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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 muscle helps maintain continence of urine along with the internal urethral sphincter which is under control of the autonomic nervous system.The external sphincter muscle prevents urine leakage as the muscle is tonically contracted via somatic fibers that originate in Onuf's nucleus and pass through sacral spinal nerves S2-S4 then the pudendal nerve to synapse on the muscle.
The pudendal nerve is the main nerve of the perineum. [1]: 274 It is a mixed (motor and sensory) nerve and also conveys sympathetic autonomic fibers.It carries sensation from the external genitalia of both sexes and the skin around the anus and perineum, as well as the motor supply to various pelvic muscles, including the male or female external urethral sphincter and the external anal sphincter.
Onuf's nucleus is the origin of innervation for the striated muscles of the rectum and urethral sphincters.The neurons of Onuf's nucleus are responsible for controlling external sphincter muscles of the anus and urethra in humans.
The external urethral sphincter is a striated muscle that allows voluntary control over urination. [5] The internal sphincter , formed by the involuntary smooth muscles lining the bladder neck and urethra, receives its nerve supply by the sympathetic division of the autonomic nervous system . [ 6 ]
The internal urethral sphincter is a urethral sphincter muscle which constricts the internal urethral orifice. It is located at the junction of the urethra with the urinary bladder and is continuous with the detrusor muscle , [ 1 ] [ 2 ] but anatomically and functionally fully independent from it. [ 3 ]
When activated, the PMC relaxes the urethral sphincter, allowing for micturition to occur. The PMC coordinates with other brain centers, including the medial frontal cortex, insular cortex, hypothalamus and periaqueductal gray (PAG). The PAG acts as a relay station for ascending bladder information from the spinal cord and incoming signals from ...
External sphincter muscle of male urethra; External sphincter muscle of female urethra; Compressor urethrae muscle in the female is sometimes included [3] Urethrovaginal sphincter in the female is sometimes included [3] other Membranous urethra in the male; proximal portion of urethra in the female; Bulbourethral gland (males).