Movement in the Colon and Defecation
The movement in the colon is similar to the small in-testine in that peristalsis and a segmentation type of contraction are present. In addition, there is a third type of contraction, mass action contraction, in which there is simultaneous contraction of the smooth muscle over a large area. This movement pushes material from one portion of the colon to an-other and into the rectum. When the rectum is dis-tended, it initiates the defecation reflex. When the pressure increases a little, there is a desire to defe-cate. Beyond a pressure of 55 mm Hg, both the inter-nal and external sphincters relax and the contents of the rectum are expelled. This is the reason for the in-voluntary expulsion seen in infants and individuals with spinal cord injury.
In humans, sympathetic stimulation causes contrac-tion of the internal anal sphincter and parasympathetic stimulation relaxation. The external anal sphincter, which is comprised of skeletal muscle, is supplied by the pudendal nerve. Defecation is primarily a spinal re-flex; however, it can be voluntarily initiated by relaxing the external sphincter and contracting the abdominal muscles to increase abdominal pressure.
Distension of the stomach by food also initiates the gastrocolic reflex. This reflex is responsible for the defecation soon after meals, often seen in chil-dren. In adults, habit and culture play an important role in determining when defecation occurs.
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