The internal anal sphincter is which of the following?

• A. Continuation of the longitudinal muscles.
• B. Contracted by rectal filling.
• C. Reflexively relaxes by rectal filling.
• D. Under volitional control.

Answer: (C)

The internal anal sphincter is smooth muscle that forms a ring at the anal canal and is a continuation of the circular muscle layer of the rectum. Its tone is kept at rest by autonomic control, but it relaxes reflexively when the rectum distends. This rectoanal inhibitory reflex allows the internal sphincter to relax in response to rectal filling, facilitating the movement of matter toward the anal canal and enabling defecation to begin. Defecation then relies on voluntary relaxation of the external anal sphincter and pelvic floor. So, the best answer is the one describing reflex relaxation in response to rectal filling.