The Nervous System comprises the central nervous system (CNS), consisting of the brain and spinal cord, protected by the skull and spine, and the peripheral nervous system (PNS), made up of spinal and cranial nerves, which extend from the CNS to other parts of the body.
Nerves are bundles of the long, tubelike extensions of nerve cells. Impulses traveling through them carry information throughout the body. Nerve impulses travel only in one direction. A message will, for instance, be sent to the brain from the fingertips through one nerve, and the message from the brain back to the fingers will be sent through another. Nerve cells are called neurons, and each has a cell body, branches called dendrites, and an axon. It is separated from the next nerve by a small gap called a synapse. Each neurone contacts the next in the nerve chain by releasing a chemical called a neurotransmitter from the end of the axon. This crosses the synapse and carries the message to the next neurone. The neurotransmitter found at the synapses of most peripheral nerves is called acetylcholine, although at some autonomic nerve synapses the transmitter is noradrenaline or dopamine.What Nerves doThe action of the nerves, which is moderated by the neurotransmitters, is a sensitive process that can be increased or decreased as needed. And because the chemical structure of many of the neurotransmitters is known, they can be used as drugs to modulate some of the most important actions of the nervous system. There are also many drugs that act by simulating the action of neurotransmitters, by modifying their action or by blocking their receptor sites.The CNS acts as a central computer to the rest of the body. It receives sensory information from the organs of the whole body, such as the ears and eyes, skin, and joints. It analyzes this information and then sends out the appropriate motor response to various muscles, such as those controlling speech or internal organs.The Two SystemsThere are two main divisions within the PNS: the somatic nervous system, which controls the muscles responsible for conscious or willed movement, such as speech or walking; and the autonomic nervous system, which controls involuntary body functions, such as sweating or shivering in response to cold. Organs and glands are influenced chiefly by the autonomic nervous system, which is divided into two areas: these are the sympathetic nervous system and the parasympatheyic nervous system. Most of the time, these two in balance, but the sympathetic system will dominate during periods of excitement or fear by stimulating functions such as the heartrate or breathing, as if preparing the body for a fight-or-flight response, as if contrast, the parasympathetic system is concerned with everyday functions and dominates during sleep.Sensory nerves carry information from the sense organs and other body receptors to the central to the central nervous system - the brain and spinal cord - for processing. Then motor nerves carry the processed information from the central nervous system to the gland, muscles to intiate a change.