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The nervous system is the master controlling and communicating system of the body.  Every thought, action, and emotion reflects its activity.  The nervous system is by far the most rapid acting and complex system of the body.  The cells of the nervous system communicate by means of electrical signals, which are rapid, specific, and usually cause almost immediate responses.  The nervous system is but one single system, but for convenience we divide it into two parts: the central nervous system (CNS) and the peripheral nervous system (PNS).  The CNS consists of the brain and the spinal cord which are located in the dorsal body cavity.  The CNS is the command center of the nervous system.  It interprets incoming signals and responds to them based on past experiences, reflexes, and current conditions.  The PNS is the part of the nervous system that is not part of the CNS.  It consists mainly of the nerves that extend from the brain and the spinal cord.  These nerves are called the cranial nerves and the spinal nerves, respectively.  These peripheral nerves serve as the communications link from the body to the CNS.

The PNS can then be farther divided into two functional subdivisions.  The sensory division is the division that contains the nerve fibers that carry impulses to the CNS from sensory receptors that are located throughout the body.  There are two types of sensory fibers.  The somatic afferents convey information from the skin , skeletal muscles, and joints.  The visceral afferents convey impulses from the visceral organs.  The other division of the PNS is the motor division.  This division transports messages from the CNS to organs, muscles, and glands.  The motor system can also be divided farther in to two parts.  The somatic nervous system is composed of motor nerve fibers that connect the CNS to the skeletal muscles and is often referred to as the voluntary nervous system.  The autonomic nervous system consists of nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands.  Since we usually cannot consciously control these activities, it is generally referred to as the involuntary nervous system.