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The external anal sphincter (or sphincter ani externus) is an oval tube of skeletal muscle fibers. [1] Distally, it is adherent to the skin surrounding the margin of the anus. [2] It exhibits a resting state of tonical contraction [1] and also contracts during the bulbospongiosus reflex. [3] [4] [5] [6]
Anal encirclement effectively supplements the anal sphincter, narrowing the anal canal and its barrier function to stool, without altering voluntary control. Since complications are common, and can be serious (fecal impaction, infection, erosion of encirclement through anal canal), modern surgeons prefer to perform colostomy.
Defects of the external anal sphincter are associated with urge incontinence. [22] The external anal sphincter is supplied by the pudendal nerve. Damage to the nerve supply of the external anal sphincter on one side may not result in severe symptoms because there is substantial overlap in innervation by the nerves on the other side. [2]
The pelvic floor are the muscles that attach to the pelvis in the abdomen. Anal sphincter dyssynergia can be caused by obstructions, but mostly improper relaxing of the anal sphincters or pelvic floor muscle during defecation. Also if there is a decrease in intrarectal pressure defecation can occur. [5]
This results in less smooth muscle contraction of the colon, increasing the colon transit time. [12] The reduced dopamine levels also causes dystonia of the striated muscles of the pelvic floor and external anal sphincter. This explains how Parkinson's disease can lead to constipation. [14]
The external anal sphincter muscle is the voluntary muscle that surrounds and adheres to the anus at the lower margin of the anal canal. This muscle is in a state of tonic contraction, but during defecation, it relaxes to allow the release of feces. Movement of the feces is also controlled by the involuntarily controlled internal anal sphincter ...
The rectococcygeal muscles are composed of smooth muscle and run from the anterior surface of the 2nd and 3rd coccygeal vertebrae down the posterior wall of the rectum as a triangular shaped muscle before branching and inserting among the various muscles and fascial structures associated with the pelvic diaphragm and anal canal and collectively called longitudinal anal muscles.
The anal wink, anal reflex, perineal reflex, or anocutaneous reflex is the reflexive contraction of the external anal sphincter upon stroking of the skin around the anus. [ 1 ] A noxious or tactile stimulus will cause a wink contraction of the anal sphincter muscles and also flexion.