It is located at the junction of the urethra with the urinary bladder and is continuous with the detrusor muscle,[1][2] but anatomically and functionally fully independent from it.

This is the primary muscle for maintaining continence of urine, a function shared with the external urethral sphincter which is under voluntary control.

[5] During urination, the preganglionic 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.

[8] Because the internal urethral sphincter is under involuntary control, it is believed to play a role in paruresis, in which a person who perceives oneself to be under observation is unable to urinate.

This article incorporates text in the public domain from page 429-431 of the 20th edition of Gray's Anatomy (1918)