Types of Neurotransmitters Secreted by Enteric Neurons
In an attempt to understand better the multiple functions of the gastrointestinal enteric nervous system, research workers the world over have identified a dozen or more different neurotransmitter substances that are released by the nerve endings of different types of enteric neurons. Two of them with which we are already familiar are (1) acetylcholine and (2)nor-epinephrine. Others are (3) adenosine triphosphate, (4) serotonin, (5) dopamine, (6) cholecystokinin, (7) substance P, (8) vasoactive intestinal polypeptide, (9) somatostatin, (10)leu-enkephalin, (11) met-enkephalin, and (12) bombesin. The specific functionsof many of these are not known well enough to justify discussion here, other than to point out the following.
Acetylcholine most often excites gastrointestinalactivity. Norepinephrine almost always inhibits gas-trointestinal activity. This is also true of epinephrine, which reaches the gastrointestinal tract mainly by way of the blood after it is secreted by the adrenal medul-lae into the circulation. The other aforementioned transmitter substances are a mixture of excitatory and inhibitory agents.
Autonomic Control of the Gastrointestinal Tract Parasympathetic Innervation. The parasympathetic sup-ply to the gut is divided into cranial and sacral divisions.
Except for a few parasympathetic fibers to the mouth and pharyngeal regions of the alimentary tract, the cranial parasympathetic nerve fibers are almost entirely in the vagus nerves. These fibers provide extensive innervation to the esophagus, stomach, and pancreas and somewhat less to the intestines down through the first half of the large intestine.
The sacral parasympathetics originate in the second, third, and fourth sacral segments of the spinal cord and pass through the pelvic nerves to the distal half of the large intestine and all the way to the anus. The sig-moidal, rectal, and anal regions are considerably better supplied with parasympathetic fibers than are the other intestinal areas. These fibers function especially to execute the defecation reflexes.
The postganglionic neurons of the gastrointestinal parasympathetic system are located mainly in the myenteric and submucosal plexuses. Stimulation of these parasympathetic nerves causes general increase in activity of the entire enteric nervous system. This in turn enhances activity of most gastrointestinal functions.
Sympathetic Innervation. The sympathetic fibers to thegastrointestinal tract originate in the spinal cord between segments T-5 and L-2. Most of the pregan-glionic fibers that innervate the gut, after leaving the cord, enter the sympathetic chains that lie lateral to the spinal column, and many of these fibers then pass on through the chains to outlying ganglia such as to theceliac ganglion and various mesenteric ganglia. Most ofthe postganglionic sympathetic neuron bodies are in these ganglia, and postganglionic fibers then spread through postganglionic sympathetic nerves to all parts of the gut.. The sympathetics innervate essentially all of the gastrointestinal tract, rather than being more extensive nearest the oral cavity and anus as is true of the parasympathetics. The sympathetic nerve endings secrete mainly norepinephrine but also small amounts of epinephrine.
In general, stimulation of the sympathetic nervous system inhibits activity of the gastrointestinal tract, causing many effects opposite to those of the parasympathetic system. It exerts its effects in two ways: (1) to a slight extent by direct effect of secreted norepinephrine to inhibit intestinal tract smooth muscle (except the mucosal muscle, which it excites) and (2) to a major extent by an inhibitory effect of norepinephrine on the neurons of the entire enteric nervous system.
Strong stimulation of the sympathetic system can inhibit motor movements of the gut so greatly that this literally can block movement of food through the gas-trointestinal tract.
Afferent Sensory Nerve Fibers from the Gut
Many afferent sensory nerve fibers innervate the gut. Some of them have their cell bodies in the enteric nervous system itself and some in the dorsal root ganglia of the spinal cord. These sensory nerves can be stimulated by (1) irritation of the gut mucosa, (2) excessive distention of the gut, or (3) presence of spe-cific chemical substances in the gut. Signals trans-mitted through the fibers can then cause excitation or, under other conditions, inhibition of intestinal move-ments or intestinal secretion.
In addition, other sensory signals from the gut go all the way to multiple areas of the spinal cord and even the brain stem. For example, 80 per cent of the nerve fibers in the vagus nerves are afferent rather than efferent. These afferent fibers transmit sensory signals from the gastrointestinal tract into the brain medulla, which in turn initiates vagal reflex signals that return to the gastrointestinal tract to control many of its functions.
The anatomical arrangement of the enteric nervous system and its connections with the sympathetic and parasympathetic systems support three types of gas-trointestinal reflexes that are essential to gastroin-testinal control. They are the following:
1.Reflexes that are integrated entirely within the gut wall enteric nervous system. These include reflexesthat control much gastrointestinal secretion, peristalsis, mixing contractions, local inhibitory effects, and so forth.
2.Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract. These reflexes transmitsignals long distances to other areas of the gastrointestinal tract, such as signals from the stomach to cause evacuation of the colon (the gastrocolic reflex), signals from the colon andsmall intestine to inhibit stomach motility and stomach secretion (the enterogastric reflexes), and reflexes from the colon to inhibit emptying of ileal contents into the colon (the colonoileal reflex).
3.Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract.
These include especially (1) reflexes from the stomach and duodenum to the brain stem and back to the stomachâ€”by way of the vagus nervesâ€”to control gastric motor and secretory activity; (2) pain reflexes that cause general inhibition of the entire gastrointestinal tract; and (3) defecation reflexes that travel from the colon and rectum to the spinal cord and back again to produce the powerful colonic, rectal, and abdominal contractions required for defecation (the defecation reflexes).