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The mechanism of defecation

Constipation

Defecation mechanism


Have you ever wondered how come defecation comes swiftly at times, and at a specific time period? Some individuals notice that the urge to defecate may happen after a dinner meal, or upon waking up in the morning. So how does defecation really work?

Defecation is actually a synchronized sequence of events associated with neural influences. There are several reflexes that are related to the physiology of defecation. The rectum is innervated with nerves that initiate reflex contractions upon its distention. These contractions altogether constitute the desire to defecate.

The human rectum has two sphincters: the external anal sphincter, and the internal anal sphincter. In the internal anal sphincter, the sympathetic nerve supply towards it is excitatory, while the parasympathetic nerve supply is inhibitory. Thus, this sphincter relaxes when the rectum is being distended. On the other hand, the external anal sphincter, which is a skeletal muscle, is innervated by a branch of the pudendal nerve, and this is maintained in a state of contraction. Even a slight increase or slight distention of the rectum allows it to increase several-fold the force of its contraction. When this rectal pressure rises to approximately 18 mmHg, the urge to defecate arises. However when this reaches 55 mmHg, this means bad news since both the external and internal anal sphincter will relax and therefore, there will be a reflex expulsion of fecal matter! This will explain why in animals, expulsion of rectal contents is not an atypical sight.

Prior to reaching this seemingly embarrassing pressure point of 55 mmHg, defecation can be made voluntarily through straining. Defecation is hence a spinal reflex that can be managed through keeping the external sphincter in its contracted state, or it can be induced through sphincter muscle relaxation and contracting the muscles of the abdomen.

Ever wondered why at times the urge to defecate happens after a hearty meal? This is because the stomach, upon distention by food, initiates contraction of the rectum. This is called the gastrocolic reflex, and there are recent studies that it is a function of gastrin on the colon, and not due to any neural influence. That is why in children, it is already a given rule that defecation should occur after meals.

In abnormalities involving the colon, these reflexes may be impaired, but not necessarily. It is important to have quite a little bit of knowledge as to how the mechanism of defecation works, so it would be easier to understand the abnormal conditions.

In to swimbi.com