Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19The Digestive System:3. GI MotilityGI Motility• Movement of food from the mouth to the anus •Mechanical mixing of food to: break it into smaller particlesto mix with the digestive substancesFunctionsMuscle types involved in GI Motility• Muscles of mouth, pharynx ,upper esophagus (including upper esophageal sphincter) and external anal sphincter are striated and innervated by somatic motor neurons •Muscles of the rest of GI tract are smooth and innervated by autonomic neuronsGI Motility includes the processes of:a) Ingestion: Taking food into the mouth. b) Mastication: Chewing the food and mixing it with saliva.c) Deglutition: Swallowing the food.d) Peristalsis and Segmentation: Rhythmic, wave-like contractions (peristalsis), and mixing contractions (segmentation) move food through the GI tract.e) Defecation: Discharge of solid or semisolid waste materials from the GI tract through the anus.Oropharyngeal stage of swallowing Oral Phase• muscles of mouth and tongue mix food with saliva and create bolus (a mass of a size to be swallowed)•Skeletal muscle pushes food to back of oral cavity -> triggering of the swallowing reflexPharyngeal Phase•Complex reflex involving nerve coordination of skeletal muscle₋closes the epiglottis₋opens the upper esophageal sphincter ₋propels food into the esophagusThis is the last voluntary act in processing food until the anal end of the GI tract. Figure 21.7•GI smooth muscles contract spontaneously (involuntarily)•GI smooth muscle cells are electrically connected through gap junctions•Different regions exhibit different types of contractions•Tonic contractions•Phasic contractions•Slow wave potentials•3-12 per min•Not all result in contractionGI smooth muscle contractionFigure 21.4Slow waves:Waves of partial depolarization that sweep along the digestive tube for a certain distance-50 to -60mV -0-60Membrane potential (mV)Tension0-60TensionMembrane potential (mV)AcetylcholineSmooth muscle action potentials and contraction•If slow wave potential reaches threshold, calcium channels open and calcium ions enter the muscle fiber.•Smooth muscle contraction is initiated by calcium entering the cell during action potentials.•Strength of contraction is graded and dependent upon the amount of calcium that entered the cell.Mechanisms of Smooth Muscle ContractionFigure 12.26aCross-section showing the layers of the gastrointestinal tract. The ICCs are the stellate cells (shaped like stars) in blue (not to scale) residing between the longitudinal and circular muscular layers. These cells normally function to help mediate gastrointestinal motility. The interstitial cells of Cajal (ICC) generate the slow waves, which pace the contractions in the GI tract.Gap Junctions Propagate Slow Wave Activity(An electrotonic pathway)Electrotonic conduction refers to the passive conduction of current from cell-to-cell via gap junctions. The electrotonic current travels via electrotonic spread, which amounts to simple diffusion of the ions along their electrochemical gradients within the cell.Figure 3.8Modified Figure 21.4c&dMainly occurring in:pharynx, esophagus, gastric body, gastric antrum, and small and large intestineMainly occurring in:small and large intestinePatterns of smooth muscle contractions:Gastric emptying and mixing as a result of peristaltic contractions (“Gastric waves”)Stomach motility is complex and subject to multiple regulatory inputs.•Filling•Storage•Mixing•Emptying•Stomach – about 3 waves per minute•Duodenum – about 12 waves per minute•Ileum – about 8 waves per minuteFeatures that accelerate the gastric wave of contraction:The number of gap junction channels in the antrum of the stomach is greater than in the proximal stomach. 1.The Interstitial Cells of Cajal network is more extensive in the antrum of the stomach.ICC are found interposed between the smooth muscle cells in the circular and longitudinal muscle layers.ICC are found within the circular muscle layer and a network of ICC are found at the intermuscular plane between the circular and longitudinal muscle layers . 2.Motility Road Map Type of Motility Function during mealsIngestionMasticationDeglutitionPeristalsis, primary and secondaryReceptive RelaxationPeristalsisMainly SegmentationLimited PeristalsisSlow SegmentationMass Movement(type of peristalsis)to propel a bolus of food to stomachdifferent movements serveeach function of the stomach:accommodation, mixing, emptying-mixing of chyme with secretions-brings chyme into contact with absorptive epitheliumkneading brings chyme into contact with epithelium forabsorption of water and saltsStarts in transverse colon andmoves fecal matterMotility Road Map Type of Motility Function after mealsempty stomach completely(relax pylorus)empty small intestinemigrating motorcomplex (MMC)(strong progressiveperistaltic waves over long distances)- start at stomach body- 6-8 over nightMotility Road MapFastSlowPropulsion speed:SlowerVery Slow50% of stomach contents emptied2.5 to 3 hoursTotal emptying of the stomach4 to 5 hours50% emptying of the small intestine2.5 to 3 hoursTransit through the colon30 to 40 hoursGI Transit TimeDefecation = Feces leaves the bodyP“Urge to defecate”RelaxContract(Skeletal muscle)External sphincter can be voluntarily relaxed at “socially appropriate times”,followed by contraction of colon, rectum and skeletal muscle under complex neural