The Open Door Web Site
Part XX: How Organisms Communicate
Homepage: The Effect of Stimuli on the Behaviour of an Organism
THE VERTEBRATE CENTRAL NERVOUS SYSTEM
The human central nervous system is made up of the brain and the spinal cord.
The brain is protected by the skull or cranium. The spinal cord is protected by the backbone or vertebrae. The spinal cord connects with the back of the brain.
The spinal cord is like a complicated motorway. Nerve cells are grouped together into nerves. Some of these nerves take information to the brain from the sensory cells of the body.
Other nerve cells are bringing information from the brain to the muscles and to other parts of the body. Every now and then there is a junction where side roads branch off from the motorway. This happens at the arms and the legs, for example. These branches also contain nerves made up of nerve cells, some are taking information to the brain while others are bringing information from the brain to the limbs.
Imagine you are walking with bare feet along a beach. Suddenly you step on a sharp stone. Immediately you pull your foot away. Then you take hold of your foot and rub it. You look down to find out what caused the pain.
When you step on the sharp stone you immediately lift up your foot. Your brain registers the pain at the same time. Once your foot has been lifted, your brain sends two messages; one to the muscles in your arm so that your hand can rub your foot. The other message goes to the muscles in your neck and eyes so that you can bend your head to look for the cause of the pain.
You will notice that the reaction to stepping on the sharp stone is immediate. The reaction was not in response to a decision made by the brain. Your brain registered the pain at the same time as your foot was lifted from the sharp stone. This is called a reflex action. Immediately after the reflex action you understand what has happened but the brain did not co-ordinate the reflex action itself. The sensory cells in the skin relayed the information to the spinal cord and the information to lift the foot went out immediately to the leg via a connecting nerve cell.
Other reflex actions in humans include the movement of the lower leg when the knee is tapped (the knee jerk) and the contraction of the iris muscles in the eye in response to light.
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