Urethral sphincters

The urethral sphincters are two muscles used to control the exit of urine in the urinary bladder through the urethra. The two muscles are either the male or female external urethral sphincter and the internal urethral sphincter. When either of these muscles contracts, the urethra is sealed shut.

The external urethral sphincter originates at the ischiopubic ramus and inserts into the intermeshing muscle fibers from the other side. It is controlled by the deep perineal branch of the pudendal nerve. Activity in the nerve fibers constricts the urethra.

Dissection of prostate showing the fibers of the external sphincter surrounding the membranous urethra and partially cradling the inferior portion of the prostate.

Sex differences
Main articles: External sphincter muscle of male urethra and External sphincter muscle of female urethra

In males and females, both internal and external urethral sphincters function to inhibit the release of urine. In males, the internal sphincter muscle of urethra functions to prevent reflux of seminal fluids into the male bladder during ejaculation.

Females do have a more elaborate external sphincter muscle than males as it is made up of three parts: the sphincter urethrae, urethrovaginal muscle, and the compressor urethrae. The urethrovaginal muscle fibers wrap around the vagina and urethra and contraction leads to constriction of both the vagina and the urethra. The origin of the compressor urethrae muscle is the right and left inferior pubic ramus and it wraps anteriorly around the urethra so when it contracts it squeezes the urethra against the vagina. The external urethrae, like in males, wraps solely around the urethra.

Congenital abnormalities of the female urethra can be surgically repaired with vaginoplasty.[2]

Function

The internal urethral sphincter provides involuntary control of urination. The external urethral sphincter provides voluntary control of urination.

References
  1. Maclean, Allan; Reid, Wendy (2011). "40". In Shaw, Robert (ed.). Gynaecology. Edinburgh New York: Churchill Livingstone/Elsevier. pp. 599–612. ISBN 978-0-7020-3120-5; Access provided by the University of Pittsburgh
  2. Hiort, O (2014). Understanding differences and disorders of sex development (DSD. Basel: Karger. ISBN 9783318025590; Access provided by the University of Pittsburgh

This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)

  • Anatomy figure: 41:06-04 at Human Anatomy Online, SUNY Downstate Medical Center - "Muscles of the female urogenital diaphragm (deep perineal pouch) and structures located inferior to it."
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