The nervous system is divided into two parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists of all afferent (sensory) neurons, which carry nerve impulses into the CNS from sensory end organs in peripheral tissues, and all efferent (motor) neurons, which carry nerve impulses from the CNS to effector cells in peripheral tissues. The peripheral efferent system is further divided into the somatic nervous system and the autonomic nervous system. The effector cells innervated by the somatic nervous system are skeletal muscle cells. The autonomic nervous system innervates three types of effector cells: (1) smooth muscle, (2) cardiac muscle, and (3) exocrine glands. While the somatic nervous system can function on a reflex basis, voluntary control of skeletal muscle is of primary importance. In contrast, in the autonomic nervous system voluntary control can be exerted, but reflex control is paramount.
Both somatic and autonomic effectors may be re-flexly excited by nerve impulses arising from the same sensory end organs. For example, when the body is exposed to cold, heat loss is minimized by vasoconstriction of blood vessels in the skin and by the curling up of the body. At the same time, heat production is increased by an increase in skeletal muscle tone and shivering and by an increase in metabolism owing in part to secretion of epinephrine.
In general terms, the function of the autonomic nervous system is to maintain the constancy of the internal environment (homeostasis). This includes the regulation of the cardiovascular system, digestion, body temperature, metabolism, and the secretion of the exocrine glands.
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