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138 Human Body

The brain, nerves, and spinal cord make up the

nervous system. It processes information from

the sense organs and controls conscious

“voluntary” actions, such as walking, and

“involuntary” processes we are not aware

of, like reflexes and breathing.

WHAT IS A REFLEX ACTION?

Reflex actions are involuntary – they happen before

you have time to think. When they happen, a nerve

signal takes a shortcut through the spinal cord

rather than involving the brain. If we touch a very

hot object, a sensory neuron sends a signal

shooting from the hand to the spinal cord.

There an association neuron transmits the

signal to a motor neuron, which tells a

muscle in the arm to move the hand.

HOW DO NEURONS WORK?

Neurons work like wires, transmitting

electric signals at speed. When the neuron

is resting, there is very little electrical

activity. If triggered, a nerve impulse

rushes to the end of the cell where a

chemical called a neurotransmitter

passes the signal to the next neuron

across a gap or synapse.

ARE ALL NEURONS THE SAME?

There are three main types of neuron. Sensory

neurons carry incoming signals from sense organs to

the central nervous system. Motor neurons carry

outgoing signals from the brain to the body, usually

to muscle cells. Association neurons form a complex

maze of connections in the brain and spinal cord,

linking sensory neurons to motor neurons.

WHAT IS A NERVE?

Nerves are the cables of the nervous system.

Each nerve is packed with hundreds of wire-like

cells called neurons, which carry electric signals

to and from the brain. The brain and spinal cord

form the central nervous system, and the nerves

outside them are the peripheral nervous system.

4 NERVE JUNCTION

This highly magnified image shows the

junction, or synapse, between two nerve

cells. In the blue cell, a nerve signal has

stimulated the release of chemicals called

neurotransmitters (pink circles). The

neurotransmitters cross the gap and bind on to

the pink receptor nerve cell to pass on the signal.

4 MOTOR NEURON

All neurons are similar in

structure to this motor

neuron. The cell body

contains the nucleus

that controls the cell.

Filaments called dendrites

branch out from the cell

body and pick up signals

from other neurons. The

nerve fibre, or axon, takes

the signal and passes it on

to other neurons, muscles,

or glands.

FIND OUT MORE. Muscle 132 • Respiration 136

Nervous System

The cerebrum

is the largest part

of the human brain,

and is responsible

for voluntary actions,

thought, language,

and consciousness

The hypothalamus

is a tiny region in the centre of the

brain that controls hormone levels

and regulates sleep, temperature, and

body water levels

4 A NETWORK OF NERVES

Nerves branch from the brain and spinal cord, forming

a tree-like network that runs throughout the body.

Spinal cord

is the nervous

system’s main

highway and

runs through

a tunnel in

the vertebrae

The cerebral cortex

is the folded surface

of the cerebrum where

information is processed

Brain

Dendrite

Axon

(nerve fibre)

Cell body

Nucleus

nervous

system