The nervous system is the body's complex
network of control and communications. Every time that you do anything
-- literally anything -- your nervous system is intimately involved.
The nervous system is essential to our senses, the control of our
movements (even automatic ones such as blinking or shivering),
unconscious body functions such as digestion and breathing and the
development of consciousness and thought.
The nervous system has two interconnected and interdependent parts.
The central nervous system (which is often abbreviated to CNS) is the
brain and spinal cord; the peripheral nervous system consists of all the
nerve tissue outside the central nervous system.
The peripheral nervous system is further split into the somatic
system and the autonomic system. The somatic system has two main
functions. It collects information from the sense organs -- eyes, nose,
ears, mouth and skin -- and coveys it to the CNS (the sensory system),
and also transmits all of the signals from the CNS to the muscles which
control movement (the motor system).
The purpose of the autonomic system is to regulate those internal
organs and glands, such as the heart, stomach, kidneys and pancreas,
that work without conscious effort.
The basic ingredient of the
nervous system is a specialized cell called a neurone or neuron, which
works in a similar way to wires in an electric circuit. It picks up a
signal in one part of the body and carries it to another, where it may
be passed on again or result in some action, such as the contraction of
There are billions of neurones in the nervous system, but they are
delicate cells, easily damaged or destroyed by injury, infection,
pressure, chemical disturbance or starvation of oxygen. Neurones do not
replace themselves, so such disorders tend to have serious consequences.
Fortunately, people start out with an excessive number of neurones, so
some wastage need have no effect.
There is another form of cell to be found in the nervous system.
Neuroglia ('nerve glue') cells exist in large numbers and work to bind,
protect, nourish and provide support for the neurones.
ANATOMY OF A NEURONE
Neurones come in various
shapes and sizes, but they all have the same basic structure. Like all
cells, they have a nucleus, contained in a roughly spherical part known
as the cell body. From this project a number of short, fine, root-like
fibres, called dendrites. The dendrites are the 'receivers' of the
Each neurone also has a single, long fibre, the axon, which act as a
'transmitter'. At the end furthest from the cell body, axons branch into
several tendrils, each ending in a number of knobs.
Each of these knobs lies close to, but does not actually touch, a
dendrite from another neurone. Messages are transmitted across these
gaps (which are known as synapses) by means of chemicals called
Axons can be anything from a fraction of a millimetre to a metre
long, and are also known as nerve fibres. A loose bundle of such fibres
which travel to a common location is called a nerve. Different nerves
are said to 'supply' a particular area or organ. A few nerves -- such as
the optic nerve to the eye -- are made up of sensory neurones alone, and
others only of motor neurones; the majority, though, have both types.
Altogether, 43 pairs of nerves emerge from the CNS. Twelve of them
connect directly to the brain, and are known as cranial nerves. The
other 31 pairs are attached to either side of the spinal cord.
The cranial nerves mainly
supply the sense organs and muscles in the head, although a very
important cranial nerve, the vagus, travels down the neck to supply the
digestive organs, the heart and air passages in the lungs.
The spinal nerves emerge from
the spinal cord at intervals, and supply all areas of the body below the
neck. Each of the spinal nerves is attached to the spinal cord by means
of two roots, one carrying motor fibres and the other sensory fibres.
Each spinal nerve splits into a number of branches a short distance from
the spinal cord.