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.
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
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. Unlike the internal sphincter muscle, the external sphincter is made of
skeletal muscle, therefore it is under voluntary control of the somatic nervous system.[1]
In males and females, both
internal and external urethral sphincters function to prevent the release of urine. The internal urethral sphincter controls involuntary urine flow from the bladder to the urethra, whereas the external urethral sphincter controls voluntary urine flow from the bladder to the urethra.[2] Any damage to these muscles can lead to
urinary incontinence. In males, the internal urethral sphincter has the additional function of preventing the flow of
semen into the male bladder during
ejaculation.[3]
Females do have a more elaborate external sphincter muscle than males as it is made up of three parts: the sphincter urethrae, the 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.[4]
Congenital abnormalities of the female urethra can be surgically repaired with
vaginoplasty.[5]
Clinical significance
The urethral sphincter is considered an integral part of maintaining
urinary continence, and it is important to understand its role in some conditions:
Stress urinary incontinence is a common problem related to the function of the urethral sphincter. Weak pelvic floor muscles, intrinsic sphincter damage, or damage to the surrounding nerves and tissue can make the urethral sphincter incompetent, and subsequently, it will not close fully, leading to stress urinary incontinence. In women, childbirth, obesity, and age can all be risk factors, especially by weakening the pelvic floor muscles.[6] In men, prostate surgery (
prostatectomy,
TURP, etc) and radiation therapy can damage the sphincter and cause stress incontinence.[7]
^Maclean, Allan; Reid, Wendy (2011). "40". In Shaw, Robert (ed.). Gynaecology. Edinburgh New York: Churchill Livingstone/Elsevier. pp. 599–612.
ISBN978-0-7020-3120-5; Access provided by the University of Pittsburgh{{
cite book}}: CS1 maint: postscript (
link)
^
abGupta S, Sharma R, Agarwal A, Parekh N, Finelli R, Shah R, Kandil H, Saleh R, Arafa M, Ko E, Simopoulou M, Zini A, Rajmil O, Kavoussi P, Singh K, Ambar RF, Elbardisi H, Sengupta P, Martinez M, Boitrelle F, Alves MG, Khalafalla K, Roychoudhury S, Busetto GM, Gosalvez J, Tadros N, Palani A, Rodriguez MG, Anagnostopoulou C, Micic S, Rocco L, Mostafa T, Alvarez JG, Jindal S, Sallam H, Rosas IM, Lewis S, AlSaid S, Altan M, Park HJ, Ramsay J, Parekattil S, Sabbaghian M, Tremellen K, Vogiatzi P, Gilani M, Evenson DP, Colpi GM (April 2022).
"A Comprehensive Guide to Sperm Recovery in Infertile Men with Retrograde Ejaculation". The World Journal of Men's Health. 40 (2): 208–216.
doi:
10.5534/wjmh.210069.
PMC8987146.
PMID34169680.
^Netter, Frank H. (2019). Atlas of Human Anatomy, Seventh Edition. Philadelphia: Elsevier.
ISBN9780323393225.
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."