Whether it is the heat from a candle or fire that is sensed by you or the pain when a pin pricks your finger, it is all because your nerves are passing the messages to the brain. The nervous system is a network of nerve cells called neurons that send information to and from all parts of the body to the brain. The messages are in the form of electrical signals or nerve impulses. They travel through the cells and are passed on to the other cells. The body has millions of nerve cells linked together to make up the nervous system. The human brain itself contains about ten thousand million nerve cells. The human nervous system is divided into two parts — the central nervous system and the peripheral nervous system. The central nervous system comprises of the brain and the spinal cord, whereas the peripheral nervous system consists of all nerve cells and nerve fibres at the periphery. The tough bones of the skull protect the brain, and the bones of the spinal column protect the spinal cord. There are two kinds of nerve cells or neurons — sensory neurons and motor neurons. Sensory neurons carry messages from distant parts of the body towards the brain. These relate to pain, temperature, pressure and other messages from the sense organs. The motor neurons carry messages to the muscles, telling them to contract; and to the glands, telling them to produce secretions. The nerves of the central and peripheral nervous system are a mixture of sensory and motor neurons travelling together. The nervous system can also be divided on the basis of the nature of work of the neurons. If the nerves are all concerned with conscious sensations, it is called the voluntary nervous system. They carry out orders given by the brain that are directed by will. When we want to walk, write or talk, we use the neurons of the voluntary system. The neurons of the autonomic system help in carrying out the routine activities we do not normally think about. The autonomic system regulates the heartbeat, respiration, digestion, blood pressure, and other unconscious activities. The autonomic nervous system has two main divisions — the sympathetic and parasympathetic systems. Their activities balance each other. Nerve impulses in the sympathetic system tend to increase the rate of heartbeat and the blood pressure. On the other hand, the impulses in the parasympathetic system tend to have the opposite effect. Under different conditions, each system takes control of the unconscious activities. With the help of prescribed physical exercises, training and to some extent will power, it is possible to control some of the activities of the autonomic nervous system.