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== Introduction ==

Nervous tissue is highly specialized to form the nervous system which is organized into two major divisions: The central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord. The peripheral nervous system consists of all the nervous tissue lying outside of the brain and the spinal cord. This is subdivided into the cerebrospinal portion (the 12 pairs of cranial nerves, 31 pairs of spinal nerves and associated ganglia) and the autonomic nervous system (sympathetic and parasympathetic components) which deals with visceral activities (smooth muscle, cardiac muscle, and glandular secretions). These parts of the nervous system are not separate entities but are structurally and functionally interdependent. The nerves of the peripheral nervous system are widely distributed throughout the body, extending into all tissues and organs.