The human nervous system oversees the activity of all other body systems. The central nervous system is composed of the brain and spinal cord. The peripheral nervous system is made up of a network of nerves connecting the brain and spinal cord with the rest of the body. Together, the central and peripheral nervous systems control both voluntary and involuntary functions in the body.
The center of the nervous system is the brain, which is composed of the brainstem, the cerebellum, and the cerebrum. The brainstem controls basic functions such as breathing rate and heartbeat and it contains nerve centers involved in hearing and vision.
The cerebellum controls balance, equilibrium, and locomotion. The cerebrum regulates such mental functions as understanding, remembering, speaking, learning, reasoning, and feeling emotions.
The spinal cord extends from the brain into the spine through a canal encased in bone. The spinal cord is a collection of nerve fibers and cells that carry sensory impulses from the outer parts of the body up to the brain. The brain responds by sending impulses down through other nerve fibers in the spinal cord to stimulate muscles that are under voluntary control.
Autonomic Nervous System
Among the motor fibers may be found groups that carry impulses to viscera. These fibers are designated by the special name of autonomic nervous system. That system consists of two divisions, more or less antagonistic in function, that emerge from the central nervous system at different points of origin. One division, the sympathetic, arises from the middle portion of the spinal cord, joins the sympathetic ganglionated chain, courses through the spinal nerves, and is widely distributed throughout the body. The other division, the parasympathetic, arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord. These two divisions control the functions of the respiratory, circulatory, digestive, and urogenital systems.
Each nerve cell consists of a central portion containing the nucleus, known as the cell body, and one or more structures referred to as axons and dendrites. The dendrites are rather short extensions of the cell body and are involved in the reception of stimuli. The axon, by contrast, is usually a single elongated extension; it is especially important in the transmission of nerve impulses from the region of the cell body to other cells. See Neuro physiology.