1. GI Motility part II
Lecture 22

2. Learning objectives
Types or patterns of motility in small intestine & functions of each type
Types of motility in the large intestine & functions of each type
Mechanism & control of defecation reflex

3. Learning outcomes
22.1 Describe the patterns of motility in small intestine and large intestine
22.2 Explain the mechanism of defecation
22.3 Explain the pathophysiology of diarrhoea and constipation
22.4 List the common gastrointestinal motility disorders
22.5 Explain the pathophysiology of irritable bowel syndrome

4. Intestine has extrinsic & intrinsic innervations.
Intrinsic myentric plexus must for motor activity. It’s activity is modulated by extrinsic nerves-
- i.e sympathetic noradrenergic generally inhibitory to smooth muscle, while causing sphincters to contract.
- parasympathetic excitatory to smooth muscle.
Hormones & drugs also affect the intestinal movements.

5. Law of Intestine (EH Starling & Bayliss)
When a bolus is placed in the small intestine, the portion behind the bolus contracts and portion ahead of it relaxes. This propels the bolus from oral to aboral direction.

6. GI transit time

7. Electrical activities of small intestine:
Basal electrical rhythm (BER)
Spontaneous fluctuation in membrane potential ranging from -65 to -45mv.
Generated by Interstitial cells of Cajal in the myenteric plexus which act as pace maker throughout GIT, frequency variable.

8. These rarely lead to muscle contraction, but spike potentials are superimposed on depolarizing phase of BER, which do increase muscle tension.
Function of BER is to coordinate peristalsis & other motor activities.
Ionic basis: Depolarizing of spike potential is due to calcium influx & repolarization is due potassium efflux.

9. MMC ( Migrating Motor Complex )
During interdigestive period or fasting this type of electrical activity originate in the stomach, terminate in terminal ileum.
Velocity 5cm / min, controlled by enteric NS
Inhibited by ingestion of food, as it is replaced by peristalsis when chyme enters.
Motilin stimulates MMC.

10. Functions of MMC:
1. Clears the digestive tract of luminal contents in preparation for the next meal (Thus called housekeeper of the small intestine)
2. Inhibits colonization of small intestine by colonic bacteria
3. Gastric, bile, pancreatic secretions increase with each MMC

11. Fundamental movements seen in the fed state in small intestine
Peristalsis (Propulsive)
Segmentation contraction (Mixing)
Tonic contractions
Movements caused by muscularis mucosa and muscle fibers of villi

12. Peristalsis already described.
Segmentation contractions
Ring like contractions that appear at fairly regular intervals along the gut, then disappear and then replaced by another set of ring contractions in the segments between the previous contractions.

13. .
Alternate segments of contraction and relaxation repeating at regular intervals when the chyme is inside

14. These contractions are determined by slow electrical waves of SI (BER), so the rate depends on it.
Normally 12 at duodenum & jejunum, weakens as moves down (9 at ileum)
If excitatory activity of enteric nervous system is blocked by atropine , these contractions become too weak.

15. Functions of segmentation contraction:
Chops the chyme 2-3times/min, helps in progressive mixing of chyme with intestinal secretions, so helps in digestion.
Slow to and fro contractions permits longer contact of the chyme with mucosal surface and permits absorption

16. Tonic contractions:
Slow and sustained contraction of large segments of intestine, relatively for prolonged period so isolates one segment of the intestine with other.
Function same as segmentation.

17. Peristaltic rushes:
these are very intense peristaltic waves not seen normally , observed during intestinal obstructions, intense irritation of mucosa or severe cases of infectious diarrhea.
function: since powerful strong peristalsis, traveling long distances, sweeps the contents of SI into colon and relieves SI of irritative chyme & excessive distension

18. Movements caused by muscularis mucosa and muscle fibers of villi:
can cause short folds to appear in the mucosa & individual muscle fibers extend into villi & cause intermittent contraction of villi which increases surface area exposed to chyme, favours absorption & also milking effect on villi (shortening & elongating) helps free flow of lymph.

19. Intestinointestinal reflex : Overdistention of one segment of intestine relaxes smooth muscle in the rest of the intestine
Gastroileal reflex : when the food leaves the stomach ileocecal sphincter opens, presumed to be a vagally mediated reflex.

20. Motility of colon
1. Peristalsis
2. Segmentation contraction (haustration)
3. Mass action contraction or mass peristalsis (unique to colon)
First 2 similar to the movements seen in small intestine.

21. Mass action contraction or mass peristalsis
Simultaneous contraction of the smooth muscle over the large confluent areas, occurs twice a day.
A ring of constriction appears near the splenic flexure, travels down sweeping the descending colon, pushing the feces into rectum.
Function- moves material from one portion of the colon to other.
Also move the material into rectum, rectal distension in turn initiates defecation reflex.

22. Transit time in the small intestine & colon
First portion of test meal reaches cecum in 4 hrs.
All of the undigested portion enter splenic flexure in 8-9hrs.
Pelvic colon in 12 hrs.
Pelvic colon to anus is much slower.
After 72 hrs also 25% still remains.

24. Defecation reflex
Stimulus- distension of the rectum with feces, following a mass peristalsis.
Normally defecation is a spinal level reflex, but to some extent voluntarily can be controlled.
Reflex consists of development of peristaltic waves in the sigmoid colon & rectum pushing the feces into anus, increased pressure in anus, relaxation of internal anal sphincter.

25. Internal anal sphincter (involuntary) supplied by ANS
Internal anal sphincter (involuntary) supplied by ANS. Symp excitatory, parasympa is inhibitory. Sphincter relaxes with distension of rectum.
External anal sphincter (voluntary) supplied by pudendal N. Normally it is at sustained contraction.

27. Urge to defecate first occurs at a rectal pressure of 18 mm Hg
Urge to defecate first occurs at a rectal pressure of 18 mm Hg. Above 55 mm Hg both internal & external sphincters relax automatically & rectal contents expelled out.
Between these pressures voluntarily defecation can be inhibited by higher centers (by spastic contraction of ext anal sphincter) or initiated by relaxing external anal sphincter.

28. In adults habits & cultural factors play a large role in determining when defecation occurs.
By voluntary efforts, straining which consists of violent expiratory efforts with glottis closed , raises intra abdominal pressure, thus aiding reflex emptying of distended rectum.

29. Gastrocolic reflex:
Distension of the stomach by food initiates contractions of the rectum, and frequently a desire to defecate. Because of the response, defecation after meals is a rule in children.

30. Intestinal dysmotility, intestinal pseudo-obstruction
Symptoms are bloating, pain, nausea, and vomiting. Results from weak or unsynchronized contractions
Some patients have contractions that are strong enough, but they are too disorganized or non-peristaltic to move food along. Cause may be neural, which coordinate (synchronize) the contractions of the intestine
Small bowel bacterial overgrowth.

31. Disorders of motility of large intestine:
Constipation:
infrequent bowel movements (less than 3 per week), passage of hard stools, and sometimes difficulty in passing stool
Diarrhea:
an excessive number of high amplitude propagating contractions. Symptoms are frequent, loose or watery stools, and a subjective sense of urgency.

32. Hirschspurng's disease or megacolon: is a rare congenital disorder, caused by absence of nerve cells (ganglion cells deficiency in the myentric plexus) in the rectum and/or colon. Here occasionally constipation so severe that bowel movements may occur only once a week or so, leads to accumulation of large quantities of fecal matter, so colon may distend to a diameter of 3-4 inches.

33. Irritable bowel syndrome : group of symptoms that occur together
Irritable bowel syndrome : group of symptoms that occur together. The normal functioning of the bowel are affected. Sometimes they move too much or too often, and sometimes they don't move enough, nerves in bowels may be more sensitive to stretch or movement and this can lead to more pain.