nervous system

The nervous system has two divisions: the central nervous system and the peripheral nervous system. The central nervous system includes the brain and spinal cord. It processes incoming sensory information and sends outgoing motor commands. The peripheral nervous system includes all neural tissue outside the central nervous system. It is divided into motor and sensory systems. Impulses go to the central nervous system through sensory nerves and are carried away from it by the motor nerves. The motor system is further divided into the somatic (or skeletal) nervous system and the autonomic nervous system. The somatic, or skeletal, motor system allows voluntary control over skeletal muscle with a few exceptions. The autonomic nervous system is largely involuntary and controls cardiac and smooth muscles and glands.The autonomic nervous system has three divisions: the enteric, the sympathetic, and the parasympathetic. The enteric nervous system is a system of nerves in the gastrointestinal tract, pancreas, and gallbladder that influences all digestive processes. The enteric system operates without input from the brain or spinal cord.
The sympathetic and parasympathetic divisions may operate together or in opposition. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs have both sympathetic and parasympathetic nerve systems. 
In such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always in opposition, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both stimulate and inhibit, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.

Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.
Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do those of special receptors concerned with sight, hearing, smell, and taste.
Muscular contractions do not always cause actual movement. A small fraction of the total number of fibers in most muscles are usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.