Internal Anal Sphincter

The internal anal sphincter is an involuntary smooth muscle structure that plays a key role in maintaining resting anal tone and continence. It is a direct continuation of the circular smooth muscle layer of the rectum and surrounds the upper two-thirds of the anal canal. Together with the external anal sphincter and puborectalis, it forms the core of the anorectal continence mechanism. It remains tonically contracted at rest and relaxes reflexively during defecation in response to parasympathetic stimulation.

Structure

The internal anal sphincter is composed entirely of circular smooth muscle fibers. It is thicker than the circular muscle of the rectum and forms a continuous, cylindrical sleeve around the upper part of the anal canal. It lacks voluntary control and is innervated by autonomic fibers. Histologically, the muscle is made up of spindle-shaped smooth muscle cells with centrally located nuclei and no visible striations under light microscopy.

• Length: Approximately 2.5–3.5 cm
• Thickness: Greater than the circular layer of the rectum
• Type: Involuntary smooth muscle

Its proximal end blends with the circular muscle of the rectum, and its distal end is demarcated from the subcutaneous part of the external anal sphincter by the intersphincteric groove.

Location

The internal anal sphincter is located in the upper two-thirds of the anal canal. It lies deep (internal) to the external anal sphincter and is separated from it by the **intersphincteric space**, which contains connective tissue, nerves, and vascular structures.

Function

• Resting Anal Tone: Maintains constant tonic contraction at rest, contributing 55–70% of resting anal pressure
• Involuntary Continence: Prevents involuntary passage of feces and gas during non-defecatory states
• Reflexive Relaxation: Undergoes reflex relaxation during the rectoanal inhibitory reflex (RAIR), triggered by rectal distension
• Complement to External Sphincter: Functions alongside voluntary muscles for layered continence control

Innervation

• Sympathetic fibers (from hypogastric plexus): Maintain tonic contraction
• Parasympathetic fibers (pelvic splanchnic nerves, S2–S4): Mediate reflex relaxation during defecation
• Visceral afferents: Relay information from stretch receptors to spinal cord

Blood Supply

• Arterial: Superior rectal artery (branch of inferior mesenteric artery), middle rectal artery, and inferior rectal artery
• Venous: Drains into superior rectal vein (portal system) and inferior rectal vein (systemic system)

Lymphatic Drainage

• Above pectinate line: Internal iliac lymph nodes
• Below pectinate line (distal interdigitation): May drain into superficial inguinal nodes

Histological Features

• Smooth muscle cells arranged concentrically
• Non-striated with central nuclei
• Often blends with surrounding connective tissue and longitudinal muscle layer

Clinical Significance

• Fecal Incontinence: Damage or degeneration of internal sphincter (e.g., aging, childbirth) can result in passive leakage
• Anal Fissures: Hypertonicity of the internal sphincter is a contributing factor in chronic fissures; lateral internal sphincterotomy is a common treatment
• Rectoanal Inhibitory Reflex (RAIR): The absence of internal sphincter relaxation is used to diagnose Hirschsprung’s disease in infants
• Surgical Considerations: Injury to the internal sphincter during anorectal surgery (e.g., hemorrhoidectomy) can result in impaired continence