IV MOTILITY OF THE SMALL INTESTINE

Function of Intestinal Motility To mix chyme with digestive secretion To bring fresh chyme into contact with the absorptive surface of the microvili To propel chyme toward the colon

Types of small intestinal movement Tonic contraction: the base of the other contractions 2. Segmentation contractions (1) def. When a portion of the small intestine becomes distended with chyme, the stretch of the intestinal wall elicits a rhythmical contraction and relaxation of localized circular muscles spaced at intervals along the intestine, (2) function: mix the chyme with the digestive juice increase its exposure to the mucosal surface Prolong spread out INTESTINAL MOTILITY & SPHINCTER CONTROL SMALL INTESTINE circular and longitudinal muscle layers function to: mix contents with secretions expose all contents to mucosal surface mucosal cell enzymes absorptive surface propel contents through intestine contractile events slow waves initiated in orad duodenum independent of slow waves in stomach independent of contractile events in pylorus have basal rate of 12/min (or 1 every 5 seconds) this rate constant for duodenum and first part of jejeunum rate decreases to 8/min in distal ileum contractions will occur if spike potentials occur on top of slow wave maximum rate of contraction in duodenum would be 12/min contractions occur at intervals which are multiples of 5 seconds spike potentials usually occur in short restricted segments each contraction covers region of 1-4 cm resulting contractions produce segmentation of the bowel effect of each contraction depends on status of adjacent segments mixing will occur with no other concurrent events locally propulsion occurs if coordinated with adjacent segments reflex responses to distension (bolus of material in lumen) local response - myogenic involving enteric plexus (law of the intestine, peristaltic reflex) contraction proximal to region of stretch, relaxation distally intestino-intestino reflex - involves extrinsic neurons response to gross distension with extreme stretch or injury, contractions cease (ileus) response to central input - psychic factors inhibition of contraction in other areas of intestine MMC originates in stomach occurs only in fasting state (postabsorptive) stimulated by motilin

Types of small intestinal movement 3. Peristalsis: propels the small intestinal contents towards the large intestines peristaltic rush：initiated by the harmful stimulation 4. MMC: Occurs during fasting state moves any undigested material still remaining in the small intestine into the large intestine prevents bacteria from remaining in the small intestine long enough to grow and multiply excessively Prolong spread out INTESTINAL MOTILITY & SPHINCTER CONTROL SMALL INTESTINE circular and longitudinal muscle layers function to: mix contents with secretions expose all contents to mucosal surface mucosal cell enzymes absorptive surface propel contents through intestine contractile events slow waves initiated in orad duodenum independent of slow waves in stomach independent of contractile events in pylorus have basal rate of 12/min (or 1 every 5 seconds) this rate constant for duodenum and first part of jejeunum rate decreases to 8/min in distal ileum contractions will occur if spike potentials occur on top of slow wave maximum rate of contraction in duodenum would be 12/min contractions occur at intervals which are multiples of 5 seconds spike potentials usually occur in short restricted segments each contraction covers region of 1-4 cm resulting contractions produce segmentation of the bowel effect of each contraction depends on status of adjacent segments mixing will occur with no other concurrent events locally propulsion occurs if coordinated with adjacent segments reflex responses to distension (bolus of material in lumen) local response - myogenic involving enteric plexus (law of the intestine, peristaltic reflex) contraction proximal to region of stretch, relaxation distally intestino-intestino reflex - involves extrinsic neurons response to gross distension with extreme stretch or injury, contractions cease (ileus) response to central input - psychic factors inhibition of contraction in other areas of intestine MMC originates in stomach occurs only in fasting state (postabsorptive) stimulated by motilin

Segmentation: mix contents to promote digestion & absorption

Peristalsis Distinctive pattern of smooth muscle contractions that propels foodstuffs distally through the esophagus and intestines Mediated by…. Local, intrinsic nervous system Ex: peristalsis is not affect to any significant degree by vagotomy or sympathectomy

Small Intestinal Motility Peristalsis: movement along the tract

Peristalsis Figure 24.4

peristalsis

Peristalsis of the small intestine http://medweb.bham.ac.uk/research/toescu/Teaching/OverviewGITY2.html

Peristalsis – law of intestine Berne et al., 2004

Peristalsis – law of intestine Berne et al., 2004

Peristalsis – law of intestine Berne et al., 2004

Peristalsis – law of intestine Berne et al., 2004

Peristalsis – law of intestine Berne et al., 2004

Peristalsis – law of intestine Bolus of food →Mechanical distension and mucosal irritation → stimulates afferent enteric neurons → 2 effects Excitatory motor neurons above the bolus activated → contraction of smooth muscle above the bolus Via Ach, substance P 2. Inhibitory motor neurons → stimulate relaxation of smooth muscle below the bolus Via nitric oxide, vasoactive intestinal peptide and ATP

Control of Intestinal Motility – Neuronal Mixing – segmentation Frequency set by slow waves (12/minute duodunum) additional control: myenteric plexus Propulsion – peristalsis Local reflex – stretch causes relaxation distal and contraction proximal (Bayliss – Starling law of the intestines) Moves bolus through intestines Intestino-intestinal reflex – extrinsic nerves Local stretch in one area inhibits contraction in rest of bowel motility controlled by neural input intrinsic (myenteric or Auerbach's plexus) tonic inhibition of circular tonic inhibition inhibited by opioids (increased segmental contraction) Extrinsic extrinsic sympathetic: inhibitory parasympathetic (vagal) vagal input increases force of contraction because: vagal input inhibits tonic inhibition (disinhibition) vagal effects inhibited by nitric oxide (relaxation

Movement in Small Intestine Peristalsis (ou propulsivos) e Segmentation (mixing) http://medweb.bham.ac.uk/research/toescu/Teaching/OverviewGITY2.html

Central and peripheral control of contractile patterns Intestinal wall Vagal centre lumenl Peptide (CCK) Receptors Glucose - Osmolality Long chain fatty acids Amino acids Sensory neurons Vago-vagal reflexes Interneurons Integrating circuits Program circuits Enteric nervous system Motorneurons Contractile patterns Luminal stimuli elicit vago-vagal reflexes which activate integrating and program circuits of the enteric nervous system. These activate specific motorneurones responsible for specific contractile patterns.

Control of Intestinal Motility – Hormonal Gastrin Secretin CCK + motility -- Glucagon 5-HT VIP Motilin GIP hormonal & paracrine influences inhibitory to contraction catecholamines secretin glucagon nitric oxide VIP excitatory to contraction serotonin gastrin cholecystokinin insulin (motilin) opioids

Ileocecal Valve What it is Opening to large intestines Function: (1) prevent the repulsion (2) control the emptying normally closed. Short-range peristalsis in terminal ileum and distension relaxes IC sphincter --> small amount of chyme is squirted into the cecum. Distension of cecum contracts IC sphincter. Gastro-ileal reflex: enhances ileal emptying after eating. The hormone gastrin relaxes ileocecal sphincter. Wall of ileum immediately preceding valve has thickened muscular coat called ileocecal sphincter Normally constricted and slows emptying of ileal contents into cecum except immediately after a meal, when gastroileal reflex intensifies peristalsis in ileum Functions to prevent backflow of fecal contents from colon to SI are forcefully closed when any excess pressure builds up in cecum, usually 50-60 cm H2O. Resistance to emptying at ileocecal valve prolongs stay of chyme in ileum and therefore facilitates absorption. When cecum distended, contraction of sphincter intensified, ileal peristalsis inhibited. When there is irritant in cecum, emptying delayed. As in acute appendicitis, which causes intense spasm of sphincter, ileal paralysis and blocks emptying of ileum.

V. GASTROINTESTINAL MOTILITY DURING FASTING STATE

Gastric motility on fasting “Migrating Motor Complex, MMC” Occurs during fasting To clear undigested food particles Peristaltic contractions sweep down stomach and duodenum – pylorus relaxes Pattern of contraction approx every 90 min Slow peristaltic waves sweeping whole of GI tract Thought to be controlled by motilin (胃动素） SWEEP approx

MMC(migrating motor complex) contents PhaseⅠ: Almost have no contractions 40-60 min PhaseⅡ: have contractions, only have few 30-45 min PhaseⅢ: have continuous contractions 5-10 min

The interdigestive motility consists of three phases Interdigestive Cycles Phases Sporadic peristaltic waves Segmenting contractions and single Motor quiescence of stomach and duodenum Contraction of reservoir Pylorus Aboral migration Accumulation of residues of chyme Phase II Phase I Stomach Duodenum Jejunum Ileum Phase III III I II Forceful The phase III of the migrating motor complex (of dog) Originates simultaneously at the stomach and duodenum Migrates within 90 to 120 minutes along the small intestine

Importance of MMC Sweep the contents of the small intestine towards the colon Housekeeper of the small intestine Inhibit the migration of colonic bacteria into the terminal ileum

VI MOTILITY OF THE COLON

Large intestine Functions Motility patterns Absorption of water and electrolytes Storage of feces In non-ruminant herbivores, fermentative digestion and absorption of nutrients Motility patterns mixing (form haustrations) propulsive (mass movements)

Segmentation in large intestine Haustration: （结肠袋）modified form of segmentation in which intense, local contraction of circular muscle causes large intestine to appear to bulge into sacs

Mass movement Occurs in colon; also known to occur in equine cecum Period of intense propulsive activity that moves entire contents of colon distally toward rectum Contractions progress for long distance such that long length of colon contracts as a unit Entry of fecal matter into recturn triggers defecation reflex

Mass Movement

Defecation Defecation Reflex initiated when rectal walls stretch ß parasympathetic reflex walls of the sigmoid colon and the rectum to contract & relaxation of the anal sphincter External sphincter control is voluntary control If defecation is delayed: the reflex stops until the next mass movement