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The internal anal sphincter is the specialised thickened terminal portion of the inner circular layer of smooth muscle of the large intestine. It extends from the pectinate line (anorectal junction) proximally to just proximal to the anal orifice distally (the distal termination is palpable). Its muscle fibres are arranged in a spiral (rather ...
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 urination. The female or male external sphincter muscle of urethra (sphincter urethrae): located in the deep perineal pouch, at the bladder's distal ...
The internal urethral sphincter is a urethral sphincter muscle which constricts the internal urethral orifice. 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. [ 3 ]
The internal anal sphincter (an involuntary muscle) responds to the pressure by relaxing, thus allowing the feces to enter the canal. The rectum shortens as feces are pushed into the anal canal and peristaltic waves push the feces out of the rectum.
Because of the high incidence of internal anal sphincter thickening with the disorder, it is thought to be a disorder of that muscle or that it is a neuralgia of pudendal nerves. In one study of 68 people with proctalgia fugax, 55 had tenderness along the course of the pudendal nerve.
Nontraumatic conditions causing anal sphincter weakness include scleroderma, damage to the pudendal nerves, and IAS degeneration of unknown cause. [5] Radiation-induced FI may involve the anal canal as well as the rectum, when proctitis, anal fistula formation, and diminished function of internal and external sphincter occur. [2]
Stimulation of the parasympathetic will cause the detrusor muscle (urinary bladder wall) to contract and simultaneously relax the internal sphincter muscle between the bladder and the urethra, allowing the bladder to void. Also, parasympathetic stimulation of the internal anal sphincter will relax this muscle to allow defecation.
The muscles of the anus (corrugator cutis ani, the internal anal sphincter and the external anal sphincter) The medial muscles of the urogenital region (the superficial transverse perineal muscle, the deep transverse perineal muscle and bulbocavernosus) The medial levator ani muscles (puborectalis and pubococcygeus) The fascia of perineum ...