Integration of gut function by sympathetic reflexes

1. The spinal sympathetic outflow that innervates the gastrointestinal tract, including its blood vessels, has an impressive representation quantitatively, yet little is known about the functions of this system and its peripheral or central organization. Electrical stimulation or section of the splanchnic nerves have variable effects on the GI tract and does not, therefore, lead to a deeper understanding of the system. 2. The targets of the sympathetic supply of the GI tract are blood vessels, nonvascular (sphincteric) smooth musculature, myenteric neurones, submucous neurones and gut associated lymphoid tissues. The corresponding functions associated with these targets are regulation of blood flow (particularly through the mucosa) and resistance to flow, of motility, of secretion and absorption and of immune responses. Little is known about the effects of the sympathetic nervous system on the latter function. 3. The sympathetic postganglionic neurones are (at least in the guinea-pig) neurochemically characterized with respect to the targets. Neurones projecting to blood vessels contain neuropeptide Y in addition to noradrenaline, while neurones projecting to the submucous plexus contain somatostatin. No neuropeptide has been detected to date in neurones projecting to the myenteric plexus. 4. Transmission through guinea-pig prevertebral ganglia in vitro have been studied electrophysiologically. The following functions have been demonstrated: (a) transmission and distribution of preganglionic impulse activity to the targets in a relay-like fashion; (b) mediation of peripheral intestinointestinal reflexes between different sections of the GI tract; (c) integration of activity from the spinal cord and from various peripheral sources. The first function may apply particularly to the sympathetic pathway innervating blood vessels. Whether the second function occurs in vivo is questionable. The third function is the most important one for pathways involved in the regulation of motility and probably secretion and absorption. 5. Only limited information is available on preganglionic neurones projecting to prevertebral ganglia. Neurones regulating blood vessels are probably located in the intermediolateral cell column, and non-vascular visceral preganglionic neurones are situated medial to this cell column in the intermediate zone of the spinal cord. Vascular (vasoconstrictor) neurones exhibit a reflex pattern which is largely dependent on the brain stem. Spinal cord transection rostral to the sympathetic outflow causes an immediate abolition of basal and reflex activity in these neurones.(ABSTRACT TRUNCATED AT 400 WORDS)