External anal sphincter

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]

External anal sphincter
Coronal section through the anal canal. B. Cavity of urinary bladder V.D. Ductus deferens. S.V. Seminal vesicle. R. Second part of rectum. A.C. Anal canal. L.A. Levator ani. I.S. Sphincter ani internus. E.S. Sphincter ani externus.
Details
Nerve	Branch from the fourth sacral and contributions from the inferior hemorrhoidal branch of the pudendal nerve
Actions	Keep the anal canal and orifice closed
Identifiers
Latin	sphincter ani externus
TA98	A04.5.04.012
TA2	2426
FMA	21930
Anatomical terms of muscle [edit on Wikidata]

Anatomy

The external anal sphincter is far more substantial than the internal anal sphincter. The proximal portion of external anal sphincter overlaps the internal anal sphincter (which terminates distally a little distance proximal to the anal orifice) superficially; where the two overlap, they are separated by the intervening conjoint longitudinal muscle.^[1]

Structure

Historically, the sphincter was described as consisting of three parts (deep, superficial, and subcutaneous). This is not supported by current anatomical knowledge. Some sources still describe it in two layers, deep (or proximal) and superficial (or distal or subcutaneous).^[1]

Some of the muscles fibres decussate at the anterior midline and posterior midline, so forming an anterior commissure and posterior commissure.^[1]

Attachments

The muscle attaches anteriorly onto the perineal body, and posteriorly onto the anococcygeal ligament.^[1]

Innervation

The sphincter receives innervation from the bilaterally paired inferior anal nerve (each a branch of the pudendal nerve which is derived from ventral rami of S2-S4). It may also receive additional motor innervation from the nerve to levator ani.^[1]

Histology

The sphincter consists mostly of slow twitch fibers that allow extended continuous contraction.^[1]

Gallery

• 
  Intestines
• 
  Anatomy of the human anus.
• 
  Muscles of male perineum.
• 
  Muscles of the female perineum.
• 
  Sagittal (vertical) section of bladder, penis, and urethra.

See also

• Internal anal sphincter
• Puborectalis muscle