SMALL INTESTINE 1 Motility and Secretion

1. SMALL INTESTINE 1Motility and Secretion DR. AMEL EASSAWI DR. Sharique Ahmed Quadri

2. SMALL INTESTINE • In small intestine most digestion and absorption takes place. • Small intestine is divided into duodenum, jejunum and ileum. • In small intestine we will study: 1. Motility 2. Secretion 3. Digestion 4. Absorption

3. SMALL INTESTINE‘MOTILITY’ Motility in small intestine includes: 1. Segmentation 2. Migrating Motility Complex [MMC]

4. SMALL INTESTINE ‘MOTILITY’ Segmentation: • Segmentation consist of oscillating, ring like contractions of smooth muscle. • Between the contracted segments are relaxed area containing small amount of Chyme. • Contractile ring occur every few centimeter, dividing the small intestine into segments. • These segmentation contraction cause mixing of Chyme and move the food.

5. SEGMENTATION

6. SMALL INTESTINE ‘MOTILITY’ Segmentation – Initiation and Control • Contraction are initiated by the small intestine pace-maker cells, which produce Basic Electrical Rhythm [BER] or slow wave. • When BER reaches the threshold levelin the circular smooth muscle segmentation are induced. • Both duodenum and ileum start to segment as meal enter the small intestine.

7. SMALL INTESTINE ‘MOTILITY’ Segmentation – Initiation and Control • Increased segmentation is caused by local distension, gastrin secreted in stomach [gastro ileal reflex]. • Parasympathetic stimulation. • Sympathetic stimulation decreases segmental activity.

8. SMALL INTESTINE ‘MOTILITY’ Functions of Segmentation • Mixing of food with digestive juices. • Exposing all Chymeto small intestine mucosa for absorption. - Propelling [moving the food]. - Segmentation occur about 12/min in duodenum and 9/min in terminal ileum. - Contents take 3 to 5 hours to move through small intestine.

9. SMALL INTESTINE ‘MOTILITY’ 2. Migrating Motility Complex [MMC] • MMC sweeps the intestine between meals. • When most of the meals are absorbed, segmentation contraction stop and they are replaced by MMC in between the meals. • What is MMC ? • It consist of weak, repetitive peristaltic wave that move a short distance in the small intestine before dying out.

10. SMALL INTESTINE ‘MOTILITY’ Functions of Migrating Motility Complex [MMC] • MMC starts at the stomach and migrate down the intestine. • Function of MMC is to move [sweep] the remaining contents of previous meal to the colon, therefore, MMC is called Intestinal House Keeper. • MMC is regulated by hormone motilin. • Motilin hormone is secreted by endocrine cells of small intestinal mucosa.

11. Ileocecal Sphincter • Ileum opens into the cecum. • Ileocecal Sphincter prevents contamination of small intestine from chronic bacteria. Two factors: • ileocecal valve: valve like folds of tissue protrude from ileum into the lumen of caecum. • Ileocecal sphincter: smooth muscle in the wall of last several centimeter of ileum is thickened to form sphincter.

12. Ileocecal Sphincter • Pushing of Ileal contents forward in cecum causes Ileocecal valve to open. • Ileocecal Sphincter [smooth muscle] is under neural [local myentric] and hormonal control. • Distension on ileal side causes the sphincter to open ---reaction mediated by intrinsic plexus. • Gastrin release at onset of meal causes relaxation.

13. Ileocecal Sphincter

14. SMALL INTESTINE‘SECRETION’ – Succus Entricus • Small intestinal secretion is called succus entricus. • Succus entricus does not contain any digestive enzymes. • But enzymes are present in brush border [micro villi] in small intestine.

15. SMALL INTESTINE‘SECRETION’ – SuccusEntricus • SuccusEntricus is secreted into the lumen by exocrine glands present in small intestine mucosa. It consist of: - Water - Mucus - Mineral Salts - pH 7.8 • Secreted 1.5 liters per day. • Secretion increases after meal due to local stimulation of small intestine mucosa by presence of Chyme.

16. SMALL INTESTINE‘DIGESTION’ • In small intestine, enzyme present in the brush border membrane complete the digestion. • In the lumen of small intestine, pancreatic enzyme are acting for fat digestion, being helped by bile, therefore, fats are completely reduced to absorble units of manocylecrides and free fatty acids. • Protein are broken down into peptide fragments and amino acids by trypsin and chymotrypsin. • Carbohydrate are reduced to disaccharides and some monosaccharides. • Therefore fat digestion is completed in small intestine lumen but carbohydrate and protein digestion is not complete yet.

17. SMALL INTESTINE‘DIGESTION’ • In the lumen surface of small intestine on the epithelial cells there are MicroVilli – hair like projection forming the brush border. • What is brush border? • It is plasma membrane and contains three types of enzyme: 1.Enterokinase – which activates pancreatic Proteolytic enzyme Trypsinogen. 2.Diasaccharidases – maltase, sucrase and lactase, which complete digestion of maltose, sucrose, and lactose into manosaccharides. 3.Amino peptidases – which hydrolyze small peptides into amino acids, thereby completing protein digestion. Therefore, carbohydrate and protein digestion are completed in brush border.

19. CLINCAL Applied Lactose Intolerance • Occurs due to deficiency of lactase enzyme, therefore, digestion of lactose [milk and milk products] are not digested properly and lead to action of bacteria in the large intestine producing large quantity of CO2 and methane gas, distension of intestine, cramps and diarrhea.

20. References • Human physiology by Lauralee Sherwood, seventh edition • Text book physiology by Guyton &Hall,11th edition • Text book of physiology by Linda .s contanzo,third edition