Intestinal movements- Dr Prafull Turerao.

Movements of intestine:their mechanism,function and regulation.

1. Movements of Small and Large Intestine Dr Prafull H Turerao. Dr D.Y.Patil Medical College, Kolhapur Tutor, Department of Physiology

2. There are two types movements of small intestine i.e. segmentation movement and peristaltic movements Movements of small intestine

3. Segmentation movements – Distension of small intestine with chyme stretches the intestinal wall and initiates segmentation movements. There are localized concentric contractions (about 1-2 cm in length) spaced at intervals along the intestine. They divide the intestine into spaced segments.

4. These contractions force the chyme back towards the stomach and towards the colon and last for 5-6 seconds. When muscle relaxes, chyme comes back to the area from which it is displaced.

5. A new set of contractions then begins at new points between previous contractions. Segmentation movements occur at a rete of 8-12 min. Maximum frequency is present in duodenum ( 12/min) and proximal jejunum . In terminal ileum it is 8 to 9/min

6. Functions – Segmentation contractions move the food back and forth as explained above. This enables chyme to become thoroughly mixed with the digestive juices and to make proper contact with the absorptive surface of the intestinal mucosa.

7. Higher frequency of segmentation in proximal intestine than in distal intestine propels the chyme slowly towards the colon .

8. Control – The segmentation contractions can occur only if the slow waves (basic electrical rhythm) produce action potential. Action potential appears on slow waves when the membrane potential is sufficiently depolarized.

9. Frequency of segmentation is directly related to the frequency of slow waves and is controlled by pacemaker cells of small intestine.

10. But segmentation contractions become very weak on blocking the nervous system by atropine. Contractions are not really effective without the background excitation by enteric nervous system (especially myenteric plexus ) though slow wave in smooth muscle control the segmentation contractions.

11. Slow wave amplitude is also increased by hormones, gastrin, cholecystokinin, motilin and insulin. Secretin and glucagon reduce the slow wave amplitude.

12. Peristalsis is a wave of contraction precede by wave of relaxation. Peristaltic wave can be initiated in any part of the small intestine. Peristaltic movements

13. It moves anal ward at a rate of 0.5-2 cm/min but it is weak and dies out after travelling only 3-5 cm very rarely up to 10 cm. So net movement of chyme in anal ward direction is slow (1cm /min)

14. Functions – Peristaltic waves propel the chyme in anal ward direction. As chyme enters the intestine, it spreads along the entire length of intestine due to peristaltic waves for proper digestion and absorption.

15. Control – Peristaltic wave is controlled by nervous and hormonal factors.

16. Nervous control – Peristalsis on the small intestine increases immediately after meals due to gastroenteric reflex. Distension of stomach due to meal initiates this reflex. Impulses pass through myenteric plexus from stomach to the small intestine along the wall of small intestine causing increase in peristaltic activity. Nervous control

17. Hormonal control – Gastrin , cholecystokinin, insulin and serotonin enhance the intestinal motility. Secretin and glucagon decrease the motility. Hormonal control

18. Intense irritation of intestinal mucosa causes very powerful and rapid peristalsis called peristaltic rush. It is partly initiated by extrinsic nervous reflex and partly by myenteric reflex. Peristaltic Rush

19. Powerful peristaltic contractions travel long distances in small intestine within minutes. It sweeps the contents of intestine into the colon and relieves small intestine of either irritative chyme or excessive distension.

20. Ileocecal valve prevents back flow of faecal contents from colon into the intestine. The valve usually resists pressure of 50-60 cm of water. Immediately preceding the ileocecal valve, the wall of ileum for several centimetre has a thickened muscular coat which is called ileocecal sphincter.

21. Tonic contractions of this sphincter slows emptying of meal contents into the caecum. The contents stay in the ileum for long time which facilitates absorption. Gastroileal reflex and gastrin cause relaxation of ileocecal sphincter. Its degree of contraction is also controlled by reflexes from caecum

22. There are two types of movements of large intestine namely haustral contraction and mass movement. Their functions are to increase the efficiency of colon for water and electrolytes absorption and promote excretion of faecal matter. Movements of Large intestine

23. Haustral contractions are similar to segmentation movements of small intestine. Large circular bands of constriction occur at regular intervals. At each of these constrictive points 2.5 cm of circular muscle contracts. Longitudinal muscles (tineae coli) also contract. This causes unstimulated portions of large intestine to bulge in a bag like sacs called haustrations. Haustral contractions

24. Mass movements only occur 3-4 times a day. They are just like peristalsis of small intestine. Much propulsion of faecal matter in caecum and ascending colon results due to haustral contractions. Mass movement of large intestine

25. From beginning of transverse colon to the sigmoid colon mass movements cause propulsion. Mass movements are initiated after breakfast or meal due to gastrocolic or duodenocolic reflexes, initiated by distension of stomach or duodenum.

26. First constrictive ring appears at irritated point (usually transverse colon), then about 20 cm colon distal to it contracts as a unit forcing faecal matter in this segment down the colon. Relaxation occurs within 2-3 minutes.

27. Then mass movement is initiated at the next point. Usually mass movements once initiated persist for about 10-30 minutes . Irritation of colon can also initiate mass movements. When mass movements force the faecal matter into the rectum, desire for defecation is felt.

28. Process by which faecal matter is excreted is called defecation. Usually the rectum is empty. Due to mass movements, faecal matter enters the rectum causing distension. Defaecation

29. A recto sphincteric reflex relaxes the anal sphincter and generate urge to defecate. Defaecation involves both voluntary and reflex activity. Ordinarily defaecation is initiated by defaecation reflexes as follows.

30. Intrinsic reflex – It is mediated by intrinsic nerve reflex. Distension of rectum with faeces initiates afferent signals which pass through myenteric plexus to descending colon to initiate a peristaltic wave. Defaecation reflex

31. As peristaltic wave approaches lower end of rectum it causes relaxation of internal anal sphincter (made up of smooth muscle ). If external sphincter is relaxed ( which is made up of skeletal muscle and is under voluntary control) defaecation occurs.

32. Spinal cord reflex – Intrinsic reflex is weak and is reinforced by this reflex. Distension of rectum due to faeces causes transmission of signals to sacral segments of spinal cord.

33. Signals are transmitted from here through pelvic nerves ( parasympathetic fibres) to colon to intensify the peristaltic waves and relaxation of inner anal sphincter. It converts intrinsic defaecation that results into emptying of large bowel from splenic flexure to anus

34. In addition to defaecation reflexes, the abdominal muscles and diaphragm contract increase the intra abdominal pressure forcing the faeces through the anal canal. Defaecation can be prevented by voluntarily contracting the external anal sphincter.

35. If defaecation does not occur, internal anal sphincter closes, rectum relaxes to accommodate faecal matter within it. In new-born babies or in person with transected spinal cord, defaecation reflex causes automatic emptying of lower bowel without normal voluntary control on external anal sphincter

36. Movements of small intestine Segmentation movements Peristaltic movements Peristaltic Rush Movements of Large intestine Haustral contractions Mass movement of large intestine Defaecation reflex Summary

37. Textbook of Medical Physiology – Guyton And Hall 13th Edition Textbook of Physiology – A K Jain 6th Edition Medical Physiology For Undergraduate Students – Indu Khurana Net Sources Images. References

38. Thank You