Which of the following statement(s) relating to anal sphincteric function is/are correct?

a. When the rectum is distended, the internal anal sphincter relaxes and the external anal sphincter contracts

d. The internal anal sphincter is responsible for resting anal pressure

The internal sphincter, because it is innervated by the autonomic nervous system, is not subject to voluntary control. This powerful muscle exists in a continuously tonic state, and is responsible for maintaining closure of the resting anal canal. The high-pressure zone of the anal canal at rest is due to the actions of the internal sphincter. The external sphincter contributes to anal pressure only when a bolus of stool is present within the anal canal. The increase in pressure during voluntary contraction (squeeze pressure) is exclusively due to the activity of the external sphincter. The high resting pressure in the anal canal acts as a barrier to prevent leakage of mucus and gas. When the rectum is distended, the internal sphincter relaxes. This relaxation allows the rectal content to move down to the anal canal. In contrast, when the rectum is distended, the external sphincter contracts. Reflex contraction of the external sphincter prevents rectal content from leaking through the anus. Although volitional contraction of the external sphincter can only be sustained for short periods, it is the most important mechanism of voluntary continence.