The sympathetic and parasympathetic divisions have both central and peripheral components. Clusters of nerve cells that contribute to the control of many organs via the ANS are located in the hypothalamus and brain stem (Fig. 2). Many of these clusters seem to be involved in regulating specific functions. For example, direct stimulation of certain areas can induce an increase in arterial blood pressure, whereas stimulation of other areas alters body temperature, gastrointestinal activity, and bladder function. To a great extent, these auto-nomic regions in the lower brain stem are influenced by neurons originating from cortical regions of the brain; thus, many human behavioral responses include auto-nomic responses that are mediated through the hypothalamus and reticular formation. Examples of such autonomic responses include increased gastric acid secretion upon the sight and smell of food and increased heart rate and dilatation of the pupils of the eye upon being frightened.
In contrast to the peripheral efferent component of the somatic nervous system, which consists only of the axons of spinal motor neurons, the peripheral efferent components (Figs. 1 and 3) of both the sympathetic and parasympathetic divisions of the ANS are rather complex, consisting of preganglionic axons, autonomic ganglia, and postganglionic neurons. The somata of efferent sympathetic preganglionic neurons are located in the intermediolateral horn of the spinal cord in regions from the first thoracic to the third or fourth lumbar segment (see Fig. 1). Axons of these preganglionic neurons leave the spinal cord and are traditionally thought to make synaptic contact with postsynaptic neurons located in either the paravertebral or pre-vertebral ganglia of the sympathetic nervous division. Axons of these postganglionic neurons then innervate via synapses the various organs to be controlled. Preganglionic axons of the parasympathetic nervous division arise from neurons whose somata are located within the motor nuclei of cranial nerves and the sacral portion of the spinal cord (see Fig. 1). These preganglio-nic axons make synaptic contact with postganglionic neurons located in parasympathetic ganglia. These ganglia are located near or in each innervated effector organ.
Most organs that are influenced by the ANS are innervated by both its sympathetic and parasympathetic divisions. The actions of these two divisions on a given organ are usually antagonistic. For example, an increase in sympathetic neural input to the sinoatrial node of the heart causes an increase in heart rate, whereas an increase in parasympathetic input to the node causes a reduction in heart rate. At first glance, such an arrangement may seem to work at cross purposes; however, the divisions usually function in a way such that an increase in the input of one is accompanied by a decrease in input from the other. On the other hand, some organs are innervated by only one division of the ANS. Two notable examples of this arrangement are innervation by the sympathetic division of the sweat glands and the peripheral blood vessels.
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