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GI Motility

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BIOL 1840U
Peter Cheung

GI Motility I. Introduction a. Function of GI Motility: to propel, grind, mix, & store its contents i. Functional segments may not always conform neatly w/ anatomical ones b. Structure: a series of pumps, conduits, reservoirs & gates c. Neural Control i. Extrinsic 1. Parasympathetic: enters thru vagus (sacral n. at end) a. NTs—acetylcholine & substance P b. stimulation => increased activity 2. Sympathetic—enters thru sympathetic nerve ganglion a. NTs—NO & VIP b. stimulation => inhibition (decreased activity) ii. Intrinsic 1. web of neurons w/in wall of GI tract; in communication w/ sympathetic & parasympathetic systems 2. extensive contact between neurons & w/ myocytes 3. key role in coordinating GI smooth mm. fxn 4. can act independently of CNS 5. Myenteric Plexus—between 2 layers of muscularis; very rich innervation iii. Neural Stimulation 1. direct contact—cells are directly synapsed upon by nn. 2. indirect contact—cells that receive electronic signals thru adjacent cells d. Types of Contractions i. Phasic contractions—result from momentary stimulation and are used for pumping ii. Tonic contractions—result in continuous pressure and must be relaxed to allow gating (sphincters) e. Patterns of Phasic Contractions i. Primarily Propulsive—progressive waves which completely occlude the lumen & propel food in a given direction ii. Mixing & Propulsive—progressive waves which fail to completely occlude the lumen & leave some food behind iii. Mixing & Grinding—interrupted progressive waves which may be discoordinated => segmental contractions; fxn is to mix & grind f. Slow Wave i. Key for generating contractions from stomach downwards ii. Generated by specialized cells (interstitial cells of Cajal (ICC)) iii. Begin in stomach and travel downward thru GI tract iv. Frequency is characteristic of different parts of GI v. Contractions => when spike potentials occur during slow waves (not every slow wave => a contraction!) g. Propagation of Contractions 1 i. Phase Locking—cells joined by tight jxns => synchronized contractions (=> progressive wave & unified contractions) ii. Phase unlocking—cells not joined tightly => uncoordinated contractions (get segmental contractions) h. Sphincters—zone of persistently high P separating sections of GI tract in order to regulate direction of motion i. Proximal distension => relaxation allowing flow of material ii. Distal Distension => contraction to prevent regurgitation II. Sections of GI a. Oropharynx—part of alimentary & respiratory tracts i. Components 1. Mouth—reservoir for chewing & mixing 2. Tongue—serves as pump 3. UES—Upper esophageal sphincter; relaxes during swallowing ii. Swallowing—initiation is under voluntary control, but once bolus enters pharynx it’s involuntary 1. normal swallow—progressive occlusive wave (primary peristalsis) down length of esophagus followed by 2 wave moving down just after the first to “clear out” the esophagus (secondary peristalsis) 2. tertiary peristalsis—uncoordinated movement of esopha
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