Motility in the small intestine

The smooth muscle lining the small intestine performs two functions. First, it is responsible for a thorough mixing of digestive juices arriving from the pancreas and liver via the common bile duct with the chyme received from the stomach. Second, it is responsible for moving the contents, usually slowly but occasionally rapidly, along the 6 m from the stomach to the ileo-caecal valve. This movement enables one meal to make way for the next. It is vitally important that food is retained in each part of the small intestine for sufficient time to allow for mixing, digestion and absorption of food.

Within the small bowel, three types of movement contribute to the mixing of chyme:

• Concentric, segmenting contractions. Segmentation helps to mix the secretions of the small intestine with the chyme particles.

• Peristaltic waves or short, propulsive contractions. These slowly push the chyme in the direction of the ileo-caecal valve. Peristaltic waves are strongest in the proximal portion of the small bowel.

• The continuous shortening and lengthening of the villi constantly stirs the intestinal contents.

As chyme approaches the large bowel, contractions in the ileum increase. Control of motility in the small intestine

Motility in the small intestine is under physiological control via several factors, including stretch, extrinsic autonomic nerves, intrinsic nerves of the intramural plexi paracrine factors and circulating hormones.

Neural control of motility in the small intestine

Activation of the intrinsic nerves in the intramural plexi can control segmentation and short peristaltic waves by influencing the basal electrical rhythm, in the absence of hormones or extrinsic nerves. Segmentation and peristalsis are increased by activation of parasympathetic nerves, and inhibited by stimulation of sympathetic nerves. Activation of the sympathetic nervous system, in response to stress for example, results in the release of adrenalin into the circulation, which inhibits intestinal motility. Sympathetic activation also results in vasoconstriction of the blood vessels in the small intestine.

There are many other transmitters, in addition to acetylcholine and catecho-lamines (adrenalin and noradrenalin), which can influence motility in the small bowel. These include peptides, amines and nucleotides. The peptides include vasoactive intestinal peptide and somatostatin.

Hormonal control of motility in the small intestine

Gastrin, which is released into the blood in response to the presence of peptides in the stomach, and secretin and cholecystokinin, released into the blood in response to the presence of fats and acids in intestinal chyme, all increase intestinal motility. Motilin, a peptide released from the walls of the small intestine into the blood when the intestinal chyme becomes alkaline, increases intestinal motility. Another peptide which is released in the presence of chyme in the small intestine is enteroglucagon. It is released in response to particles of glucose and fat in the chyme. This hormone inhibits peristalsis, and its role allows additional time for absorption of glucose and fat before the chyme reaches the ileo-caecal valve.

Reflex control of motility in the small intestine

Activation of pressure receptors by the distension of the intestinal walls is involved in the reflex control of intestinal motility. A bolus of food placed in the small intestine will cause smooth muscle behind it to contract, and in front of it to relax. When food is present in the stomach, motility increases in the ileum and the ileo-caecal sphincter relaxes. This is known as the gastro-ileal reflex. This reflex appears to be mainly under the control of external nerves to the mucosa of the intestine; however, gastrin released into the blood in response to chyme in the stomach may augment this reflex.

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