1 IV MOTILITY OF THE SMALL INTESTINE

2 Function of Intestinal Motility (1)To mix chyme with digestive secretion (2)To bring fresh chyme into contact with the absorptive surface of the microvili (3)To propel chyme toward the colon

3 Contractions of Intestinal Smooth Muscles Occur automatically in response to endogenous pacemaker activity. Rhythm of contractions is paced by graded depolarizations called slow waves. Slow waves produced by interstitial cells of Cajal. Slow waves spread from 1 smooth muscle cell to another through nexuses.

4 Cells and Electrical Events in the Muscularis Insert fig

5

6 When slow waves above threshold, it triggers APs by opening of VG Ca 2+ channels. Inward flow of Ca 2+ : Produces the upward depolarization phase. Stimulates contraction of smooth muscle. Repolarization: VG K + channels open. Slow waves decrease in amplitude as they are conducted. May stimulate contraction in proportion to the magnitude of depolarization. Contractions of Intestinal Smooth Muscles

Vg Ca ++ Channel Vg K + Channel Intestinal Smooth Muscle Action Potential

8 Parasympathetic NS, stretch and gastrin increase the amplitude of slow waves. Stimulate APs. SNS decrease APs. Contractions of Intestinal Smooth Muscles

9 1.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 Types of small intestinal movement

10 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 Types of small intestinal movement

11 Segmentation: mix contents to promote digestion & absorption

12 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

13 Small Intestinal Motility Peristalsis: movement along the tract

14 Peristalsis Figure 24.4

15 peristalsis

16 Peristalsis of the small intestine

17 Berne et al., 2004 Peristalsis – law of intestine

18 Berne et al., 2004 Peristalsis – law of intestine

19 Berne et al., 2004 Peristalsis – law of intestine

20 Berne et al., 2004 Peristalsis – law of intestine

21 Peristalsis – law of intestine Berne et al., 2004

22 Peristalsis – law of intestine Bolus of food →Mechanical distension and mucosal irritation → stimulates afferent enteric neurons → 2 effects 1.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

23 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

24 Movement in Small Intestine Peristalsis (ou propulsivos) e Segmentation (mixing)

25 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. Central and peripheral control of contractile patterns Intestinal wall Vagal centre Intestinal 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

26 Gastrin Secretin CCK + motility -- Glucagon 5-HT VIP Motilin GIP Control of Intestinal Motility – Hormonal

27 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.

28 V. GASTROINTESTINAL MOTILITY DURING FASTING STATE

29 Gastric motility on fasting “Migrating Motor Complex, MMC” Occurs on 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

30 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 Interdigestive Cycles Phases Sporadic peristaltic waves Segmenting contractions and single peristaltic waves Motor quiescence of stomach and duodenum Contraction of reservoir Pylorus Aboral migration Accumulation of residues of chyme Phase IIPhase I Stomach Duodenum Jejunum Ileum Phase III III I II III Phase III Phase II Phase I Forceful peristaltic waves Motor quiescence The interdigestive motility consists of three phases

31 MMC(migrating motor complex) Phase Ⅰ : Almost have no contractions Phase Ⅱ : have contractions, only have few Phase Ⅲ : have continuous contractions

32 Migrating myoelectric Complex

33

34 The antral waves are associated with a wide opening of the pylorus and inhibition of duodenal contractions followed by duodenal peristaltic waves occurring at maximal frequency. Middle Antrum Pyloric diameter Duodenal bulb Duodenum Gastric phases III 1 min P A P P 0 mm 6 mm Stomach is cleaned of residues of chyme and secretions. Gastric phase III consisting of forceful contractions

35 Phase III of the interdigestive” (MMC) oral aboral Aboral migration of phase III Velocity of the peristaltic waves 1 minute Jejunal phase III (MMC) Rectangles: strain gauge transducers, Data of dog.

36 Phase III (MMC) of the small intestine Intestinal phase III oral Successsive peristaltic waves Chyme Slow aboral migration of phase III aboral Time (about 20 sec) The peristaltic waves clean the intestinal segment from chyme which accumulates aborally. Because the successive waves start and end further aborally the phase III slowly migrates distally

37 Importance of MMC 1.Sweep the contents of the small intestine towards the colon Housekeeper of the small intestine 2.Inhibit the migration of colonic bacteria into the terminal ileum

38 Postprandial motility is characterised by  a lower amplitude of the antral waves occurring at maximal frequency,  rhythmic pyloric opening and closure  co-ordinated duodenal contractions occurring in sequence with the antral waves Ingestion of a meal suppresses the interdigestive motility and induces a fed motor pattern

39 VI MOTILITY OF THE COLON

40 Large intestine Functions Absorption of water and electrolytes Storage of feces In non-ruminant herbivores, fermentative digestion and absorption of nutrients Motility patterns Segmentation Antiperistalsis Mass movement

41 Large intestinal motility patterns Segmentation Antiperistalsis Antiperistalsis moves large intestinal contents cranially

42 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

43 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

44 Defecation

45 VII GI Reflexes Gastrocolic Increase in colonic activity after a meal Distention of the stomach stimulates evacuation of the colon Enterogastric Distention and irritation of the small intestine results in suppression of secretion and motor activity in the stomach Colocolonic Propels stool caudally by proximal muscle constriction and distal dilatation Mediated by myenteric plexus

46 Rectocolic Colonic peristalsis due to stimulation of rectum Mediated by pelvic nerve Intestinointestinal Reflex: When a part of the intestine becomes overdistended or its mucosa becomes excessively irritated, activity in other parts of the intestine is inhibited as long as the distention persists. Gastroileal Reflex: An increase in ileal motility and opening of the ileocecal valve when food enters the empty stomach