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Lecture 24 Notes.pdf

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

1Lecture 24 Nervous and Hormonal Regulation of Digestion1 GI Tract Nerve PlexesGut motility and the secretion of certain substances are controlled by two nerve plexes Innervating the submucosa layer is the submucosal nerve plexis and innervating the smooth muscle is the myenteric nerve plexis The submucosal plexis influences secretory activitythis is why it is located closer to the intestinal lumen The myenteric plexis influences smooth muscle activity The smooth muscle can contract and relax without any nervous or hormonal regulationhowever it is often influenced by these factors 2 Sensory Pathways and ReceptorsThere are a host of different receptor types within the gut that can sense the various chemical stimuli that arise from the breakdown of foodstuffs These receptors are generally located on the walls of the stomach lumen and are connected to the nerve plexes via a sensory afferent itself connected to an interneuron which then connects to a motor or enteric neuron which then sends nervous signals to the gut muscles or to secretory cellsThere are three general types of receptors in the gut mechanoreceptors which sense the distension or stretching of the gut as it fills with food there are chemoreceptors that sense changes in pH sugars amino acids and fatty acids There are also osmoreceptors that sense changes in osmotic pressure as digestion progressesas food gets broken down into smaller and smaller pieces the osmotic pressure of the stomach chyme will increase2 The osmoreceptors in the gut are stretchinhibited as osmotic pressure increases around the cells due to the presence of solutes water will be drawn out of the osmoreceptors and into the stomach lumen This causes the receptor cells to shrink open up stretchsensitive ion channels and finally depolarize which sends a signal to either the motor neurons or to the enteric neurons lefthand diagram below Mechanoreceptors are stretchactivated Distension causes ion ie Na channels to open leading to membrane depolarisation and receptor activation righthand diagram below
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