Motility of the Colon

The motor response of the human colon depends on the caloric density of the ingested meal [15]. A 1000-kcal meal stimulates colonic motor activity but a 350-kcal meal does not. Fat stimulates human colonic motor activity when administered orally, but proteins and amino acids do not [71,72], Gastric or duodenal mucosal contact with the meal is necessary for colonic motor response because the intravenous administration of lipids, like sham feeding, 

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Rhubarb is a laxative, primarily owing to its influence on the motility of the colon, inhibiting stationary and stimulating propulsive contractions. This results in an accelerated intestinal passage and, because of the active chloride secretion, an increase in the water and electrolyte content of stool. 

There are two different mechanisms of action for the various anthrones which are seen as the main active metabolites with specific activity on the colon. One mechanism involves the anthrone influencing the motility of the colon through an inactivation of the enzyme Na-r/K-i-ATP-ase which results in an inhibition of the Na-r pump and of Cl-channels in the colonic membrane. This results in the speeding up of transit time through the bowel. A second mechanism involves enhanced fluid secretion into the lumen of the bowel by stimulation of mucous and chloride. The resulting net transfer of fluid into the intestinal lumen triggers reflex defecation. There is a delay of 6-12 hours from administration of the laxative until defecation occurs due to the time taken for the glycosides to reach the colon and be transformed into the active anthrones. 

Stimulant laxatives increase the motility of the colon, induce changes in the surface cells of the colon, and cause... 

A second form of motility in the large intestine is mass movement. Three or four times per day, typically after a meal, a strong propulsive contraction occurs that moves a substantial bolus of chyme forward toward the distal portion of the colon. Mass movements may result in the sudden distension of the rectum that elicits the defecation reflex. 

Intestinal motility can be altered by reduced contact time in the small intestine, premature emptying of the colon, and by bacterial overgrowth. 

Saline laxatives like MgS04, Mg(OH)2, Mg2 Citrate and Na+ Phosphates act via their osmotic pressure to retain water in the colon. Other osmotic laxatives are carbohydrates such as lactulose, glycerin, sorbitol, and mannitol. They are not absorbed and are resistant to digestion in the small intestine. Most agents are orally administered. It should be noted however that glycerin, sodium phosphates and sorbitol are formulated for rectal use. From lactulose lactic and acetic acids are formed by intestinal bacteria and apart from its osmotic effects it thus acidifies the content of the colon. The reduction of the pH stimulates motility and secretion. 

Q3 The muscular wall of the colon is quiescent for much of the time. The major type of motility is segmentation but this is less frequent than in the small intestine. It mixes and moves colonic contents around to promote reabsorption of water and electrolytes. Peristaltic movements also occur and promote colon emptying. The myenteric plexus in the wall of the colon coordinates both motor and secretory activity and affects activity of the internal sphincter at the junction of the colon and rectum. This sphincter is usually contracted and is maintained closed by sympathetic stimulation. Stimulation of parasympathetic nerves increases motor activity throughout the colon and relaxes the internal sphincter, allowing material to enter the rectum. 

The gastrocolic reflex stimulates motility in the colon during or just after eating, when chyme enters the colon from the ileum. The content of the distal colon may be stored there for variable periods until defecation occurs, which may be 24 hours or more after eating. The rectum is normally empty and movement of faecal material into the sigmoid colon and rectum stimulates the defecation reflex. 

Q5 Diverticula are sacs, or pouches, of the mucosa which form in the wall of the colon and bulge through the muscular wall of the intestine, often where arteries penetrate the intestinal wall. Their formation is associated with diets of low fibre content and is thought to be related to abnormal colon motility... 

Diverticula, which are pouches of the mucosa, can form and bulge through the muscular wall of the colon. These pouches may become infected and inflamed (diverticulitis), causing abdominal pain. Development of diverticula is associated with low-fibre diets and is thought to be related to abnormal colonic motility and high intraluminal pressures. The chance of developing diverticula increases with age. 

Motility patterns of the colon determine the rate of transit throngh the colon and hence the residence time of a drug and its absorption. 

Altered intestinal motility produces diarrhea by three mechanisms reduction of contact time in the small intestine, premature emptying of the colon, and bacterial overgrowth. Chyme must be exposed to intestinal epithelium for a sufficient time period to enable normal absorption and secretion processes to occur. If this contact time decreases, diarrhea results. Intestinal resection or bypass surgery and drugs (such as metoclopramide) cause this type of diarrhea. On the other hand, an increased time of exposure allows fecal bacteria overgrowth. A characteristic small intestine diarrheal pattern is rapid, small, coupling bursts of waves. These waves are inefficient, do not allow absorption, and rapidly dump chyme into the colon. Once in the colon, chyme exceeds the colonic capability to absorb water. 

Morphine reduces the activity of the entire gastrointestinal tract in that it reduces the secretion of hydrochloric acid, diminishes the motility of the stomach, and increases the tone of the upper part of the duodenum. These actions may delay passage of the stomach contents into the duodenum. Both pancreatic and biliary secretions are diminished, and this may also hinder digestion, hi the large intestine, the propulsive peristaltic wave in the colon is reduced, the muscle tone including that of the anal sphincter is increased, and the gastrocohc reflex (defecation reflex) is reduced. These actions, in combination, cause constipation, which seems to be a chronic problem among addicts. 

Under normal circumstances, these quantities are well within the range of the total absorptive capacity of the small bowel ( 16 L) and colon (4-5 L). Neurohumoral mechanisms, pathogens, and drugs can alter these processes, resulting in changes in either secretion or absorption of fluid by the intestinal epithelium. Altered motility also contributes to this process, as the extent of absorption parallels transit time. With decreased motility and excess fluid removal, feces can become inspissated and impacted, leading to constipation. When the capacity of the colon to absorb fluid is exceeded, diarrhea will occur. 

Constipation can be due to primary and secondary causes (Table 18-1). Primary or idiopathic constipation is typified by normal-transit constipation, slow-transit constipation, and dyssynergic defecation. In the normal-transit type, colonic motility is unchanged and patients tend to experience hard stools despite normal movements. In the slow-transit type, motility is decreased leading to infrequent harder, drier stools. In dyssynergic defecation (also known as pelvic floor dysfunction), patients have lost the ability to relax the anal sphincter while coordinating muscle contractions of the pelvic floor. Some causes of secondary constipation are listed in Table 18-1. 

Disturbed intestinal motility appears to be a central feature of IBS, which leads to altered stool consistency. Studies suggest that the colon of IBS sufferers is abnormally sensitive to normal stimuli.13 This enhanced visceral sensitivity manifests as pain, especially related to gut distention. 

V. cholerae is a gram-negative bacillus. Vibrios pass through the stomach to colonize the upper small intestine. Vibrios have filamentous protein extensions that attach to receptors on the intestinal mucosa, and their motility assists with penetration of the mucus layer.2 The cholera enterotoxin consists of two subunits, one of which (subunit A) is transported into the cells and causes an increase in cyclic AMP, which leads to a deluge of fluid into the small intestine.20 This large volume of fluid results in the watery diarrhea that is characteristic of cholera. The stools are an electrolyte-rich isotonic fluid, the loss of which results in blood volume depletion followed by low blood pressure and shock.2 Of note, the diarrheal fluid is highly infectious. 

An indwelling catheter is commonly used in various health care settings, and is associated with UTIs. Bacteria may be introduced into the bladder via the catheter in several ways. These include direct infection introduction during catheterization (via colonization and subsequently traveling the length of the catheter through bacterial motility or capillary action). UTIs as a result of an indwelling catheter are common and occur at a rate of 5% per day of catheter presence.25... 

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