External sphincter muscle of male urethra
The external sphincter muscle of male urethra, also sphincter urethrae membranaceae, sphincter urethrae externus, surrounds the whole length of the membranous urethra, and is enclosed in the fascia of the urogenital diaphragm.
|Sphincter urethrae membranaceae muscle|
The male urethra laid open on its anterior (upper) surface. (Region visible, but muscle not labeled.)
Coronal section of anterior part of pelvis, through the pubic arch. Seen from in front. (Region visible, but muscle not labeled.)
|Origin||Junction of the inferior rami of the pubis and ischium to the extent of 1.25 to 2 cm.|
|Nerve||Somatic fibers from S2-S4 through Pudendal nerve|
|Actions||Constricts urethra, maintains urinary continence|
|Latin||musculus sphincter urethrae externus urethrae masculinae, musculus sphincter urethrae membranaceae|
|Anatomical terms of muscle|
Its innermost fibers form a continuous circular investment for the membranous urethra.
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.
Voiding urine begins with voluntary relaxation of the external urethral sphincter. This is facilitated by inhibition of the somatic neurons in Onuf's nucleus via signals arising in the pontine micturition center and traveling through the descending reticulospinal tracts. During ejaculation, the internal urethral sphincter contracts to prevent mixing between urine and semen and backward flow of semen into the bladder (retrograde ejaculation).
- Jung J, Ahn HK, and Huh Y (September 2012). "Clinical and Functional Anatomy of the Urethral Sphincter". Int Neurourol J. 16 (3): 102–106. doi:10.5213/inj.2012.16.3.102. PMC 3469827. PMID 23094214.
- Shah AP, Mevcha A, Wilby D, Alatsatianos A, Hardman JC, Jacques S, Wilton JC (November 2014). "Continence and micturition: An anatomical basis" (PDF). Clin. Anat. 27 (8): 1275–1283. doi:10.1002/ca.22388. PMID 24615792.
- Shafik A (July–August 1997). "Pelvic floor muscles and sphincters during erection and ejaculation". Arch Androl. 39 (1): 71–78. doi:10.3109/01485019708987904. PMID 9202836.
- J. B. Macalpine (November 1934). "The Musculature of the Bladder-neck of the Male in Health and Disease". Proceedings of the Royal Society of Medicine. 28 (1): 39–56. doi:10.1177/003591573402800112. PMC 2205523. PMID 19990023.
- 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."