Organ
Internal anal sphincter
An involuntary smooth-muscle contributor to closure
High-yield
Resting tone is not maintained solely by sympathetic firing. Distension-induced relaxation uses inhibitory enteric pathways, prominently nitric oxide signaling.
Overview
The internal sphincter is the thickened terminal circular muscle of the bowel. It lies within the external sphincter and supplies much of resting anal pressure. Persistent activity has an intrinsic myogenic basis that neural pathways modify.
Reference facts
- Location
- Encircles the proximal anal canal, deep to the external sphincter, and ends above the anal verge.
- Parts
- A thickened circular smooth-muscle sleeve; its lower edge contributes to the intersphincteric groove.
- Blood supply
- Communicating branches of superior, middle and inferior rectal arterial systems.
- Innervation
- Intrinsic smooth-muscle activity is modulated by enteric inhibitory neurons and extrinsic autonomic fibers through pelvic plexuses; nitric oxide is an important relaxation transmitter.
- Relations
- Longitudinal muscle and intersphincteric tissues intervene between it and the external skeletal-muscle sphincter.
- Function
- Provides much of resting closure and relaxes reflexly during rectal filling, permitting sensory sampling and coordinated evacuation.
Clinical correlation
Rectal distension normally elicits the rectoanal inhibitory reflex. Its absence is important in Hirschsprung disease, where affected bowel lacks enteric ganglion cells. Cryptoglandular infection can spread between the internal and external sphincters.
Frequently asked questions
Where is the internal anal sphincter located?
The internal anal sphincter lies encircles the proximal anal canal, deep to the external sphincter, and ends above the anal verge.
What does the internal anal sphincter do?
Provides much of resting closure and relaxes reflexly during rectal filling, permitting sensory sampling and coordinated evacuation.
What are the parts of the internal anal sphincter?
A thickened circular smooth-muscle sleeve; its lower edge contributes to the intersphincteric groove.
What is the blood supply of the internal anal sphincter?
The internal anal sphincter is supplied by the communicating branches of superior, middle and inferior rectal arterial systems.