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Urethral sphincter

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Title: Urethral sphincter  
Author: World Heritage Encyclopedia
Language: English
Subject: Urethra, Internal sphincter muscle of urethra, Detrusor urinae muscle, Urinary system, Overflow incontinence
Collection: Perineum, Urethra, Urinary System
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Urethral sphincter

The term urethral sphincter refers to one of two muscles used to control the exit of urine in the urinary bladder through the urethra. The two muscles are the 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.

  • The external sphincter muscle of urethra (sphincter urethrae): located at the bladder's distal inferior end in females and inferior to the prostate (at the level of the membranous urethra) in males is a secondary sphincter to control the flow of urine through the urethra. Unlike the internal sphincter muscle, the external sphincter is made of skeletal muscle, therefore it is under voluntary control of the somatic nervous system.
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

Despite common misconceptions, human males do not have stronger urethral sphincter muscles than females, nor are there any noticeable capacity differences in the bladders. Anatomically, females have to reproductive organs.

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.

Function

In addition to the internal and external sphincters, in males the urethra extends to the end of the penis, where it opens up to the outside.

The levator ani, the voluntary muscle of the pelvic floor, can be used to control urination. In females this muscle may be damaged, most commonly by childbirth, leading to weakness of the sphincter mechanism and stress incontinence. Kegel exercises are a form of exercise intended to strengthen the pelvic floor muscles. They are also known as Pelvic Floor Muscle Training.

External links

  • Origin, insertion and nerve supply of the muscle at Loyola University Chicago Stritch School of Medicine
  • urethral+sphincter at eMedicine Dictionary
  • 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."

This article incorporates text from a public domain edition of Gray's Anatomy.

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