Which statement about rectal filling and the internal anal sphincter is accurate?

Which statement about rectal filling and the internal anal sphincter is accurate?

• A. Rectal filling triggers contraction of the internal anal sphincter.
• B. Rectal filling triggers reflex relaxation of the internal anal sphincter.
• C. Rectal filling has no effect on the internal anal sphincter.
• D. Rectal filling strengthens the external anal sphincter.

Answer: (B)

Rectal filling distends the rectum, triggering a reflex that relaxes the internal anal sphincter. The internal anal sphincter is smooth muscle under involuntary control, and its relaxation reduces its resting tone to allow stool to move toward the anal canal and to permit coordinated defecation when appropriate. This reflex helps maintain continence by keeping the IAS tight at rest but permitting relaxation in response to rectal distension signals. This is why the statement about rectal filling causing reflex relaxation of the internal anal sphincter is the best description. The external anal sphincter is skeletal muscle under voluntary control, and rectal distension does not inherently strengthen it; its contraction is a voluntary action used to maintain continence when desired.

Rectal filling distends the rectum, triggering a reflex that relaxes the internal anal sphincter. The internal anal sphincter is smooth muscle under involuntary control, and its relaxation reduces its resting tone to allow stool to move toward the anal canal and to permit coordinated defecation when appropriate. This reflex helps maintain continence by keeping the IAS tight at rest but permitting relaxation in response to rectal distension signals.

This is why the statement about rectal filling causing reflex relaxation of the internal anal sphincter is the best description. The external anal sphincter is skeletal muscle under voluntary control, and rectal distension does not inherently strengthen it; its contraction is a voluntary action used to maintain continence when desired.