External Anal Sphincter (Subcutaneous, Superficial and Deep Parts)
Pelvis & Perineum · Trunk
The external anal sphincter is the voluntary muscular ring that encircles the anal canal and gives conscious control over defecation. It is described in three continuous parts: a subcutaneous part surrounding the anal orifice, a superficial part, and a deep part that blends with the puborectalis above. Posteriorly it is anchored to the anococcygeal ligament and the tip of the coccyx, and anteriorly its fibres attach to the perineal body, so that the whole muscle forms an encircling collar around the canal. Its action is to close the anal canal and provide voluntary continence, working with the involuntary internal sphincter and the puborectalis sling. It is supplied by the inferior rectal branch of the pudendal nerve together with the anterior ramus of S4.
Origin, Insertion, Action & Nerve
Origin
Anococcygeal ligament and tip of the coccyx (posteriorly)
Insertion
Perineal body anteriorly, encircling the anal canal
Action
Closes the anal canal and provides voluntary control of defecation
Nerve
Inferior rectal branch of the pudendal nerve and anterior ramus of S4
Attachments explained
The External Anal Sphincter (Subcutaneous, Superficial and Deep Parts) is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Anococcygeal ligament and tip of the coccyx (posteriorly)
Insertion
Perineal body anteriorly, encircling the anal canal
Action & function
When the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts) contracts, it produces the following movement: Closes the anal canal and provides voluntary control of defecation.
As part of the Pelvis & Perineum group of the Trunk, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The External Anal Sphincter (Subcutaneous, Superficial and Deep Parts) receives its nerve supply from the Inferior rectal branch of the pudendal nerve and anterior ramus of S4. This nerve carries fibres from spinal segment(s) S4.
Because a muscle can only contract when its nerve is intact, injury to the Inferior rectal branch of the pudendal nerve and anterior ramus of S4 or to its spinal roots (S4) can weaken or paralyse the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts), impairing the movements it normally produces (closes the anal canal and provides voluntary control of defecation). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts) (the OIANS method)
Anchor the external anal sphincter by its two midline attachments, the coccyx behind and the perineal body in front, which explain how a ring of muscle can be tethered fore and aft while still encircling the canal. Learn its three parts as one functional collar rather than memorising each separately; the key distinction is simply that it is voluntary, unlike the smooth-muscle internal sphincter deep to it.
Distinguish this voluntary sphincter from the puborectalis it blends with: the sphincter squeezes the canal closed, the puborectalis kinks the bowel above, and continence depends on both. Its innervation is the exam-worthy detail, coming from the inferior rectal branch of the pudendal nerve plus S4, and this same pudendal territory supplies the neighbouring perineal muscles, which helps you cluster them together.
Frequently asked questions
Where is the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts) located?
The External Anal Sphincter (Subcutaneous, Superficial and Deep Parts) is a muscle of the Pelvis & Perineum group, located in the Trunk.
What is the origin of the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts)?
Anococcygeal ligament and tip of the coccyx (posteriorly)
What is the insertion of the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts)?
Perineal body anteriorly, encircling the anal canal
What movements does the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts) produce?
Closes the anal canal and provides voluntary control of defecation
What nerve supplies the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts)?
Inferior rectal branch of the pudendal nerve and anterior ramus of S4
Is the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts) free to study in OIANS?
The External Anal Sphincter (Subcutaneous, Superficial and Deep Parts) is always free to browse. Its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory. Quiz and Flashcard practice for the Trunk is part of the one-time Lifetime upgrade, though; only the Upper Limb decks are free to test yourself on.
Related muscles
Study the External Anal Sphincter (Subcutaneous, Superficial and Deep Parts)
Test yourself on Trunk muscles with interactive quizzes and flashcards in the free OIANS app.
Quiz yourself → Flashcards Open the OIANS app
Browsing is always free. A one-time Lifetime unlock adds every region's quizzes, flashcards, progress tracking and explore tools.