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Nervous system
 The
function of the nervous system in all animals is internal communication
and co-ordination. It co-ordinates transmission of messages to facilitate
rapid responses to changes both within the organism and in its surrounding
environment, thus greatly enhancing survival.
The human nervous system consists of two parts, the central nervous
system and the peripheral nervous system. Both are made entirely of
nerve cells.
Central nervous system (CNS)
The central nervous system consists of the brain
and the spinal cord.
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The brain
The brain is the control centre or "computer" of the
body, co-ordinating and directing all the other systems. It consists
of over ten billion nerve cells (neurones) in an interconnected
network. It receives and interprets messages from within the body
as well as outside it.
(see Human senses A:
Sight and sound and Human
senses B: Smell, taste and touch)
- The cerebrum is the largest part of the brain. It controls
conscious body movements and interprets information from the
sensory organs. The two interconnected halves of the cerebrum
are known as the left and right cerebral hemispheres. The left
side controls speech, logical thought and mathematical ability.
The right side is more involved with emotions and artistic and
musical ability.
- The cerebellum ("little brain") coordinates and
fine-tunes the movement of skeletal muscles and helps the body
to maintain balance.
- The medulla, or brain stem, controls many involuntary actions
such as breathing, heart rate and blood pressure.
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Spinal cord
The spinal cord, extending from the base of the brain, is protected
by the spinal vertebrae. The spinal cord facilitates transmission of
information to and from the rest of the body, like a super highway.
It connects with nerves all over the body through spinal nerves.
Peripheral nervous
system (PNS)
The peripheral nervous system is a network of nerves carrying messages
between the CNS and the rest of the body. It consists of cranial nerves
which are connected directly to the brain and are found in the face
area, and spinal nerves which connect the spinal cord to all other body
parts. For a finger or an arm to move, a message must originate in the
brain, travel down the spinal cord and through the spinal nerves to
the muscle which then responds by moving.
Neurons (nerve cells - also
called neurones)
Nerves are made of neurons, arranged like wires in a telephone
cable.
The neuron consists of a cell body with projections from
it called axons and dendrites. A neuron's structure reflects its function.
The numerous branched dendrites receive messages from adjacent neurons
or sensory organs. The long axon transmits an electrical impulse to
an effector (a muscle or gland) or to another neuron. The insulating
myelin sheath around the axon speeds up electrical transmission.
At the synapse (the fluid-filled gap between neurons),
the electrical impulse causes chemicals called neuro-transmitters to
be released into the gap. These generate another electrical impulse
in the next neuron, and so the message travels. Some psychological disorders
involve an imbalance in these chemicals, preventing normal message transmission.
Sensory neurones carry signals from sense receptors (eg.
heat receptors in our skin) to the CNS.
Motor neurones carry signals the other way from the
brain and spinal cord to effectors (muscles or glands) to cause a response
(the muscle contracts or gland releases secretion).
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