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The Central Nervous
System consists of the Brain and Spinal Cord. It contains millions of neurones (nerve cells). If you slice through some
fresh brain or spinal cord you will find some areas appear grey whilst other ares appear
rather white. The white matter consists of axons, it appears white because it contains a lot of
fatty material called myelin. The myelin sheath insulates an axon from its neighbours.
This means that nerve cells can conduct electrical messages without interfering with one
another. The grey matter consists of cell bodies and
the branched dendrites which effectively connect
them together. So this area is mainly cytoplasm of nerve cells which is why it appears
white.
Different areas of the brain are concerned with
different functions. If I drilled a hole in your head with my Black & Decker, and then
put a piece of copper wire in and wiggled it about, I could give your brain a little
electric shock; not enough to kill you of course, but enough to make something happen. So
if the electrode was put into your taste centre you might taste something even though
there was nothing in your mouth. We know exactly where to put the wires to make different
things happen. So an electric shock in another area might make you wiggle your toes. That
explains why you "see stars" when you bang your head and stir up the visual
centre which is at the back of your brain. There are areas of the brain which deal with
speech, hearing, smell, sight, movements, salivating, and so on. Some of these centres are
concerned with the information coming into the brain (sensory areas) and others are
concerned with making something happen (motor centres).
If your brain is anything like mine, the sensory
areas and motor areas are connected up so that when you are stimulated, you do something
sensible. What do you do when you bite into a ripe apple? Do you wiggle your toes or
salivate? Some responses are very simple and everyone makes the same response: e.g. we all
sneeze when our noses are tickled. Other stumuli are much more complicated and we do not
all react or respond in the same way. Do you run away or go and stroke a lion when you see
one in the playground? Well it all depends on whether you know the lion and if you thought
he was hungry. Some people make a big big fuss when they see a fly because they think that
it is a wasp and it will sting them to death. Other people have learnt the difference
between a wasp and a fly!
The Peripheral Nervous
System consists of all the sensory nerves (these feed information into
the spinal cord and brain) and the motor nerves (these carry messages to other parts of
the body from the brain and spinal cord). Sensory nerves contain sensory neurones. Motor
nerves contain motor neurones. Mixed nerves contain both sensory and motor neurones.
Sensory neurones are usually connected to motor neurones by intermediate neurones
(sometimes called inter neurones). Sensory, intermediate and motor nerves have gaps
between them called synapses.
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Neurones:
a neurone is a nerve cell; it has a cell body, a very long axon sheathed
in myelin, and many tiny branches called dendrites. There are three kinds of neurones: sensory,
intermediate and motor neurones.
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Axons: these
are long cytoplasmic tubes, they carry electric impulses from one part of
the body to another. They are insulated from each other by their myelin sheathes.
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Dendrites:
these are tiny branches on the cell body and at the ends of all neurones. The
dendrites of
one cell do not actually touch the dendrites of any other cell. There are very tiny gaps
between them called synapses.
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Synapses: these
are the gaps between the dendrites of one neurone and the cell body of another one. There
is no electrical connection between nerve cells. when one neurone stimulates another it
does so by secreting a chemical into the synapse. Many drugs work by interfering
with these chemical transmitters.
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Grey Matter: this
is the material in the brain and spinal cord which contains the cell bodies and dendrite
of nerve cells. It is mainly cytoplasm. It appears grey to the naked eye. So if
someone says you have not got much grey matter, they are being very rude.
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White Matter:
this is the material in the brain and spinal cord which contains the axons and
myelin sheathes of nerve cells. It is mainly myelin which is a fat, so it appears
white to the naked eye.
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