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the splanchnic nerves This is an exception to the rule that organs innervated by the autonomic nervous system receive only postganglionic bers This special arrangement can be explained by the fact that cells of the adrenal medulla are the morphologic homologues of the postganglionic sympathetic neurons and secrete epinephrine and norepinephrine (the postganglionic transmitters) directly into the bloodstream In this way, the sympathetic nervous system and the adrenal medulla act in unison to produce diffuse effects as one would expect from their role in emergency reactions By contrast, the parasympathetic effects, as in the pupil and urinary bladder, are more discrete There are three cervical (superior, middle, and inferior, or stellate), eleven thoracic, and four to six lumbar sympathetic ganglia The head receives its sympathetic innervation from the eighth cervical and rst two thoracic cord segments, the bers of which pass through the inferior to the middle and superior cervical ganglia Postganglionic bers from cells of the superior cervical ganglion follow the internal and external carotid arteries and innervate the blood vessels and smooth muscle as well as the sweat, lacrimal, and salivary glands of the head Included among these postganglionic bers, issuing mainly from T1, are the pupillodilator bers and those innervating Muller s muscle of the upper eyelid The arm receives its postganglionic innervation from the inferior cervical ganglion and uppermost thoracic ganglia (the two are fused to form the stellate ganglion) The cardiac plexus and other thoracic sympathetic nerves are derived from the upper thoracic ganglion and the abdominal visceral plexuses, from the fth to the ninth or tenth thoracic ganglia The lowermost thoracic ganglia have no abdominal visceral connections; the upper lumbar ganglia supply the descending colon, pelvic organs, and legs The terminals of autonomic nerves and their junctions with smooth muscle and glands have been more dif cult to visualize.

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and study than the motor end plates of striated muscle As the postganglionic axons enter an organ, usually via the vasculature, they ramify into many smaller branches and disperse, without a Schwann cell covering, to innervate the smooth muscle bers, the glands, and, in largest number, the small arteries, arterioles, and precapillary sphincters (see Burnstock) Some of these terminals penetrate the smooth muscle of the arterioles; others remain in the adventitia At the ends of the postganglionic bers and in part along their course there are swellings that lie in close proximity to the sarcolemma or gland cell membrane; often the muscle ber is grooved to accommodate these swellings The axonal swellings contain synaptic vesicles, some clear and others with a dense granular core The clear vesicles contain acetylcholine, and those with a dense core contain catecholamines, particularly norepinephrine (Falck) This is well illustrated in the iris, where nerves to the dilator muscle (sympathetic) contain dense-core vesicles and those to the constrictor (parasympathetic), clear vesicles A single nerve ber innervates multiple smooth muscle and gland cells Visceral Afferents Somewhat arbitrarily, anatomists have declared the autonomic nervous system to be purely efferent motor and secretory in function However, most autonomic nerves are mixed, also containing afferent bers that convey sensory impulses from the viscera and blood vessels The cell bodies of these sensory neurons lie in the posterior root sensory ganglia; some central axons of these ganglionic cells synapse with lateral horn cells of the spinal cord and subserve visceral re exes; others synapse in the dorsal horn and convey or modulate impulses for conscious sensation Secondary afferents carry sensory impulses to certain brainstem nuclei, particularly the nucleus tractus solitarius, as described below, and the thalamus via the lateral spinothalamic and polysynaptic pathways The Central Regulation of Visceral Function Integration of autonomic function takes place at two levels, the brainstem and the cerebrum In the brainstem, the main visceral afferent nucleus is the nucleus tractus solitarius (NTS) Cardiovascular, respiratory, and gastrointestinal afferents, carried in cranial nerves X and IX via the nodose and petrose ganglia, terminate on speci c subnuclei of the NTS The caudal subnuclei are the primary receiving site for viscerosensory bers; other less well de ned areas receive baroreceptor and chemoreceptor information The caudal NTS integrates these signals and projects to a number of critical areas in the hypothalamus, amygdala, and insular cortex, involved primarily in cardiovascular control, as well as to the pontine and medullary nuclei controlling respiratory rhythms The NTS therefore serves a critical integratory function for both circulation and respiration, as described further on An important advance in our understanding of the autonomic nervous system occurred with the discovery of autonomic regulating functions of the hypothalamus Small, insigni cant-appearing nuclei in the walls of the third ventricle and in buried parts of the limbic cortex, formerly judged to have purely olfactory functions, are now known to have rich bidirectional connections with autonomic centers in various parts of the nervous system In fact, the hypothalamus serves as the integrating mechanism of the autonomic nervous system and limbic system, as indicated in Chap 25 The regulatory activity of the hypothalamus is accomplished in two ways through direct pathways that descend to particular groups of cells in the brainstem and spinal cord and through the pituitary and thence to other endocrine glands The supranuclear regulatory apparatus of the hypothalamus includes three main cerebral struc-.

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