Internal urethral sphincter

Function

This is the primary muscle for maintaining continence of urine, a function shared with the external urethral sphincter which is under voluntary control. It prevents urine leakage as the muscle is tonically contracted via sympathetic fibers traveling through the inferior hypogastric plexus and vesical nervous plexus. Specifically, it is controlled by the hypogastric nerve, predominantly via the alpha-1 adrenergic receptor.

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. During ejaculation, the muscle contracts to prevent reflux of semen into the urinary bladder, a phenomenon called retrograde ejaculation.

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