Defecation reflexes

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. 

Ordinarily, defecation results from the defecation reflexes, which can be described as follows. When the feces enter the rectum, distension of the rectal wall initiates afferent signals that spread through the myenteric plexus to initiate peristaltic waves in the descending colon, sigmoid, and rectum, forcing feces toward the anus. As the peristaltic wave approaches the anus, the internal anal sphincter is inhibited by the usual phenomenon of receptive relaxation, and if the external anal sphincter is relaxed, defecation will occur. This overall effect is the intrinsic defecation reflex of the colon itself. 

FIGURE 12.1 The afferent and efferent pathways of the parasympathetic mechanism for enhancing the defecation reflex. 

However, the intrinsic defecation reflex itself is usually weak, and to be effective in causing defecation, it must be fortified by another type of defecation reflex, a parasympathetic defecation reflex that involves the sacral segments of the spinal cord, as illustrated in Figure 12.1 and Figure 12.2. 

However, despite the defecation reflexes, other effects are also necessary before actual defecation occurs because relaxation of the internal sphincter and forward movement of feces toward the anus normally initiates an instantaneous contraction of the external sphincter, which still temporarily prevents defecation. Except in babies and mentally challenged persons, the conscious mind then takes over voluntary control of the external sphincter and either inhibits it to allow defecation to occur or further contracts it if the moment is not socially acceptable for defecation. When the contraction is maintained, the defecation reflexes die out after a few minutes and usually will not return until an additional amount of feces enters the rectum, which may not occur until several hours thereafter. 

When it becomes convenient for the person to defecate, the defecation reflexes can sometimes be excited 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 to elicit new reflexes. Unfortunately, reflexes initiated in this way are never as effective as those that arise naturally, for which reason people who too often inhibit their natural reflexes become severely constipated. 

In the newborn baby and in some persons with transacted spinal cords, the defecation reflexes cause automatic emptying of the lower bowel without the normal control exercised through contraction of the external anal sphincter. 

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. 

There are several modes of action of laxatives. One group acts as softeners, which can be administered orally or as an enema. Another group is the bulk producers these are taken orally and swell in the intestine, and thus stimulate the defecation reflex. A large group are the stimulant laxatives with an action in the colon by inhibiting the absorption of electrolytes and water through a specific pharmacological mechanism. 

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. 

Large Intestine Propulsive peristaltic waves in the colon are diminished or abolished after administration of morphine, and tone is increased to the point of spasm. The resulting delay in the passage of bowel contents causes considerable desiccation of the feces, which, in turn, retards their advance through the colon. The amplitude of the nonpropulsive type of rhythmic contractions of the colon usually is enhanced. The tone of the anal sphincter is augmented, and reflex relaxation in response to rectal distension is reduced. These actions, combined with inattention to the normal sensory stimuli for defecation reflex owing to the central actions of the drug, contribute to morphine-induced constipation. 

Bulk-forming agents absorb water and swell, thns increasing the size of the fecal mass. The larger the fecal mass, the more the defecation reflex is stimulated and the passage of stool is promoted. 

Compared to oral administration, rectal administration encounters some specific problems. Degree and rate of absorption of the active substance are more difficult to predict and depend largely on the never predictable residence time in the rectum. Both irritation of the rectal mucosa by the active substance or the excipients and a large (liquid) volume in the rectum may cause a defecation reflex terminating the absorption process of the active substance. Also the degree of filling of the colon and sometimes the rectum influences the release and the absorption of the active substance. In favour of rectal administration would be... 

For rectal administration, the osmotic value may vary within wide limits. An osmotic value corresponding to a 0-1.8 % sodium chloride solution is tolerated. Adjusting the iso-osmotic value of an enema, by addition of, for example, sodium chloride, offers no advantage regarding activity and irritation, and can be omitted. StrrMig hyperosmotic solutions must be avoided in enemas, except for those intended as laxative. A strrMigly hyperosmotic solution, for example a phosphate enema, induces a defecation reflex. The osmotic value of this enema is about seven times higher than that of a normal saline solution. 

The rectum is the lowest part of the large intestine. It is 15-20 cm long with a diameter of about 5 cm. In the rectum 1-5 mL of viscous fluid with a pH between 6.4 and 7.4 and a small buffer capacity is present. The temperature (under physiological conditions) is 36.2—37.6 °C. The rectum is a flat tube, because of the pressure of the bowels. After administration of a fluid into the rectum the Uquid will spread due to this pressure. Larger volumes (enemas) are spread into the colon as well. A defecation reflex will occur when volumes exceeding 100 mL are applied, which limits the volume of enemas. 

Rectal solutions have water or oil as a vehicle. If necessary to enhance the solubility of poorly soluble active substances, aqueous rectal solutirms may contain cosolvents, such as ethanol and propylene glycol. However, cosolvents and surfactants should only be used in limited amounts because of the potential irritation and the defecation reflex they may cause. For the rectal absorption of active substances from enemas the same mechanisms as for suppositories apply. A major advantage of a rectal solution over a suppository may be the fact that the active substance is already in a dissolved state which may increase the absorption rate. Increasing the volume of a rectal solution to dissolve a poorly water-soluble active substance will enhance the dissolution rate and thereby increase the absorption rate. Because of the higher volume more active substance will be dissolved and the membrane surface over which absorption occurs, is increased as well. 

Extrinsic and intrinsic nervous systems as well as local and circulating gastrointestinal hormones contribute to local and centrally mediated reflex mechanisms that regulate GI function, ingestion, secretion, mixing and propulsion, and absorptimi of food and defecation reflexes that force the fecal material into the rectum by intense mass movements (detailed description can be found in Guyton and HaU Text Book of Medical Physiology Hall 2011). 

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