Enteric Reflexes that Influence Motility
In Innervation of the Gastrointestinal Tract. eds Costa, M. & Brookes, S.H. pp. 1-55. London: Taylor & Francis.
J.C. Bornstein1, J.B. Furness2, W.A.A. Kunze2 and P.P. Bertrand1
1-Department of Physiology and 2-Department of Anatomy and Cell Biology, University of Melbourne, Victoria 3010, Australia
The gut exhibits a variety of movements that depend on the region studied and the timing and composition of the last meal. Primary control of these movements is exerted by the enteric nervous system. Except in the oesophagus and the sphincters, external inputs modulate enteric neural activity, rather than directly controlling muscle movements. In isolated intestine, localised mechanical and chemical stimuli excite enteric neurons to produce stereotypic reflexes in the smooth muscle. The reflex circuits form the basis of the circuitry responsible for more complex motor patterns evoked by more generalised stimuli. The enteric neural circuitry responsible for stereotyped reflexes in the guinea-pig ileum has been studied in detail and the neurons involved have been identified. These include at least three types of intrinsic primary afferent neurons (IPANs), a population of ascending interneurons, three populations of descending interneurons, excitatory and inhibitory motor neurons innervating the circular muscle and longitudinal muscle motor neurons. Synaptic transmission in the ascending excitatory reflex pathway is primarily via fast excitatory synaptic potentials (EPSPs) mediated by acetylcholine acting at nicotinic receptors; slow EPSPs mediated via NK3 tachykinin receptors may also be involved. Acetylcholine is less important in descending pathways. Transmission from interneurons to inhibitory motor neurons depends on ATP acting at P2X receptors and tachykinins play a significant, but not exclusive, role in transmission from IPANs to descending interneurons. Transmitters at other synapses in this pathway remain to be identified. These properties of simple reflexes will be important for understanding more complex behaviour.