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 external sphincter muscle of male urethra, male or external sphincter muscle of female urethra, fe ...

muscle which constricts the

internal urethral orifice The internal urethral orifice is the opening of the urinary bladder into the urethra. Anatomy It is usually somewhat crescent-shaped. Relations It is formed by the neck of the urinary bladder. It opens at the apex/inferior angle of the tri ...

. It is located at the junction of the

urethra The urethra (: urethras or urethrae) is the tube that connects the urinary bladder to the urinary meatus, through which Placentalia, placental mammals Urination, urinate and Ejaculation, ejaculate. The external urethral sphincter is a striated ...

with the

urinary bladder The bladder () is a hollow organ in humans and other vertebrates that stores urine from the Kidney (vertebrates), kidneys. In placental mammals, urine enters the bladder via the ureters and exits via the urethra during urination. In humans, the ...

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 s ...

, but anatomically and functionally fully independent from it., page 29
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It is composed of

smooth muscle Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...

, so it is under the 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 ...

, specifically the

sympathetic nervous system The sympathetic nervous system (SNS or SANS, sympathetic autonomic nervous system, to differentiate it from the somatic nervous system) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous sy ...

Function

This is the primary muscle for maintaining 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 ...

, 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 Inferior may refer to: * Inferiority complex * An anatomical term of location * Inferior angle of the scapula, in the human skeleton * ''Inferior'' (book), by Angela Saini * '' The Inferior'', a 2007 novel by Peadar Ó Guilín * Inferior good: ...

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 fund ...

. Specifically, it is controlled by the hypogastric nerve, predominantly via the

alpha-1 adrenergic receptor alpha-1 (α1) adrenergic are G protein-coupled receptors (GPCRs) associated with the Gq alpha subunit, Gq heterotrimeric G protein. α1-adrenergic receptors are subdivided into three highly homologous subtypes, i.e., α1A-adrenergic, α1A-, α1B ...

. During urination, the

preganglionic In the autonomic nervous system, nerve fibers from the central nervous system to the autonomic ganglion, ganglion are known as preganglionic nerve fibers. All preganglionic fibers, whether they are in the sympathetic nervous system, sympathetic d ...

neurons of this sympathetic pathway are inhibited via signals arising in the pontine micturition center 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 penis through the urethra. It is the final stage and natural objective of male sexual stimulation, and an essential component of natural conception. ...

, the muscle contracts to prevent reflux of

semen Semen, also known as seminal fluid, is a bodily fluid that contains spermatozoon, spermatozoa which is secreted by the male gonads (sexual glands) and other sexual organs of male or hermaphrodite, hermaphroditic animals. In humans and placen ...

into the

, 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, inhibiting urination and preventing a reflux of semen ...

. 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 urination, urinate in the real or imaginary presence of others, such as in a public restroom. The analogous condition that affects bowel movement i ...

, in which a person who perceives oneself to be under observation is unable to urinate.

References

{{Authority control Urinary system Urethra