Just as we have blood vessels that go to every part of the body, we also have nerves that supply every bit of our skin and all the organs beneath the skin. Some nerves are called sensory nerves. They allow us to feel sensations, such as heat and cold or pressure of pain. Without these sensory nerves, we wouldn’t be able to feel anything at all. Other nerves are motor nerves. They go to the muscles and give them the signals to contract or relax. These motor nerves are responsible for our movements and motions. Without them, we would not be able to move at all.
These two sets of nerves work together like a well-trained team. If you should accidentally stick your finger with a sharp pin, the sensation of pain would travel-like lightning-from the sensory nerves in your fingertip up your arm to the nerves of the spinal cord inside your spine. In a flash, the sensation of pain would go from the sensory nerves in your spinal cord to the motor nerves in your spinal cord. Immediately, instantly, in no time at all, the motor nerves send the signal to the muscles of your arm and hand and you would immediately pull your hand away from the pin.
These nerves act in the space of time it takes an electric bulb to turn on when you flick the light switch. Naturally, we have to be made that way. Imagine what it would be like if you had a sharp pin sticking into you and didn’t realize it for a few minutes, or if your nerves were so slow that it took you a long time to pull your finger away? It could be serious, too, if you accidentally leaned against a hot radiator or stove and didn’t know it for a while. You could get pretty badly hurt if your nerves didn’t act quickly.
Although the sensory nerves tell us about pain and heat and cold, not all of them act in exactly the same way. For example, if you were to feel pain in the skin of your stomach or thigh, it wouldn’t hurt as much as if you were feeling it in the tip of one of your fingers. That is because the fingers have special nerve endings that are much more sensitive than the ends of the nerves in the stomach or thighs. These nerve endings in the fingertips allow us to feel things that practically no other nerves can feel.
You can see for yourself just how sensitive the nerve endings in your fingers are. Have someone blindfold you and ask him to take something familiar-a nickel or quarter, a pencil or pen, a spoon or fork, or anything else you are used to seeing and feeling -and put it between your bare knees. Now, rub your knees together and see if you can recognize what he has placed there. You probably won’t be able to tell, or if you can, it won’t be easy. Now, have him take the same thing and, while you are still blindfolded, put it in your hand. In a second, you will probably recognize what it is. That’s because the nerves in your fingertips are so highly developed with their special nerve endings that they can tell one thing from another much better than nerves anywhere else in your body.
When we automatically pull our hand away the second we feel something painful, we call this action a reflex. A reflex is something we do without thinking. But our nerves don’t always act by reflex. For example, if we feel something unfamiliar, our nerves may study what it feels like and send a signal to the brain. We don’t jump away from something that feels smooth or round or comfortably warm or cool. Instead we feel it more thoroughly in order to decide what it is. When we do this, the signal from the nerve endings travels from our fingers to the nerves in our spine, and, instead of ending there, keeps on going up the nerves of the spinal cord to the brain. The brain then decides what we are feeling, and we can identify the object as a spoon or a nickel or a pencil.
Motor nerves from the brain cross over in the spinal cord. Thus a nerve from the right side of the brain supplies a muscle on the left side of the body.
Motor nerves work just as fast as sensory nerves. With lightning speed they direct muscles and tendons to contract or relax, thus moving various parts of the body away from danger. Think how wonderfully muscles and tendons work when we pull away from a sharp pin or a hot stove. And how remarkably they work at difficult jobs such as playing a piano or a violin. None of these things could be done if the motor nerves failed to give the proper signals to the muscles and tendons of the hands and fingers.
The signals for many of our motor nerves come directly from the brain. Here is an example:
Think of raising your arm-quickly -but don’t move it yet. Get ready, get set, RAISE IT. How did it happen? Your brain told the nerves in your arm that you were going to raise your arm, butt you didn’t. Then, when you saw the words horse IT, your brain gave the command to the nerves in your arm, and bingo!-up went your arm. In other words, the nerves obeyed your brain, and the muscles obeyed your nerves, and the bones obeyed your muscles. The boss of the whole thing was your brain. If your brain hadn’t said “I’m going to raise my arm,” nothing would have happened.
The Nervous System. Every part of the body Is supplied by nerves, and all of these nerves connect with the brain. Some nerves carry impulses toward the brain: these are called afferent nerves. Other nerves carry impulses away from the brain: these are called efferent nerves. Nerve tissue shows very little tendency to grow again once it has been destroyed.
In addition to the motor and sensory nerves, we have what is called an involuntary nervous system. This is made up of a bunch of nerves that act by themselves, without the nerves in the spinal cord or brain having anything directly to do with them. This system of nerves is just as necessary as other nerves, because they control some very important actions of our body. Here are a few actions of our involuntary nerves:
They allow us to keep on breathing without having to think about it. They make the small blood vessels in our skin contract when it is cold. This holds in heat and keeps us warm. And they cause the small vessels in our skin to relax when it is hot. This permits more blood to go to our skin, and, in this way, we get rid of extra body heat.
Signals sent to our heart by involuntary nerves make our heart beat faster when we run and play. When we run or play hard, our muscles need more blood. The faster the heart beats, the more blood it will pump to our muscles.
Involuntary nerves tell us when we have to go to the bathroom. They are responsible for the contractions of our stomach when we get hungry, too. Also, without the action of the involuntary nerves, our foods wouldn’t pass along from the stomach to the intestines and our digestion would get out of order.
Many other signals are given by the involuntary nerves, and all their actions are automatic. Our brains don’t have to think about them. For instance, we don’t have to say to ourselves, “Now I am going to perspire,” or “Now I am going to see to it that the food leaves my stomach and goes into my intestines.” These things happen all by themselves as a result of the actions of our involuntary nerves.
One of the great things about our nerves is that they are able to work without our doing much to keep them healthy. However, they work at their best when we exercise regularly and eat a diet containing plenty of vitamins and minerals. If we don’t exercise and get fat and lazy, our nerve reactions may become slow and sluggish. And if we don’t eat a proper diet, our nerves may suffer and won’t work as well as they should.