Movements of Small Intestine Mixing contractions Propulsive Contractions
Movements of Small Intestine Mixing contractions Propulsive Contractions All movements of the small intestine cause at least some degree of both mixing and
propulsion. Mixing Contractions (Segmentation Contractions) When a portion of the small intestine becomes distended with chyme, stretching of
the intestinal wall elicits localized concentric contractions spaced at intervals along the intestine and lasting a fraction of a minute. As one set of segmentation contractions
relaxes, a new set often begins but the contractions this time occur mainly at new points between the previous contractions. The segmentation contractions chop the chyme two to three times per minute, in this
way promoting progressive mixing of the food with secretions of the small intestine. The maximum frequency of the segmentation contractions in the small intestine is determined by the frequency of electrical slow
waves in the intestinal wall Propulsive Contractions(Peristalsis) Chyme is propelled through the small intestine by peristaltic waves. Peristasis can occur in any part of the small intestine
It moves toward the anus at a velocity of 0.5 to 2.0 cm/sec Faster in the proximal intestine and slower in the terminal intestine. They normally are very weak and usually die out after traveling only 3 to 5 centimeters
Net movement along the small intestine normally averages only 1 cm/min 3 to 5 hours are required for passage of chyme from the pylorus to the ileocecal valve.
Control of Peristalsis by Nrevous and Hormonal Signals Peristaltic activity of the small intestine is greatly increased after a meal. Stretch
Gastroenteric reflex Hormones Gastrin, CCK, Motilin, Insulin Secretin, Glucagon The function of the peristaltic waves in the
small intestine is not only to cause progression of chyme toward the ileocecal valve but also to spread out the chyme along the intestinal mucosa. Propulsive Effect of Segmentation
Contractions The segmentation movements although lasting for only a few seconds at a time often also travel 1 centimeter or so in the anal direction and during that time help propel the
food down the intestine. Peristaltic Rush Although peristalsis in the small intestine is normally weak intense irritation of the intestinal mucosa as occurs in some severe
cases of infectious diarrhea can cause both powerful and rapid peristalsis called the peristaltic rush. Movements Caused by the Muscularis Mucosae and Muscle Fibers of the Villi
The muscularis mucosae can cause short folds to appear in the intestinal mucosa The mucosal folds increase the surface area exposed to the chyme thereby increasing absorption.
Individual fibers from this muscle extend into the intestinal villi and cause them to contract intermittently. Contractions of the villishortening, elongating, and shortening againmilk the villi so that lymph flows freely from the central
lacteals of the villi into the lymphatic system These movements are initiated by submucosal plexus Function of the ileocecal valve Function of the ileocecal valve is to prevent
backflow of fecal contents from the colon into small intestine. The wall of the ileum for several centimeters immediately upstream from the ileocecal valve has a thickened circular muscle called
the ileocecal sphincter This sphincter normally remains mildly constricted and slows emptying of ileal contents into the cecum Immediately after a meal a gastroileal reflex
intensifies peristalsis in the ileum and emptying of ileal contents into cecum proceeds Resistance to emptying at the ileocecal valve prolongs the stay of chyme in the ileum and
facilitates absorption Normally only 1500 to 2000 milliliters of chyme empty into the cecum each day The degree of contraction of the ileocecal sphincter and the intensity of peristalsis in the terminal ileum
are controlled significantly by reflexes from the cecum When the cecum is distended contraction of the ileocecal sphincter becomes intensified and ileal peristalsis is inhibited both of which greatly delay emptying of additional chyme into the cecum from
the ileum. Any irritant in the cecum delays emptying. The reflexes from the cecum to the ileocecal sphincter and ileum are mediated both by way of the myenteric plexus in the gut wall and by
the extrinsic autonomic nerves Functions of Large Intestine Absorption of water and electrolytes from the chyme to form solid feces (proximal half) Storage of fecal matter until it can be expelled
(distal half) Movements of Large Intestine Mixing movements (Haustrations) Propulsive movements (Mass movements)
Haustrations Large circular constrictions occur in the large intestine At each of these constrictions about 2.5 cm of the circular muscle contracts At the same time the longitudinal muscle of the colon, which is aggregated into three longitudinal strips called
the teniae coli, contracts. These combined contractions of the circular and longitudinal strips of muscle cause the unstimulated portion of the large intestine to bulge outward into baglike sacs called haustrations. Each haustration usually reaches peak
intensity in about 30 seconds and then disappears during the next 60 seconds. After another few minutes, new haustral contractions occur in other areas nearby
Mass Movements These movements usually occur only one to three times each day A mass movement is a modified type of peristalsis
First a constrictive ring occurs in response to distension Then rapidly the 20 or more cm of colon distal to the constrictive ring lose their haustrations and instead contract as a unit propelling the fecal material further down the colon. The contraction develops progressively more force for about 30 seconds and relaxation occurs during the next 2 to 3 minutes.
Then another mass movement occurs. A series of mass movements usually persists for 10 to 30 minutes. Then they cease but return perhaps a half day later. When they have forced a mass of feces into the rectum the desire for defecation is felt
Irritation in the colon can also initiate intense mass movements Appearance of mass movements after meals is facilitated by gastrocolic and duodenocolic reflexes. These reflexes result from distention of the
stomach and duodenum. Defecation The process by which faeces are emptied from the rectum through the anus.
When it becomes convenient for the person to defecate the defecation reflexes can purposely be activated by taking a deep breath to move the diaphragm downward and then contracting the abdominal muscles to increase the pressure in the abdomen thus forcing fecal
contents into the rectum Defecation signals entering the spinal cord initiate other effects such as taking a deep breath, closure of the glottis and contraction of the abdominal wall muscles
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