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The female or male external sphincter muscle of urethra (sphincter urethrae): located in the deep perineal pouch, at the bladder's distal inferior end in females, and inferior to the prostate (at the level of the membranous urethra) in males. It is a secondary sphincter to control the flow of urine through the urethra.
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. [1] [2] Voiding urine begins with voluntary relaxation of the external urethral sphincter.
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 sphincter is present both in males ...
The human urinary system, also known as the urinary tract or renal system, consists of the kidneys, ureters, bladder, and the urethra. The purpose of the urinary system is to eliminate waste from the body, regulate blood volume and blood pressure, control levels of electrolytes and metabolites , and regulate blood pH .
During urination, the detrusor muscle contracts, the external urinary sphincter and muscles of the perineum relax, and urine flows through the urethra [13] and exits the penis or vulva through the urinary meatus. [11] The urge to pass urine stems from stretch receptors that activate when between 300 - 400 mL urine is held within the bladder. [13]
A rhabdosphincter (from Greek rhabdos meaning "rod") is a sphincter consisting of striated muscle fibres. [1] The muscle is a part of the external urethral sphincter that continues superiorly as a trough-like extension running vertically across the anterior aspect of the prostate to reach the neck of the bladder. [2] [3]
Diagram of ileocaecal valve and sphincter. The sphincter pupillae, or pupillary sphincter, belonging to the iris in the eye. The orbicularis oculi muscle, a muscle around the eye. The upper oesophageal sphincters; The lower esophageal sphincter, or cardiac sphincter, at the upper portion of the stomach.
The membranous urethra or intermediate part of male urethra is the shortest, least dilatable, and, with the exception of the urinary meatus, the narrowest part of the urethra. [citation needed] It extends from the apex of the prostate proximally to the bulb of urethra distally. It measures some 12 mm in length. It traverses the pelvic floor.