Muscles use electricity and energy
Look at this star *. Find your mouse and move it until the cursor is over the star. What happened in your body to make this movement? Let us have a closer look. Your eyes received light from the screen. There is a black hole in the centre of each eye. It is called the student. The light from the screen passed through the student. Behind the student, the eye is filled with a clear jelly. The light passed through this and struck the back wall of the eye. Here the light formed a tiny picture of the screen.

In the back wall of the eye are thousands of tiny patches which are sensitive to light. When light strikes them they make tiny currents of electricity. Behind each patch in the wall is a very long fine thread called a nerve fibre. It carries the electrical currents away round the wall of the eye and meets with other fibres to form a thick white cord that leaves the back of the eye. This cord is called the optic nerve and it is full of nerve fibres carrying their electrical currents. The optic nerve is connected to the brain.

Each electrical current carries a message to the brain. The brain uses the messages to build up a picture of the screen inside your head. When the brain has built up the picture, and this only takes a fraction of a second, it sends messages along other nerve fibres down the spine. The nerve fibres in the spine form an even thicker white cord than the optic nerve. This cord is called the spinal cord and it passes through a hole in every bone in the spine. There are also spaces between the bones, which let nerves leave the spinal cord and run out into all parts of the body.

When your brain had sent its messages, they travelled down the spinal cord and out along a nerve into your arm. Some of the messages went to the biceps muscle on top of your upper arm and told it to contract or shorten. At the same time messages went into the triceps muscle on the lower side of the forearm and told it to relax. These messages made the muscles raise the forearm. Other messages from other nerves reached the muscles in your back and told them to move the whole of your arm so the hand swung over the mouse. When the hand was in position, more messages arrived at the biceps and triceps. This time they told the biceps to relax and the triceps to contract. When the muscles followed these orders your hand came down gently on the mouse.

Still more messages arrived. This time they went to the muscles in the forearm to move the fingers. As your fingers moved, your mouse rolled across its mat and the cursor came over the star. It may have taken a couple of minutes to read this but the whole action would take place in only a few seconds.

Every moment of your waking day electrical currents are shooting along your nerves carrying messages to the brain and the muscles. Energy is needed for this. It comes from your food. The energy is locked in your food but your body has a way of getting the energy out. It uses oxygen. Inside your body the blood picks up food from the intestines and oxygen from the lungs. When they flow into a muscle that is about to contract a change takes place between the food and the oxygen and energy is released. The muscles take up the energy and use it to pull on bones. In the process of releasing energy, carbon dioxide is made. It travels in the blood to the lungs and escapes into the air when we breathe out.

If moving your mouse across its mat is brought about by simply looking at the screen and moving a few muscles in your arm, think of the messages that shoot through your body, the muscles that contract and relax, and the food and oxygen that change to release energy when you play a game of football or netball.

How strong are the electrical currents in the nerves?
They are very, very weak and you need complicated equipment to detect them. The nerves, however, can carry strong electrical currents, which can damage or even kill. This means that you must take great care with all electrical equipment and follow the rules for its use carefully.

Does the brain get messages from other parts of the body besides the eyes?
Yes, it does. It receives messages from the skin, ears, nose and tongue. It also gets messages from the muscles themselves. These muscles tell the brain how much a muscle is contracting or relaxing.

Do you have to think about every message that you send out to the muscles?
No. If you did you would not have time to think of anything else. The action of some muscles takes place automatically. For example, the beating of the heart muscle takes place without you thinking about it. As you sit reading this, muscles all over your body are holding up your head and body. Each one is sending messages to the body but part of your brain deals with them and sends out messages to control them without you being aware of it. This makes other parts of the brain free to think. In fact most movements of the body are dealt with by the brain in such a way that you can think and move at the same time. Only when you are really concentrating on a movement, such as trying to unscrew something that is stuck, do you really think about the movements you are making.

Are there nerve fibres inside your brain?
Nerve fibres make up some parts of the brain. In the top wrinkly surface of the brain are tiny boxes of living material called brain cells. They look like spiders with very many legs. These 'legs' connect up to many other brain cells. It is thought that the way these brain cells connect up helps you store your memories and allows you to think quickly.

Is the brain hard like bone?
No, it is soft. It is protected by a hard layer of thin bone in the skull. This gives the brain some protection from damage but you must always take care not to damage you head when taking part in games or playing outside.

What happens to your nerves and muscles when you are asleep?
The brain still needs energy to keep the brain cells alive when you are asleep. Some messages are still being sent out to the muscles. For example, the muscles between the ribs have to be kept working so that you can breathe. The muscles which move your arms and legs do not need to work so messages are not sent to them and they relax. However, if you turn over in the night this is due to your brain sending out messages to operate your muscles while you are asleep.

Do other animals have nerves to control their muscles?
Yes, they do. Even animals such as earthworms and slugs have simple brains and nerves to control their muscles. A starfish has a brain in the shape of a ring in the centre of its body.

Do plants have muscles to move their stems?
No, they do not. Plants do not have nerves either. They move by growing. They have substances inside them which are sensitive to light and the force of gravity, and they help the plant grow to find the things it needs.