External Anal Sphincter Innervation
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External Anal Sphincter Innervation: Anatomy and Function
The external anal sphincter is a short tube of skeletal muscle that surrounds the lower part of the anal canal. It is one of the three structures that form the anal sphincter complex, which controls defecation. The external anal sphincter is largely under voluntary control by the somatic nervous system, which allows it to stay in a contracted state. The muscle is voluntarily relaxed during defecation to allow the passage of feces.
In this article, we will discuss the anatomy and function of the external anal sphincter innervation, as well as some clinical implications.
Anatomy of External Anal Sphincter Innervation
The external anal sphincter receives somatic innervation from the inferior anal nerve, a branch of the pudendal nerve (S2-S4). The pudendal nerve originates from the sacral plexus and passes through the greater sciatic foramen, then reenters the pelvis through the lesser sciatic foramen. It then runs along the lateral wall of the ischiorectal fossa and gives off several branches, including the inferior anal nerve.
The inferior anal nerve enters the perineum through Alcock's canal, a fascial tunnel formed by the obturator fascia on the medial side of the ischial tuberosity. It then divides into several branches that supply the external anal sphincter and the perianal skin. The inferior anal nerve also communicates with the perineal branch of the fourth sacral nerve (S4), forming an anococcygeal nerve that innervates the anococcygeal ligament and skin.
As a result of this innervation, the external anal sphincter is under voluntary control and can be contracted or relaxed at will. However, there are also reflex mechanisms that regulate its activity. For example, when feces enter the rectum and distend it, stretch receptors send signals to the spinal cord and trigger a reflex contraction of both internal and external anal sphincters. This is called the rectoanal inhibitory reflex and it prevents involuntary defecation. Conversely, when there is a conscious desire to defecate, signals from the brain override this reflex and relax both sphincters.
Function of External Anal Sphincter Innervation
The main function of the external anal sphincter innervation is to maintain fecal continence and prevent leakage of feces or gas from the anus. The external anal sphincter works together with other structures of the pelvic floor, such as the puborectalis muscle and the levator ani muscle, to form an angle between the rectum and the anal canal. This angle helps to keep feces in place until defecation occurs.
The external anal sphincter also plays a role in defecation by relaxing to allow feces to pass through. Defecation is a complex process that involves several factors, such as abdominal pressure, colonic motility, rectal distension, and voluntary control. When feces accumulate in the rectum, they stimulate stretch receptors that initiate a mass movement of feces towards the anus. At the same time, abdominal muscles contract to increase intra-abdominal pressure and push feces downwards. The internal anal sphincter relaxes reflexively in response to rectal distension, while
the external anal sphincter remains contracted until there is a conscious decision to defecate. Then, both sphincters relax and feces are expelled through
Clinical Implications of External Anal Sphincter Innervation
The external anal sphincter innervation can be affected by various conditions that impair its function or damage its structure. Some examples are:
Trauma: Injury to the pudendal nerve or its branches can occur during childbirth, pelvic surgery, pelvic fracture, or blunt trauma. This can result in partial or complete loss of voluntary control over
the external anal sphincter, leading to fecal incontinence or difficulty in defecation.
Neurological disorders: Diseases that affect
the central or peripheral nervous system can also impair
the external anal sphincter innervation. For example,
spinal cord injury, stroke, multiple sclerosis,
Parkinson's disease, diabetes mellitus,