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Publisher Summary This chapter presents an overview of the autonomic nervous system (ANS). The ANS regulates the action of smooth muscle and glands. Its efferent nerves supply all tissues except skeletal muscle and are involved in the regulation of heart contraction, blood vessel diameter, sweating, saliva secretion, pupil diameter, intestinal secretions and movement, kidney function, piloerection, urination, sexual function, and lipolysis. Thus, it is responsible for orchestrating much of the automatic life-sustaining processes of circulation, temperature regulation, metabolism, digestion, fluid balance, and adrenergic function. Neurons in the ANS are predominantly nonmyelinated in contrast to neurons in the somatic motor system, which are myelinated. A second marked difference is the location of the last synaptic junction. In the somatic motor system, all interneurons and their connections occur within the central nervous system (CNS). In the ANS, the last interneuron actually traverses either an anterior (ventral) root or a cranial nerve and forms a synapse with one or more efferent fibers in an ANS ganglion outside the CNS. Thus, every ANS efferent neuron has its origin in an ANS ganglion. The ANS interneurons that terminate in the ganglion and form synapses with efferent neurons are called preganglionic neurons, and ANS effector neurons are called postganglionic neurons. |
