The internal urethral sphincter is a
urethral sphincter
The urethral sphincters are two muscles used to control the exit of urine in the urinary bladder through the urethra. The two muscles are either the male or female external urethral sphincter and the internal urethral sphincter. When either of th ...
muscle which constricts the
internal urethral orifice
The internal urethral orifice is the opening of the urinary bladder into the urethra. It is placed at the apex of the trigonum vesicae, in the most dependent part of the bladder, and is usually somewhat crescent-shaped; the mucous membrane immedi ...
. It is located at the junction of the
urethra
The urethra (from Greek οὐρήθρα – ''ourḗthrā'') is a tube that connects the urinary bladder to the urinary meatus for the removal of urine from the body of both females and males. In human females and other primates, the urethra con ...
with the
urinary bladder
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
and is continuous with the
detrusor muscle
The detrusor muscle, also detrusor urinae muscle, muscularis propria of the urinary bladder and (less precise) muscularis propria, is smooth muscle found in the wall of the bladder. The detrusor muscle remains relaxed to allow the bladder to stor ...
,
but anatomically and functionally fully independent from it.
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It is composed of
smooth muscle
Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...
, so it is under the 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 viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...
, specifically the
sympathetic nervous system
The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of th ...
.
Function
![Urinary Sphincter](https://upload.wikimedia.org/wikipedia/commons/e/ed/Urinary_Sphincter.png)
This is the primary muscle for maintaining
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 synony ...
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 excretion, excreted from the body through the urethra.
Cel ...
, a function shared with the
external urethral sphincter which is under
voluntary
Voluntary may refer to:
* Voluntary (music)
* Voluntary or volunteer, person participating via volunteering/volunteerism
* Voluntary muscle contraction
See also
* Voluntary action
* Voluntariness, in law and philosophy
* Voluntaryism
Volunt ...
control. It prevents urine leakage as the muscle is tonically contracted via
sympathetic fibers traveling through the
inferior hypogastric plexus
The inferior hypogastric plexus (pelvic plexus in some texts) is a network () of nerves that supplies the organs of the pelvic cavity. The inferior hypogastric plexus gives rise to the prostatic plexus in males and the uterovaginal plexus in female ...
and
vesical nervous plexus
The vesical nervous plexus arises from the forepart of the pelvic plexus. The nerves composing it are numerous, and contain a large proportion of spinal nerve fibers. They accompany the vesicle arteries, and are distributed to the sides and fundu ...
. Specifically, it is controlled by the hypogastric nerve, predominantly via the alpha-1 receptor.
During urination, the
preganglionic
In the autonomic nervous system, fibers from the CNS to the ganglion are known as preganglionic fibers. All preganglionic fibers, whether they are in the sympathetic division or in the parasympathetic division, are cholinergic (that is, these f ...
neurons of this sympathetic pathway are inhibited 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, allowing the muscle to relax. During
ejaculation
Ejaculation is the discharge of semen (the ''ejaculate''; normally containing sperm) from the male reproductory tract as a result of an orgasm. It is the final stage and natural objective of male sexual stimulation, and an essential componen ...
, the muscle contracts to prevent reflux of
semen
Semen, also known as seminal fluid, is an organic bodily fluid created to contain spermatozoa. It is secreted by the gonads (sexual glands) and other sexual organs of male or hermaphroditic animals and can fertilize the female ovum. Semen i ...
into the
urinary bladder
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
, a phenomenon called
retrograde ejaculation
Retrograde ejaculation occurs when semen which would be ejaculated via the urethra is redirected to the urinary bladder. Normally, the sphincter of the bladder contracts before ejaculation, sealing the bladder which besides inhibiting the releas ...
.
Spasms of the internal urethral sphincter are associated with penile erection.
Because the internal urethral sphincter is under involuntary control, it is believed to play a role in
paruresis
Paruresis, also known as shy bladder syndrome, is a type of phobia in which a person is unable to urinate in the real or imaginary presence of others, such as in a public restroom. The analogous condition that affects bowel movement is called p ...
, in which a person who perceives oneself to be under observation is unable to urinate.
See also
*
Detrusor muscle
The detrusor muscle, also detrusor urinae muscle, muscularis propria of the urinary bladder and (less precise) muscularis propria, is smooth muscle found in the wall of the bladder. The detrusor muscle remains relaxed to allow the bladder to stor ...
*
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 ...
*
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.
Its external fibers ...
*
Urination#Physiology
References
{{Authority control
Urinary system
Urethra