Muscle
External anal sphincter
Voluntary skeletal muscle surrounding the anal outlet
High-yield
The inferior rectal nerve supplies this skeletal sphincter. Contrast it with the internal anal sphincter, which is smooth muscle under involuntary control.
Overview
External anal sphincter adds voluntary control to anal closure. Its subcutaneous, superficial and deep portions surround the canal at different levels; superiorly it works closely with puborectalis and the internal sphincter in the continence mechanism.
Reference facts
- Origin
- Posterior superficial fibers attach in the coccygeal and anococcygeal region.
- Insertion
- Anterior superficial fibers join the perineal body; subcutaneous fibers encircle the outlet, while deeper fibers interrelate with puborectalis.
- Innervation
- Mainly the inferior rectal branch of the pudendal nerve, carrying S2–S4 fibers.
- Action
- Augments anal closure voluntarily and relaxes as part of coordinated defecation.
- Blood supply
- Inferior rectal branches of the internal pudendal artery provide the main arterial supply.
Mnemonic
External means extra voluntary squeeze; internal maintains involuntary tone.
Clinical correlation
Childbirth injury or damage involving the anal sphincter complex can impair continence. Continence also depends on puborectalis, internal-sphincter tone and sensory function, so it is not determined by one muscular ring alone.
Frequently asked questions
What does the external anal sphincter do?
The external anal sphincter augments anal closure voluntarily and relaxes as part of coordinated defecation.
What nerve innervates the external anal sphincter?
Mainly the inferior rectal branch of the pudendal nerve, carrying S2–S4 fibers.
Where does the external anal sphincter attach?
Origin: Posterior superficial fibers attach in the coccygeal and anococcygeal region. Insertion: Anterior superficial fibers join the perineal body; subcutaneous fibers encircle the outlet, while deeper fibers interrelate with puborectalis.
What is the blood supply of the external anal sphincter?
Inferior rectal branches of the internal pudendal artery provide the main arterial supply.