The nervous system enables the body to respond to continuous changes in its external and internal environment. It controls and integrates the functional activities of the organs and organ systems. Anatomically, the nervous system is divided into the following:  The central nervous system (CNS) consists of the brain and the spinal cord, which are located in the cranial cavity and spinal canal, respectively.
 The peripheral nervous system (PNS) consists of cranial, spinal, and peripheral nerves that conduct impulses from (efferent or motor nerves) and to (the afferent or sensory nerves of ) the CNS, collections of nerve cell bodies outside the CNS called ganglia, and specialized nerve endings (both motor and sensory). Interactions between sensory (afferent) nerves that receive stimuli, the CNS that interprets them, and motor (efferent) nerves that initiate responses create neural pathways. These pathways mediate reflex actions called reflex arcs. In humans, most sensory neurons do not pass directly into the brain but instead communicate by specialized terminals (synapses) with motor neurons in the spinal cord.
Functionally, the nervous system is divided into the following:  The somatic nervous system (SNS) consists of somatic [Gr. soma, body] parts of the CNS and PNS. The SNS controls functions that are under conscious voluntary control with the exception of reflex arcs. It provides sensory and motor innervation to all parts of the body except viscera, smooth and cardiac muscle, and glands.  The autonomic nervous system (ANS) consists of autonomic parts of the CNS and PNS. The ANS provides efferent involuntary motor innervation to smooth muscle, the conducting system of the heart, and glands. It also provides afferent sensory innervation from the viscera (pain and autonomic reflexes). The ANS is further subdivided into a sympathetic division and a parasympathetic division. A third division of ANS, the enteric division, serves the alimentary canal. It communicates with the CNS through the parasympathetic and sympathetic nerve fibers; however, it can also function independently of the other two divisions of the ANS (see page 378).
Nerve tissue consists of two principal types of cells: neurons and supporting cells.
The neuron or nerve cell is the functional unit of the nervous system. It consists of a cell body, containing the nucleus, and several processes of varying length. Nerve cells are specialized to receive stimuli from other cells and to conduct electrical impulses to other parts of the system via their processes. Several neurons are typically involved in sending impulses from one part of the system to another. These neurons are arranged in a chainlike fashion as an integrated communications network. Specialized contacts between neurons that provide for transmission of information from one neuron to the next are called synapses.
Supporting cells are nonconducting cells that are located close to the neurons. They are referred to as neuroglial cells or simply glia. The CNS contains four types of glial cells: oligodendrocytes, astrocytes, microglia, and ependymal cells (see page 367). Collectively, these cells are called the central neuroglia. In the PNS, supporting cells are called peripheral neuroglia and include Schwann cells, satellite cells, and a variety of other cells associated with specific structures. Schwann cells surround the processes of nerve cells and isolate them from adjacent cells and extracellular matrix. Within the ganglia of the PNS, peripheral neuroglial cells are called satellite cells. They surround the nerve cell bodies, the part of the cell that contains the nucleus, and are analogous to Schwann cells. The supporting cells of the ganglia in the wall of the alimentary canal are called enteric neuroglial cells. They are morphologically and functionally similar to central neuroglia (see page 367).