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Lecture 30

PSL301H1 Lecture 30: L30 Motility

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Michelle French

Lecture 30 Motility Silverthorn 7E: pp. 661-663, 668-669, 685-688 Monday, March 27 Lecture Outline: I. What is the function of GI motility? II. Describe the physiology of GI smooth muscle. III. How does food move in the GI tract? IV. What happens during emesis? Recall: Functions of Digestive System • Motility describes movement of material, primarily through digestive tract from mouth to anus • Specific motility patterns through each segment, so it’s not always from A to B • Motility pattern that we see in specialized regions have special functions, as well • When we talk about receptive relaxation and segmentation and mass movement, we’ll be focusing on this Describe the Physiology of GI Smooth Muscle. GI Smooth Muscle • Cells connected by gap junctions to create a functional electrical syncytium • Action potential travels in all directions • Smooth muscle cells make up majority of the muscle • Recall, the last two thirds of esophagus and rest of the digestive tract is smooth muscle • ANS impinges on smooth muscle, postganglionic fibres end in these varicosities – we have a string of swellings (varicosities) that release the neurotransmitter into the space above the smooth muscle, rather than a single synapse • We have a broad release of NTs that can then be taken up by a wide number of receptors all on those smooth muscles facing those varicosities • NTs bind receptors and fire action potentials at the muscle cells themselves • Smooth muscle cells that make up GI tract are joined by gap junctions o Action potentials that are initiated along cells that receive neurotransmitter signals can move and be dispersed throughout the interconnected smooth muscle cells, which can act as an electrical syncytium (all responding at the same time) o These APs are moving throughout that section of tissue Activity in Smooth Muscle Cells • Slow wave potentials: slow, undulating changes that • Specialized cells can undergo rhythmic depolarization and repolarization of the membrane potential don’t always reach threshold • Undulating changes that we see are called slow wave potentials – • Action potentials: occur when membrane potential depolarization and repolarization that are initiated by these specialized reaches threshold muscle cells o Frequency dependent on the slow wave • They don’t always reach threshold, but when they do, they fire off a typical action potential • The width or size of the slow wave, when it hits the threshold to fire an AP, controls the frequency of action potentials o Larger the slow wave, the more APs will be fired • It’s the APs that cause contraction of those muscles • The wider or larger the slow wave potential, the more APs are fired, and the longer the contraction and greater force of contraction of the muscle cells Specialized Smooth Muscle Cells Have Pacemaker Activity Interstitial cells of Cajal: • Slow wave potentials (regular cycling of depolarization and • Generate slow wave potentials repolarization that may or may not hit threshold to fire an AP in smooth muscle cells) come from specialized pacemaker cells called • Electrically coupled to smooth muscle cells via gap junctions interstitial cells of Cajal (ICCs) • Lie at interface between nerve fibres and smooth muscle cells • Found throughout GI tract, but have their own frequencies in specific regions • ICCs are connected with smooth muscle cells through gap junctions, so the slow waves that are being released from the ICCs are moving throughout the entire syncytium – if high enough, they can cause APs throughout the entire system, independent of external stimuli • Slow waves are generated by ICCs and aren’t regenerated as they move through the smooth muscle cells because (1) first generated in ICCs and then smooth muscle cells (temporal), and (2) slow waves generated in ICCs degrade in amplitude as they move through the smooth muscle cells • ICCs important in maintaining pace of contraction of smooth muscle cells Comparison of Pacemaker Activity in Heart vs. GI Tract Heart pacemaker cells: • Heart pacemaker cells have a regular continuous repolarization (pacemaker potential) until it hits the threshold to fire an AP • Frequency approx. 80/min • Slow wave potentials generated by ICCs don’t always hit • Point sources or nodes of threshold potential, so they don’t always fire an AP resulting in a activity contraction Interstitial cells of Cajal: • ICCs have various frequencies depending on where you are in the GI tract • Frequency varies throughout o Fewer in stomach, more in duodenum • SA node and AV node in heart, but ICC pacemaker cells in the tract gut aren’t congregated in nodes, but are spread diffusely o E.g., 3/min stomach to 12/min duodenum throughout GI tract, explaining why they’re hard to find in the first place • No nodes, any region is capable 2 Modulation of ICC Membrane Potential • ICCs can also be influenced by external stimuli (i.e., autonomic function) o Stretch of material in the gut, release of hormones, and then depending on the innervation, we have different influences • Parasympathetic innervation and stretch cause depolarization of membrane potential – easier for slow waves to hit threshold – easier to fire an action potential and cause contraction of those muscles (e.g., rest and digest) • CCK and sympathetic innervation cause hyperpolarization, bringing membrane potential down away from threshold (more negative), so it’s harder or impossible for a slow wave to hit threshold and fire an action potential (e.g., fight-or-flight) Modulation of GI Muscle Parasympathetic: • Plexuses in the wall of the gut (submucosal and myenteric), with local and long influence on them – short reflex are things like stretch • Increase gut muscle activity and mechanoreceptors, digested products and chemoreceptors, • Relaxes sphincters osmoreceptors etc. that impinge on things like those networks of Sympathetic: nerves contained in submucosal layer or between the muscles • Parasympathetic and sympathetic innervation also comes in for rest • Inhibit gut movements and digest or fight or flight movements • Specialized ICCs are always localized at interface between plexus and • Constricts sphincters smooth muscle cells, never buried deep within muscle, so they can Hormones: communicate what is coming out of the ENS • Either increase (motilin) or decrease (CCK, secretin) activity 3 How Does Food Move in the GI Tract? Interdigestive State: Migrating Motor Complex These are all the motility patterns that occur throughout the GI tract. • Movement through is very distinct based on where it is in the GI tract, and function of motility pattern makes sense for the job that needs to be done in that segment • Know where, the function, and the result, as well as the influences • There are motility patterns in interdigestive state (when there’s no food)  MMC • MMC is a specific motility pattern during interdigestive state Migrating Motor Complex (MMC) MMC has three phases: • MMC is a housekeeping feature • It sweeps undigested material out from stomach, along • Phase 3: rhythmic contraction of circular muscle that propagates small intestine, and deliver to ileum to be dumped into large along length of small intestine every 80-100 minutes intestine for removal • Phase 1 – quiescence, no muscle movement • Phase 2 – irregular phasic contractions • Phase 3 – regular, rhythmic concentrated contractions that moves from stomach through to the ileum, and starts up again to sweep material down, Learning Catalytics 1. On the recording of intraluminal pressure activity in the small intestine below, indicate which section you would identify as Phase 3, and indicate the direction of movement. (Note: recording probe is located at the bottom of the small intestine) |
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