External Anal Sphincter

The external anal sphincter is a voluntary, striated muscle that encircles the anal canal and provides fine motor control over fecal continence. Unlike the internal anal sphincter, it is under conscious control and is innervated by the somatic nervous system. The muscle is cylindrical, highly dynamic, and functionally subdivided into three parts: deep, superficial, and subcutaneous. It acts in coordination with the internal sphincter and puborectalis muscle to maintain anal tone and enable voluntary control of defecation.

Structure

The external anal sphincter is composed of skeletal muscle fibers and appears as a broad, elliptical muscular band. It is arranged concentrically around the lower two-thirds of the anal canal. Though commonly described as three parts — deep, superficial, and subcutaneous — these divisions are not sharply demarcated and blend together. The muscle maintains tonic contraction at rest, with the ability to increase tension voluntarily.

• Muscle type: Striated skeletal muscle
• Fiber orientation: Circular
• Subdivisions:
  • Deep part: Fused with puborectalis muscle, surrounds upper anal canal
  • Superficial part: Anchored to perineal body and coccyx via anococcygeal ligament
  • Subcutaneous part: Surrounds anal orifice, just beneath the skin

Location

The external anal sphincter surrounds the inferior portion of the anal canal and lies external to the internal anal sphincter. It extends from just below the anorectal junction to the anal verge. It is situated in the perineum and is part of the muscular components of the pelvic floor.

Function

• Voluntary Continence: Provides voluntary control over the expulsion of feces and flatus
• Supportive Role: Acts as a muscular barrier that compresses the anal canal and aids closure
• Active Contraction: Enhances pressure during coughing, sneezing, or increased intra-abdominal pressure
• Postural Contribution: Contracts reflexively with changes in posture or exertion

Innervation

• Somatic motor supply: Inferior rectal nerve (branch of pudendal nerve, S2–S4)
• Voluntary control: Conscious contraction and relaxation
• Reflex arcs: Involved in reflex continence (e.g., guarding reflex)

Blood Supply

• Arterial: Inferior rectal artery (branch of internal pudendal artery)
• Venous: Inferior rectal vein → internal pudendal vein → internal iliac vein

Lymphatic Drainage

• Superficial inguinal lymph nodes

Histological Features

• Striated muscle fibers with peripheral nuclei
• Intermixed connective tissue and neurovascular bundles
• Fibers organized in circular layers

Clinical Significance

• Fecal Incontinence: Damage to the external sphincter (e.g., obstetric injury, nerve damage) results in voluntary control loss
• Pudendal Neuropathy: Compression or stretch injury to pudendal nerve affects external sphincter function
• Episiotomy Complications: Improper extension may involve the external sphincter in third- or fourth-degree perineal tears
• Electromyography (EMG): Used to assess function in patients with continence disorders
• Surgical Repair: Sphincteroplasty is performed in sphincter defects due to trauma or surgery