urethral sphincterとは 意味・読み方・使い方

The urethral sphincter is a collective name for the muscles used to control the flow of urine (micturition) from the urinary bladder. These muscles envelop the urethra, so that when they contract, the urethra is sealed shut. The muscles originate at the pubic ramus with the insertion point at the median raphe. The function of the sphincter urethrae (external sphincter) is controlled by the pudendal nerve which acts to constrict the urethra. There are actually two urethral sphincters in the human body:

• The internal sphincter muscle of urethra: located at the bladder's inferior end and the urethra's proximal end at the junction of the urethra with the urinary bladder. The internal sphincter is a continuation of the detrusor muscle and is made of smooth muscle, therefore it is under involuntary or autonomic control. This is the primary muscle for prohibiting the release of urine.
• The external sphincter muscle of urethra (sphincter urethrae): located at the bladder's distal inferior end in females and inferior to the prostate (at the level of the membranous urethra) in males is a secondary sphincter to control the flow of urine through the urethra. Unlike the internal sphincter muscle, the external sphincter is made of skeletal muscle, therefore it is under voluntary control of the somatic nervous system.