Internal urethral sphincter

Definition
noun
The urethral sphincter made up of smooth muscle cells found at the inferior end of the urinary bladder and the proximal end of the urethra, and is involved primarily in the maintenance of continence of urine
Supplement
The urethral sphincters are the muscles that control the discharge of urine from the urinary bladder through the urethra. In males, they are also involved in controlling the exit of semen during ejaculation. There are two urethral sphincters: (1) internal urethral sphincter and (2) external urethral sphincter (i.e. the urethral sphincter that is at the distal end of the urinary bladder in females and inferior to the prostate in males).
The internal urethral sphincter is a sphincter muscle that is found at the inferior end of the urinary bladder and the proximal end of the urethra. It is a complete collar of smooth muscle cells of the neck of the urinary bladder which extend distally to surround the pre-prostatic portion of the male urethra. There is not a comparable structure in the neck of the femoral bladder.¹
The internal urethral sphincter is made up of smooth muscle. It is the primary muscle that prohibits the release of urine. In males, the internal urethral sphincter may exist to prevent reflux of semen into the bladder. It is kept contracted tonically by the lumbar plexus and relaxes during micturition via the branches from the inferior hypogastric plexus of the parasympathetic nervous system.
Synonym(s):

• sphincter vesicae
• annulus urethralis
• musculus sphincter vesicae
• preprostate urethral sphincter
• proximal urethral sphincter
• sphincter muscle of urinary bladder

See also:

Reference(s):

¹ internal urethral sphincter. (n.d.) Farlex Partner Medical Dictionary. (2012). Retrieved from ://medical-dictionary.thefreedictionary.com/internal+urethral+sphincter