= Internal anal sphincter
The <internal anal sphincter> is a ring of <smooth muscle> at the lower end of the <rectum> and around the upper <anal canal>. It contributes involuntary resting closure. The surrounding <external anal sphincter> is a different muscle system with voluntary control, so the two should not be treated as one anatomical ring.<Small and Large Intestine in SEER Training by the National Cancer Institute>[Anal canal: internal smooth-muscle sphincter and involuntary control]{ref}
Rectal distension can trigger relaxation of the <internal anal sphincter>, the <rectoanal inhibitory reflex>. Researchers can inflate a rectal balloon while recording anal <pressure>, separating this relaxation from other contractile responses and from reported sensation. A study of 23 subjects observed inhibitory and contractile reflexes during graded balloon distension.<Rectoanal sensorimotor response in humans during rectal distension by Sherrie De Ocampo and others>[2007, abstract: distension protocol and pressure/sensory responses]{ref} A continuous pressure trace therefore reveals coordinated components of continence that are difficult to infer from static anatomy alone.
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