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
• Central nervous system (CNS), consisting of the brain and the spinal cord, located in the cranial cavity and spinal canal, respectively.
• Peripheral nervous system (PNS), consisting of cranial, spinal, and peripheral nerves that conduct impulses from (efferent or motor nerves) and to (afferent or sensory nerves) the CNS, collections of nerve cell bodies outside the CNS called ganglia, and specialized nerve endings (both motor and sensory).
Functionally, the nervous system is divided into the
• Somatic nervous system (SNS), consisting of somatic [Gr.soma, body] parts of the CNS and PNS. It provides sensory and motor innervation to all parts of the body except viscera, smooth muscle, and glands.
• Autonomic nervous system (ANS), consisting of autonomic parts of the CNS and PNS. It 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 element, the enteric division, is sometimes incorporated into the ANS subdivision (see page 310).
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