How the nervous system works
The nervous system is made up of billions of special cells called neurons or nerve cells. Cordlike bundles of neuron fibres are called nerves. The nerves form a network of pathways that conduct information rapidly throughout the body. A person's reaction to a situation may take only an instant, but it involves many complicated processes within the nervous system. For example, what happens in the nervous system of a person who sees a wild tiger and, an instant later, turns and runs away? Specialized neurons called receptors are located in the ears and eyes and the other sense organs of the body. The receptors translate events in a person's surroundings--such as the sight of a tiger--into nerve messages, which are known as impulses. Nerve impulses travel along nerve fibres at speeds of 1 to 90 metres per second.
The receptor cells in the eyes respond to light rays that reflect off the tiger and translate the rays into a pattern of nerve impulses. These impulses then travel through neurons called sensory neurons and association neurons. The sensory neurons carry information from receptors in the sense organs to the association neurons, which are located in the brain and within the spinal cord. The neurons in the brain receive the impulses, analyse and interpret the message, and decide what action should be taken. A message consisting of the sight of a wild tiger is, of course, interpreted as danger. The person's brain immediately sends out a message--"Run!"--in the form of nerve impulses. Next, the impulses travel through motor neurons. These nerve cells carry messages from the brain to the muscles and glands, which are called effectors. The effectors carry out the brain's instructions. Thus, the leg muscles respond and the person runs away. At the same time, the brain sends messages to various other parts of the body. For example, it sends messages to the heart to beat faster and send more blood to the leg muscles.