尿道括約筋（ にょうどうかつやくきん、英：sphincter urethrae ）
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よってここからは外肛門括約筋の解説となる。
尿道口から距離は、女性で約1～2㎝、男性は成人で約15～18㎝となる。（男性は個人差が大きい）
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【 停 止 】：尿道をぐるりと取り囲むように走り、反対側の筋線維と交差して環状を成します。
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排尿 (urination) をコントロールする。
・神 経 ： 陰部神経（S2〜S4）。脳の命令（随意支配）を受けて収縮します。
・脈 管 ： 内陰部動脈 （の枝である尿道球動脈など）
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「The urethral sphincter is one of two muscles used to control the exit of urine in the urinary bladder through the urethra. The two muscles are the male or female external urethral sphincter and the internal urethral sphincter. When either of these muscles contracts, the urethra is sealed shut.
The external urethral sphincter originates at the ischiopubic ramus and inserts into the intermeshing muscle fibers from the other side. It is controlled by the deep perineal branch of the pudendal nerve. Activity in the nerve fibers constricts the urethra.
- 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 female or male 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.
Dissection of prostate showing the fibers of the external sphincter surrounding the membranous urethra and partially cradling the inferior portion of the prostate.
Sex differences
Main articles: External sphincter muscle of male urethra and External sphincter muscle of female urethra
In males and females, both internal and external urethral sphincters function to inhibit the release of urine. In males, the internal sphincter muscle of urethra functions to prevent reflux of seminal fluids into the male bladder during ejaculation.
Females do have a more elaborate external sphincter muscle than males as it is made up of three parts: the sphincter urethrae, urethrovaginal muscle, and the compressor urethrae. The urethrovaginal muscle fibers wrap around the vagina and urethra and contraction leads to constriction of both the vagina and the urethra. The origin of the compressor urethrae muscle is the right and left inferior pubic ramus and it wraps anteriorly around the urethra so when it contracts it squeezes the urethra against the vagina. The external urethrae, like in males, wraps solely around the urethra.
Function
The internal urethral sphincter provides involuntary control of urination. The external urethral sphincter provides voluntary control of urination.」
【 語 句 】
・urine：尿 ・urinary bladder：膀胱 ・urethra：尿道 ・external urethral sphincter：外尿道括約筋 ・internal urethral sphincter：内尿道括約筋 ・contract：収縮する ・ ischiopubic ramus：坐骨恥骨枝 ・perineal：会陰の ・pudendal nerve：陰部神経 ・proximal：近位の ・detrusor：排尿筋 ・smooth muscle：平滑筋 ・membranous urethra：尿道膜様部 ・ involuntary：無意識な、不随意の ・autonomic：自立の ・prostate：前立腺 ・skeletal muscle：骨格筋 ・somatic nervous system：体性神経系 ・dissection：解剖 ・cradle：抱くように持つ.？・inhibit：抑制する ・reflux：逆流 ・seminal：精液の ・ejaculation：射精 ・elaborate：精巧な ・urethrovaginal muscle：尿道膣筋？・compressor urethrae：尿道圧迫筋 ・vagina：膣 ・constriction：締め付け ・inferior pubic ramus：下恥骨枝
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