What is the external anal sphincter muscle?
May 16, 2022
Source: Cainiu Health
Dr. Cheng Yicheng
Introduction
The subcutaneous layer consists of a circular muscular bundle surrounding the lower end of the anal canal and lying beneath the skin.
It is situated slightly lateral and superior to the subcutaneous layer, positioned between the deep and subcutaneous portions of the sphincter.
It originates from the coccyx and encircles the anal canal, terminating at the perineal body.
The deep portion lies superior to the superficial portion and forms a circular muscular bundle.
The posterior fibers of this muscular bundle merge with the puborectalis muscle and extend anteriorly as free fibers; some muscle fibers intermingle with the deep transverse perineal muscle at the ischial tuberosity.
The anal sphincter is part of the anorectal musculature and is conventionally divided into three components: the subcutaneous, superficial, and deep portions. Then, what muscle constitutes the external anal sphincter?
What Muscle Constitutes the External Anal Sphincter?
The subcutaneous portion consists of a circular muscular band encircling the lower end of the anal canal, located immediately beneath the skin. The superficial portion lies slightly lateral and superior to the subcutaneous portion, situated between the deep and subcutaneous sphincter layers. It originates from the coccyx and inserts into the central tendon of the perineum (perineal body), encircling the anal canal. The deep portion lies superior to the superficial portion and also forms a circular muscular band. Its posterior fibers merge with the puborectalis muscle and extend anteriorly as free fibers; some muscle fibers blend with the deep transverse perineal muscle and insert onto the ischial tuberosity.
The anal sphincter comprises three components: subcutaneous, superficial, and deep. The subcutaneous portion is a circular muscular band encircling the lower end of the anal canal and lies directly beneath the skin. The superficial portion is located slightly lateral and superior to the subcutaneous portion, lying between the deep and subcutaneous sphincter layers; it originates from the coccyx and encircles the anal canal anteriorly before inserting into the central tendon of the perineum. The deep portion—the longest and strongest component of the sphincter—is situated superior to the superficial portion and forms a circular muscular band. Its posterior fibers merge with the puborectalis muscle, making the two structures difficult to separate anatomically. Most fibers of the deep sphincter and the puborectalis muscle unite anteriorly to form the anterior portion of the anorectal ring.
The anal sphincter represents a continuation of the circular smooth muscle layer of the rectum. It appears pearly-white and functions primarily to maintain anal closure and regulate defecation. Normally, it remains in a state of tonic contraction to prevent involuntary leakage of feces, fluid, or gas from the anal canal. This smooth-muscle sphincter fatigues minimally but is prone to spasm. In contrast, the external anal sphincter—comprising the subcutaneous, superficial, and deep skeletal-muscle components—provides voluntary control over defecation through its contraction. However, being composed of striated muscle, the external sphincter fatigues more readily; sustained contraction typically lasts only about one minute. We hope this explanation proves helpful!
