GLOSSARY

The internal anal sphincter is a thickening of the rectum’s circular smooth-muscle layer in the upper two-thirds of the anal canal, about 2.5–4 centimetres long; it is involuntary and, through continuous tonic contraction, is the main factor keeping the anus closed at rest.

Characteristics

• Smooth muscle: involuntary, in constant contraction (fatigue-resistant).
• Contribution: about 70–85 percent of anal canal resting pressure.
• Innervation: sympathetic (contraction) and parasympathetic (relaxation), as well as the enteric nervous system.

The Rectoanal Inhibitory Reflex (RAIR)

Distension of the rectum by faeces or gas causes transient relaxation of the internal sphincter, allowing sampling of the contents by anal canal sensation to distinguish solid, liquid, or gas; at the same time the external sphincter contracts to prevent