The external sphincter muscle of the male urethra, also sphincter urethrae membranaceae, sphincter urethrae externus, surrounds the whole length of the
membranous urethra
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. It extends from the apex of the prostate proximally to the bulb of ur ...
, and is enclosed in the
fascia
A fascia (; : fasciae or fascias; adjective fascial; ) is a generic term for macroscopic membranous bodily structures. Fasciae are classified as superficial, visceral or deep, and further designated according to their anatomical location.
...
of the
urogenital diaphragm
Older texts have asserted the existence of a urogenital diaphragm, also called the triangular ligament, which was described as a layer of the pelvis that separates the deep perineal sac from the upper pelvis, lying between the inferior fascia of ...
.
Its external fibers arise from the junction of the
inferior pubic ramus
In vertebrates, the pubis or pubic bone () forms the lower and anterior part of each side of the hip bone. The pubis is the most forward-facing (ventral and anterior) of the three bones that make up the hip bone. The left and right pubic bones ar ...
and
to the extent of 1.25 to 2 cm., and from the neighboring fascia.
They arch across the front of the urethra and
bulbourethral glands, pass around the urethra, and behind it unite with the muscle of the opposite side, by means of a tendinous
raphe
Raphe ( ; from ;Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie.'' Oxford: Clarendon Press. : raphae or raphes) has several differe ...
.
Its innermost fibers form a continuous circular investment for the membranous urethra.
Function
The muscle helps maintain
continence of
urine
Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...
along with the
internal urethral sphincter
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, but anatomically and f ...
which is under control of the
autonomic nervous system
The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...
. 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
The pudendal nerve is the main nerve of the perineum. It is a Mixed nerve, mixed (motor and sensory) nerve and also conveys Sympathetic nervous system, sympathetic Autonomic nervous system, autonomic fibers. It carries sensation from the exter ...
to
synapse
In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...
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 external sphincter opens and the internal sphincter closes.
Additional images
File:Gray236.png, Right hip bone. Internal surface.
See also
*
Levator ani
The levator ani is a broad, thin muscle group, situated on either side of the pelvis. It is formed from three muscle components: the pubococcygeus, the iliococcygeus, and the puborectalis.
It is attached to the inner surface of each side of the ...
*
External sphincter muscle of female urethra
*
Internal urethral sphincter
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, but anatomically and f ...
*
Prostatic urethra
The prostatic urethra, the widest and most dilatable part of the urethra canal, is about 3 cm long.
It runs almost vertically through the prostate from its base to its apex, lying nearer its anterior than its posterior surface; the form of ...
References
External links
* - "Muscles of the female urogenital diaphragm (deep perineal pouch) and structures located inferior to it."
Urinary system
Male urethra
{{muscle-stub
de:Musculus urethralis