The Nervous System

CNS

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!

PNS

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.

Glossary

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.

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. This is just part of a neurone.

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. These are parts of neurones.

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.

Grey Matter: this is the material in the brain and spinal cord which contains the cell bodies and dendrite of nerve cells. Grey matter is an example of a tissue. 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.

White Matter: this is the material in the brain and spinal cord which contains the axons and myelin sheathes of nerve cells. This is another tissue. It is mainly myelin which is a fat, so it appears white to the naked eye. I understand all this now, do you? ~ if you don’t, ask a question below.

7 Responses to The Nervous System

  1. Celma says:

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  2. Elisabeth says:

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  3. Carla Corazon says:

    I like this subject and it will help me a lot with my homework. Thanks for helping.

  4. Cleones Franca says:

    That was a lovely article. I liked reading it. Thanks for sharing.

  5. hussein says:

    nice stuff it realy updates

  6. truong says:

    So in my psych textbook, it stated that “the myelin sheath is insulating material that encases SOME axons” so my question is why don’t all axons have the myelin sheath?

    • The myelin sheath serves two main purposes. Firstly it makes the axon transmit very much faster; the nerve impulse literally jumps from node to node. Secondly it insulates the axon from other axons.

      If an axon is surrounded by other axons that do have myelin sheathes, it does not need its own insulation. If it does not have a sheath, it will conduct more slowly and this is needed so that the impulse arrives at the nerve ending at the right time: a tiny fration of a second later than it would with a sheath. The result is that two impulses starting at the same time in two axons with different lengths arrive at thir endings at the same time.

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