The nervous system is an extension of the brain, though the brain can also be considered to be part of the nervous system – it is a matter of definition. The brain communicates with the rest of the body through the nerves of the spine and a multitude of others outside the spine. The brain is like the telephone exchange and the nerves are like the thousands of telephone lines that connect the telephone exchange to individual telephones. Innumerable messages pass to and from the brain, bringing all organs, muscles and all parts of the body into direct communication with the “brain. Most of these messages occur below the level of conscious perception.
There are two main groups of nerves. One group is the sensory nerves which transmit information to the brain concerning the state and happenings in the outside environment and about the physical condition of the body, such as temperature, pain, etc. Each type of sensory perception requires a special receptor. The sensations connected with pain, pleasure and touch of objects depend on specific sensors; these cannot detect other sensations. There are thousands of these different sensors throughout the whole body. Nerve impulses from these detectors travel to the brain along specific nerve pathways; from there they are redirected to the particular brain centre connected with that type of sense. They are then interpreted according to previous experience.
The second group of nerves are called motor nerves. These come into play if action is required in response to the interpretation of nerve impulses from the sensory nerves. The motor nerves carry explicit commands from the brain to the muscles telling them to move in a certain manner. Let us give an example. You are reading a book. Your eyes are attracted to something interesting. Sensory information is transmitted to the brain. The brain responds by sending a series of messages to the muscles of the fingers, hands and arms, directing them to turn the pages of the book for more information. The sensory and motor nerves act as a continuous feedback system.
This same feedback operates with the nerves of taste and smell located in the mouth and nose. If we taste some food, we immediately know whether we like it or not. The taste buds on the tongue are sensory receptors that immediately flash a message to the brain; from this we know whether to continue eating or not. The pleasant odour of a well-cooked meal acts in the same way. Both of these sensations not only allow one to enjoy food, they also cause the digestive organs to prepare for the process of digestion.
It is the nervous system that allows the brain to receive information from the eyes. Within each eve there are millions of small receptors that are sensitive to light; they detect all the different colours. When the eyes look at something, then a picture or image is formed on these sensors in the screen (retina) at the back of the eye. These sensors transmit messages to the brain where they are interpreted. In this way, one understands the nature of the object.
It is the same with the ears; they contain thousands of tiny sensory nerves that are sensitive to differences of pitch, tone and quality of sound. All the different sounds that occur at any one moment are transmitted to the brain, where they are interpreted and recognized as being a sound that has been heard before. If it is a pleasant sound such as beautiful music, it will produce a joyful emotional response. Without the brain we would never be able to recognize any sounds.
A most important section of the nervous system is the autonomic nervous system, comprising the sympathetic and parasympathetic nerves. They are essential for maintaining perfect balance and harmony in the functioning of all physical organs.
It should be abundantly clear that without the brain and the nervous system, we would be totally unable to experience the outside world; also the functioning of the inner organs would not be synchronized so that they work together for the overall health and well-being of the entire body. The degree to which we can live to the full depends on the efficiency of the brain. Various yogic practices, especially sirshasana, help the brain to function as a perfect instrument.