SMALL INTESTINE 1Motility and Secretion

DR. AMEL EASSAWIDR. Sharique Ahmed Quadri

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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

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SMALL INTESTINE‘MOTILITY’

Motility in small intestine includes: 1. Segmentation 2. Migrating Motility Complex [MMC]

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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.

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SEGMENTATION

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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 level in the circular smooth muscle segmentation are induced.

• Both duodenum and ileum start to segment as meal enter the small intestine.

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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.

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SMALL INTESTINE ‘MOTILITY’

Functions of Segmentation- Mixing of food with digestive juices.- Exposing all Chyme to 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.

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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.

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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.

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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.

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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.

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ILEOCECAL SPHINCTER

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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.

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SMALL INTESTINE‘SECRETION’ – Succus Entricus

• Succus Entricus 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.

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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.

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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.

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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.

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

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