Urogenital diaphragm and the external urethral sphincter
The urogenital diaphragm is the traditional name for the deep perineal pouch, a space in the front part of the muscular floor of the pelvis.
Remember
The external urethral sphincter sits in the deep perineal pouch (the traditional urogenital diaphragm) and is skeletal muscle, so it is the sphincter you can squeeze at will to hold urine in, unlike the detrusor (the smooth muscle of the bladder wall) and the internal anal sphincter, which work automatically.
The urogenital diaphragm is the traditional name for the structures of the deep perineal pouch. This pouch is a space in the front part of the pelvic floor (the muscular layer that closes the bottom of the pelvis), and it holds muscle and fascia (sheets of connective tissue) around the urethra, the tube that carries urine out of the body.
The muscle to know in the deep perineal pouch is the external urethral sphincter, a ring of skeletal muscle around the urethra. Skeletal muscle is the kind you control consciously, like the muscles of your arm, so this ring is under voluntary control. You can squeeze it to hold urine back and relax it to begin micturition (passing urine). It is supplied by the pudendal nerve, a nerve that carries voluntary motor fibres to the perineum.
In the classic textbook description, the principal muscle of the deep perineal pouch is the deep transverse perineal muscle, which runs transversely (side to side) and sits alongside the external urethral sphincter. Some modern anatomical studies doubt whether the muscle forms a distinct sheet, which is why many newer texts prefer the term deep perineal pouch.
The levator ani (the main muscle of the pelvic diaphragm, the funnel-shaped floor of the pelvic cavity) lies above the deep pouch, and the superficial perineal pouch lies below it. The detrusor (the smooth muscle of the bladder wall) and the internal anal sphincter are smooth muscle and work automatically.
Weakness or damage of the external urethral sphincter, or of the pelvic floor muscles around it, is a recognised cause of stress incontinence (leakage when coughing or lifting).
Key facts
- Name
- urogenital diaphragm is the traditional term for the structures of the deep perineal pouch, in the front part of the pelvic floor.
- Voluntary sphincter
- the external urethral sphincter is a ring of skeletal muscle in the deep perineal pouch that can be contracted voluntarily to keep the urethra closed.
- Nerve supply
- the pudendal nerve (S2 to S4) supplies the external urethral sphincter and gives voluntary control over micturition.
- Classic muscle
- the deep transverse perineal muscle is described in the deep perineal pouch together with the external urethral sphincter.
- Layer above
- the levator ani and the coccygeus (a small muscle of the pelvic floor behind the levator ani) form the pelvic diaphragm above the deep pouch.
- Layer below
- the bulbospongiosus (beside the erectile tissue of the penis or clitoris), the ischiocavernosus (a muscle that acts on the erectile tissue of the clitoris or penis) and the superficial transverse perineal muscle lie in the superficial perineal pouch.
- Automatic muscles
- the detrusor and the internal anal sphincter are smooth muscle and work automatically.
Exam traps
- The levator ani is the main muscle of the pelvic diaphragm, and some of its fibres help with urinary continence (holding urine in), but it forms the layer above the deep perineal pouch, so it is not part of the urogenital diaphragm.
- The detrusor is central to passing urine, but it lies in the bladder wall and works automatically, so it is not the voluntary muscle of the urogenital diaphragm.
- The superficial transverse perineal muscle has almost the same name as the deep transverse perineal muscle, but it lies in the superficial perineal pouch, below the perineal membrane (the tough fascial sheet under the deep pouch), so it is not part of the urogenital diaphragm.
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