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The external anal sphincter (or sphincter ani externus ) is a flat plane of skeletal muscle fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus. Its action is entirely involuntary, and it is in a state of continuous maximal contraction. It helps the sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces. Sympathetic fibers from the superior rectal and hypogastric plexuses stimulate and maintain internal anal sphincter contraction. Its contraction is inhibited by parasympatheticfiber stimulation. The external anal sphincter receives somatic innervation from the inferior anal nerve, a branch of the pudendal nerve (s2-s4). Theres a quick and fun way to remember the innervation of the external anal sphincter! Just use the phrase s2, 3, 4 keep the poo off the floor. other articles where external anal sphincter is discussed anal canal the external sphincter is a layer of voluntary (striated) muscle encircling the outside wall of the anal canal and anal opening. One can cause it to expand and contract at will, except during the early years of life when it is not yet fully developed. Though resting sphincter tone is predominantly attributed to the internal anal sphincter, studies under general anesthesia or after pudendal nerve block suggest the external anal sphincter generally accounts for approximately 25, and up to 50, of resting anal tone. 9, 33, 40, 50, 144, 170 when continence is threatened, the external sphincter contracts to augment anal tone, preserving continence. External anal sphincter voluntary muscle that surrounds the lower 23 of the anal canal (and so overlaps with the internal sphincter). Somatic innervation via the inferior rectal nerves (branches of the pudendal nerve) sensitive to pain, temperature, touch and pressure. other articles where internal anal sphincter is discussed anal canal the internal sphincter is part of the inner surface of the canal it is composed of concentric layers of circular muscle tissue and is not under voluntary control. The distribution of the sympathetic innervation to the internal anal sphincter (ias) and rectum and the occurrence of different types of adrenergic receptors in the two organs were investigated in anaesthetized cats. anal sphincter emg may also be useful in the evaluation of patients with multisystem atrophy (msa). 11, 12 patients with msa have degenerative changes in interomediolateral cell columns and onufs nucleus in the spinal cord, innervating both urethral and anal sphincters. Abnormal emg of the eas is strongly suggestive of an atypical parkinsonian syndrome such as msa.