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    Related Topics

    From Digestive System

    Cecum
    First part of large intestine.
    Jejunum
    Second portion of the small intestine.
    Body
    Main central region of the stomach.
    Nasopharynx
    Superior region of pharynx behind the nasal cavity.
    Hepatic Flexure
    Bend between ascending and transverse colon.
    Anus
    Opening through which feces are expelled.
    Lingual Frenulum
    Fold of mucous membrane anchoring the tongue to the floor of the mouth.
    Esophagus
    Muscular tube conveying food from the pharynx to the stomach.
    Fundus
    Upper curved portion of the stomach.
    Parotid Glands
    Largest salivary glands located near the ear.
    Minor Duodenal Papilla
    Opening for accessory pancreatic duct.
    Epiploic Appendages
    Fat-filled pouches attached to colon.
    Caudate Lobe
    Lobe of liver near inferior vena cava.
    Hard Palate
    Bony anterior portion of the roof of the mouth.
    Oral Cavity
    Entry point of the digestive system; includes teeth, tongue, and salivary openings.
    Rectum
    Straight section of the colon leading to anus.
    Ileocecal Valve
    Controls flow from ileum to cecum.
    Descending Colon
    Vertical segment of the colon on the left side.
    Lower Esophageal Sphincter
    Muscle at the junction of esophagus and stomach.
    Major Duodenal Papilla
    Opening for bile and pancreatic ducts into duodenum.
    Liver
    Largest gland in the body with roles in metabolism and bile production.
    External Anal Sphincter
    Voluntary muscle around anus.
    Cardia
    Upper opening of the stomach.
    Soft Palate
    Muscular posterior part of the roof of the mouth.
    Cheeks
    Lateral walls of the oral cavity composed of muscle and fat.

    Internal Anal Sphincter

    Reviewed by our medical team

    Involuntary muscle around anal canal.

    Overview

    The internal anal sphincter is an involuntary smooth muscle structure that forms the inner portion of the anal canal’s sphincter complex. It plays a crucial role in maintaining resting anal tone and ensuring fecal continence under autonomic control. It functions alongside the external anal sphincter and pelvic floor muscles to regulate defecation.

    Location

    The internal anal sphincter is located in the lower rectum and upper anal canal. Specifically, it:

    • Is a continuation of the inner circular layer of the rectal muscularis externa

    • Lies deep to the external anal sphincter and just beneath the mucosa of the anal canal

    • Extends from the upper anal canal to approximately the level of the pectinate (dentate) line

    It is encased within the anal canal and closely related to the longitudinal muscle layer and intermuscular septum.

    Structure

    The internal anal sphincter is composed of:

    • Smooth muscle: Involuntary muscle fibers continuous with the circular smooth muscle of the rectum

    • Encircling fibers: Completely surround the anal canal in a cylindrical fashion, though more developed anteriorly and laterally

    • Innervation: Primarily supplied by the autonomic nervous system—sympathetic fibers maintain contraction, and parasympathetic fibers mediate relaxation

    It is separated from the external anal sphincter by the intermuscular groove, a useful surgical landmark.

    Function

    The internal anal sphincter performs several key functions:

    • Maintains resting anal tone: Responsible for approximately 70–85% of resting pressure in the anal canal

    • Prevents involuntary leakage: Remains tonically contracted at rest to prevent the passage of gas or feces

    • Coordinates with rectal distension: Undergoes reflex relaxation during the rectoanal inhibitory reflex (RAIR) to permit sampling and defecation

    Physiological Role(s)

    The internal anal sphincter plays vital roles in anorectal physiology:

    • Continence control: Constantly contracted under sympathetic control to ensure continence, even during sleep or unconscious states

    • Rectoanal inhibitory reflex: Reflex relaxation in response to rectal filling allows the anal canal to sense contents (solid, liquid, gas)

    • Coordination with defecation: Relaxes as part of the defecation reflex, working with abdominal pressure, pelvic floor descent, and external sphincter relaxation

    This sphincter is particularly important in unconscious or passive continence, especially in infants or patients with neurological impairments.

    Clinical Significance

    The internal anal sphincter is involved in several significant clinical conditions:

    • Fecal incontinence: Injury or degeneration of the internal sphincter (e.g., due to aging, childbirth, or surgery) can lead to passive fecal leakage

    • Anal fissures: Hypertonicity of the internal sphincter is often associated with chronic fissures and pain due to reduced blood flow

    • Hirschsprung disease: In neonates, failure of the internal sphincter to relax due to lack of enteric innervation in the distal colon leads to obstruction

    • Botulinum toxin therapy: Used to induce temporary relaxation of the internal anal sphincter to relieve pain and promote healing in anal fissures

    • Anorectal manometry: A diagnostic test used to evaluate internal sphincter tone and reflexes in patients with incontinence or chronic constipation

    Proper functioning of the internal anal sphincter is essential for bowel control and quality of life. Disruption of its tone, reflexes, or innervation can significantly impair continence and rectal sensation.

    Did you know? The small intestine is responsible for absorbing most of the nutrients from food.