connections disrupts existing memory circuits the cortex. This might
by competing with them. This means there is explain why we are
an optimal level of neurogenesis, which balances unable to remember
learning ability with retaining older memories. our infancy.

Memory forms
in hippocampus
NORMAL
MEMORY PATH Memory Memory
consolidated and retained
HIPPOCAMPUS transferred to cortex CORTEX long-term

DISRUPTED Consolidation
MEMORY PATH disrupted by new

neurons

New neurons Memory
develop in poorly
hippocampus
retained

The Aging Normal size of Ventricles are
Brain subarachnoid regular-sized
hollow spaces
space

With age, some abilities decline as neurons
degenerate and the brain decreases in
volume. In those neurons that remain,
impulses may travel more slowly.

The shrinking brain White-matter Healthy basal
tracts are in ganglia free of
As we age, there is a natural reduction of neurons abnormalities
as they degenerate, and the brain as a whole shrinks good condition
5 to 10 percent in volume. This is partially due
to decreased blood flow to aging brains. The fatty Young brain
myelin that insulates the axons of neurons also decays Young brains look plump; the ridges
with age, leaving brain circuits less efficient at
transmitting information, which can lead to problems covering the surface of the cortex
with memory recall and maintaining balance. almost touch. The fluid-filled ventricles

KEY Basal in the center of the brain are small,
ganglia and the subarachnoid space, which
Gray surrounds and cushions the brain,
matter Ventricles
forms a thin layer.
White
matter

Aging and happiness ALZHEIMER’S DISEASE

Aging might seem like a bad thing, but studies have Alzheimer’s disease, the most common form of dementia
shown that as we get older, our feelings of happiness (see p.200), is linked to the buildup of proteins in the
and well-being increase, while levels of stress and brain, which clump into plaques and tangles. Eventually,
worry decrease. Older adults’ brains seem to be better affected brain cells die, causing memory loss and other
at focusing on the positive. They are more likely to symptoms. Scientists do not know yet whether the
remember happy than sad pictures and spend more proteins cause the disease or are a symptom of it, and
time looking at happy faces than angry or upset ones. drugs to break them down have not helped patients.

Enlarged ventricles

Ups and downs WELL-BEING Severe
A study found younger cortical
and older people shrinkage
reported higher levels
of well-being than AGE HEALTHY BRAIN ALZHEIMER’S BRAIN
those in middle age.
Happiness levels
rose steadily from
age 50 onward.