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The largest organ in the nervous system is the

brain. It controls vital involuntary processes,

such as breathing, as well as our thoughts,

emotions, memories, and sensations.

Brain scans can monitor brain

activity, called. BRAINWAVES.

WHY IS THE HUMAN BRAIN SPECIAL?

Relative to body weight, the human brain is one of

the largest in the animal kingdom. Its surface, the

cerebral cortex, is also the wrinkliest. During human

evolution, the cerebral cortex grew, and so became

wrinklier to fit inside the skull. The large cortex, site

of unique abilities such as language, is probably

what makes humans unusually intelligent.

WHAT IS THE BRAIN MADE OF?

The brain contains more than 100 billion neurons,

and a multitude of helper cells, which nourish and

support the neurons. The cells are organized into

different areas, the largest of which is the cerebrum.

The whole brain is surrounded by protective

membranes and cushioned by fluid-filled hollows.

Doctors can monitor a person’s brain by looking at brainwaves,

the pattern of electrical activity produced by all the brain’s

neurons. There are three main types of wave: alpha, beta, and

delta. They are detected by an EEG (electroencephalograph).

IS THE BRAIN ACTIVE WHEN WE SLEEP?

The brain is always active, but the level of

activity varies. In deep sleep, when delta waves

occur, the brain is working but at its least

active. During dreams, the eyes dart about

and the brain produces alpha waves and is

just as active as during waking hours.

HOW DOES THE BRAIN WORK?

Scientists used to think that each part of the cerebral

cortex carried out a specific function. Recent research

has shown the brain to be more complicated. During

speech, for instance, large areas of the cerebral cortex

work together in an ever-changing pattern.

FIND OUT MORE. Nervous System 138 • Skeleton 130–131

4 BRAIN SCAN

This MRI scan

shows how the

brain fills the space

inside the skull. The

cerebrum takes up most

of the room, and accounts

for nearly 70 per cent of the

weight of the nervous system.

BRAINWAVES

WAVE PATTERNS 3

When a person is awake but

resting, the brain produces a

regular pattern of medium-length

waves – alpha waves. When alert

and concentrating, the brain

produces shorter, quicker waves –

beta waves. During deep sleep,

very long, slow delta waves occur.

Brain

4 PET SCANS OF THE BRAIN

Active parts of the cerebral

cortex show as flashes of

colour in a PET scan.

Scientists use these

scanners to carry out

research into how

the brain works.

1 NEURONS IN THE BRAIN

Brain cells send signals to each

other via spindly connections.

Each cell has an average of 10,000

connections, which change as the

brain learns and adapts.

The cerebellum

is involved in the

involuntary control

of balance and

posture, especially

during movement

The brain stem
is an extension
of the spinal cord
and controls vital
involuntary functions
such as breathing
and heart rate

BETA: ALERT AND CONCENTRATING

IN CONVERSATION

DELTA: SLEEPING DEEPLY

LISTENING

Spinal cord

ALPHA: AWAKE BUT RESTING

brain