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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]
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 external anal sphincter can be relaxed by the application of glyceryl trinitrate creams, and constipation is managed with laxatives and improving hydration. [9] Some fissures may require botulinum toxin injection; worst cases may require surgical intervention, such as "lateral internal anal sphincterotomy or advancement anoplasty ...
In some cases, the fibers of the deeper layer of the external anal sphincter cross over in front of the anus and are continued into this muscle. Occasionally it gives off fibers, which join with the bulbocavernosus of the same side. There are some variations: it may be absent or double, or insert into the bulbocavernosus or the external sphincter.
The sphincter urethrae, or urethral sphincter, controlling the exit of urine from the body. At the anus, there are two anal sphincters which control the exit of feces from the body, the internal anal sphincter and external anal sphincter. The inner sphincter is involuntary and the outer is voluntary.
To top it off, the anal sphincter will contract once the skin is irritated on the outside. “This is an involuntary muscle, so even with bowel movements, the muscle will not relax,” Pannell said.
The external anal sphincter, by contrast, is made up of skeletal (or striated muscle) and is therefore under voluntary control. It can contract vigorously for a short time. Contraction of the external sphincter can defer defecation for a time by pushing stool from the anal canal back into the rectum.
The deep branch of the perineal nerve supplies the muscles of the perineum. [2] These include superficial transverse perineal muscle, bulbospongiosus muscle, ischiocavernosus muscle, the bulb of penis. [1] It also supplies levator ani, and the external anal sphincter. [1]