Internal sphincter is smooth muscle and involuntary; external is skeletal muscle and voluntary
Mnemonic
“Internal is involuntary, external is under your control”:
- Internal anal sphincter - smooth muscle, a thickening of the inner circular layer, involuntary, sympathetic contraction and parasympathetic relaxation. Responsible for resting tone, about 80 per cent of it
- External anal sphincter - skeletal muscle, voluntary, supplied by the inferior rectal branch of the pudendal nerve (S2 to S4). Responsible for squeeze pressure
“S2, 3, 4 keeps the faeces off the floor” for the pudendal supply, the same roots that keep the bladder from emptying.
Puborectalis completes the mechanism, slinging around the anorectal junction to maintain the anorectal angle; its relaxation on defaecation straightens the angle.
Obstetric injury typically damages the external sphincter and puborectalis, causing urge incontinence, while internal sphincter damage after fistula surgery causes passive soiling.
Expansion
Internal anal sphincter
- Smooth muscle - a thickening of the inner circular layer
- Involuntary, supplied by the autonomic nervous system
- Contributes about 70 to 85 per cent of resting tone
- Surrounds the upper two-thirds of the anal canal
External anal sphincter
- Skeletal muscle, in subcutaneous, superficial and deep parts
- Voluntary, supplied by the inferior rectal branch of the pudendal nerve (S2 to S4)
- Provides squeeze pressure
- Surrounds the lower two-thirds, overlapping the internal sphincter
The puborectalis sling maintains the anorectal angle, which is essential for gross continence and relaxes during defaecation.
Obstetric injury typically damages the external sphincter, causing urge incontinence; a lateral internal sphincterotomy for fissure risks passive soiling by reducing resting tone.