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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 contract, 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.
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. In males and females, this muscle functions to prevent 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. In males the bladder does not share space with any 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.
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.
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