Urethral sphincter
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The urethral sphincter is a collective name for the muscles used to control the flow of urine (micturition) from the urinary bladder. These muscles envelope the urethra, so that when they contract, the urethra is sealed shut. The muscles originate at the pubic ramus with the insertion point at the median raphe. The function of the sphincter urethrae (external sphincter) is controlled by the pudendal nerve and acts to constrict the urethra. There are actually two urethral sphincters in the human body:
- The internal sphincter muscle of urethra: located at the bladder's inferior end and the urethra's proximal end at the junction of the urethra with the urinary bladder. The internal sphincter is a continuation of the detrusor muscle and is made of smooth muscle, therefore it is under involuntary or autonomic control. This is the primary muscle for prohibiting the release of urine.
- 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.
[edit] Gender 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 urinate more frequently because their bladders share space with the uterus and vagina in the anterior wall, which causes the involuntary interior sphincter muscle to give off the nerve impulses more frequently even when the bladder is not at capacity. In males the bladder lies posteriorly against the rectum, where it does not share space with any reproductive organs. Females do have a more elaborate external sphincter muscle than men 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.
[edit] 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.
Both sexes can use the levator ani, the voluntary muscle of the pelvic floor, to control urination. In females this muscle may be damaged, particularly by pregnancy, 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.
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