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
A fascia (; plural fasciae or fascias; adjective fascial; from Latin: "band") is a band or sheet of connective tissue, primarily collagen, beneath the skin that attaches to, stabilizes, encloses, and separates muscles and other internal organs ...
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 pubic region ( la, pubis) is the most forward-facing (ventral and anterior) of the three main regions making up the coxal bone. The left and right pubic regions are each made up of three sections, a superior ramus, inferior ra ...
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 Greek ῥαφή, "seam"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.) has several diffe ...
.
Its innermost fibers form a continuous circular investment for the membranous urethra.
Function
The muscle helps maintain
continence
Continence may refer to:
*Fecal continence, the ability to control defecation, see Fecal incontinence
*Urinary continence, the ability to control urination, see Urinary incontinence, the involuntary excretion of urine
*Sexual continence, a synon ...
of
urine
Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra.
Cellular m ...
along with the
internal urethral sphincter which is under control of the
autonomic nervous system
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies internal organs, smooth muscle and glands. The autonomic nervous system is a control system t ...
. The external sphincter muscle prevents urine leakage as the muscle is tonically contracted via
somatic fibers that originate in
Onuf's nucleus
Onuf's nucleus is a distinct group of neurons located in the ventral part ( laminae IX) of the anterior horn of the sacral region of the human spinal cord involved in the maintenance of micturition and defecatory continence, as well as muscular ...
and pass through
sacral spinal nerves S2-S4 then the
pudendal nerve
The pudendal nerve is the main nerve of the perineum. 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 fem ...
to
synapse
In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell.
Synapses are essential to the transmission of nervous impulses from ...
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
The Pontine micturition center (PMC, also known as Barrington's nucleus) is a collection of neuronal cell bodies located in the rostral pons in the brainstem involved in the supraspinal regulation of micturition. When activated, the PMC relaxes th ...
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
The external sphincter muscle of female urethra is a muscle which controls urination in females. The muscle fibers arise on either side from the margin of the inferior ramus of the pubis. They are directed across the pubic arch in front of the uret ...
*
Internal urethral sphincter
*
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