External Urethral Sphincter
Pelvis & Perineum · Trunk
The external urethral sphincter is the voluntary ring of muscle that surrounds the membranous urethra in the deep perineal pouch and provides conscious control over urine flow. It arises from the inferior ramus of the pubis and the adjacent fascia and encircles the membranous urethra, its fibres meeting those of the opposite side. By constricting the urethra it maintains urinary continence and can voluntarily interrupt the urinary stream, complementing the involuntary internal urethral sphincter at the bladder neck. It is supplied by the perineal branch of the pudendal nerve (S2-S4).
Origin, Insertion, Action & Nerve
Origin
Inferior ramus of the pubis and adjacent fascia, surrounding the membranous urethra
Insertion
Encircles the membranous urethra, meeting its fellow of the opposite side
Action
Constricts the urethra to maintain urinary continence
Nerve
Perineal branch of the pudendal nerve (S2–S4)
Attachments explained
The External Urethral Sphincter 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
Inferior ramus of the pubis and adjacent fascia, surrounding the membranous urethra
Insertion
Encircles the membranous urethra, meeting its fellow of the opposite side
Action & function
When the External Urethral Sphincter contracts, it produces the following movement: Constricts the urethra to maintain urinary continence.
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 Urethral Sphincter receives its nerve supply from the Perineal branch of the pudendal nerve (S2–S4). This nerve carries fibres from spinal segment(s) S2, S4.
Because a muscle can only contract when its nerve is intact, injury to the Perineal branch of the pudendal nerve or to its spinal roots (S2, S4) can weaken or paralyse the External Urethral Sphincter, impairing the movements it normally produces (constricts the urethra to maintain urinary continence). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the External Urethral Sphincter (the OIANS method)
Anchor the external urethral sphincter as the voluntary counterpart to the internal sphincter: the internal one sits at the bladder neck and is involuntary, while this external one surrounds the membranous urethra and is under conscious control through the pudendal nerve. Pairing the two by location and control type makes both easy to recall.
Tie it to a practical detail: because it is skeletal muscle supplied by the perineal branch of the pudendal nerve (S2-S4), it is the muscle you use to voluntarily stop mid-stream, and it is the structure at risk during prostate surgery in males. Linking that function and clinical vulnerability to its position around the membranous urethra keeps origin, action, and nerve bound together.
Frequently asked questions
Where is the External Urethral Sphincter located?
The External Urethral Sphincter is a muscle of the Pelvis & Perineum group, located in the Trunk.
What is the origin of the External Urethral Sphincter?
Inferior ramus of the pubis and adjacent fascia, surrounding the membranous urethra
What is the insertion of the External Urethral Sphincter?
Encircles the membranous urethra, meeting its fellow of the opposite side
What movements does the External Urethral Sphincter produce?
Constricts the urethra to maintain urinary continence
What nerve supplies the External Urethral Sphincter?
Perineal branch of the pudendal nerve (S2–S4)
Is the External Urethral Sphincter free to study in OIANS?
The External Urethral Sphincter 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
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