It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.

[2][3][4][5] The internal anal sphincter is the specialised thickened terminal portion of the inner circular layer of smooth muscle of the large intestine.

[6] Transient relaxation of its proximal portion occurs with rectal distension and post-prandial rectal contraction (the recto-anal inhibitory reflex and sampling reflex, respectively) while the distal portion of the sphincter remains contracted and the external anal sphincter becomes contracted to maintain continence; this transient relaxation allows passage of stool into the proximal anal canal - this filling is sensed.

When the rectum fills beyond a certain capacity, the rectal walls are distended, triggering the defecation cycle.

This is thought to allow a small amount of rectal contents to descend into the anal canal where specialized mucosa samples whether it is gas, liquid or solid.