Anorectal Physiology

Pasithorn A. Suwanabol, Scott E. Regenbogen
Anorectal Physiology is a topic covered in the ASCRS Textbook of Colon and Rectal Surgery.

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Key Concepts

  • Maintenance of fecal continence and defecation are complex processes requiring both voluntary and involuntary reflexes that have yet to be fully characterized.
  • Normal continence is dependent on coordination between neuronal reflexes, sensory and motor pathways, the rectum, anal sphincters, and pelvic floor and requires adequate rectal compliance and competence of the anal sphincter.
  • During defecation, sensory mechanisms allow the rectum to stretch to accommodate feces, the pelvic floor muscles relax, and intraabdominal pressure increases. Simultaneously, the puborectalis relaxes and straightens the anorectal angle, the anal canal shortens, and the pelvic floor descends. Finally, the anal sphincters relax and evacuation is initiated.
  • Anatomy and physiology of the rectum and anus are intrinsically related, allowing physiologic testing to be exceedingly useful for diagnosis and management of anorectal pathologies.
  • Disorders of continence can derive from deficits of mental, anatomic, and physiologic functions, including reflexes, sensory and motor nerves, and the muscles of the rectum, sphincters, and pelvic floor.
  • Functional defecatory disorders frequently coexist with urogynecologic conditions likely due to the shared musculature of the pelvic floor and urogenital diaphragm, as well as from the overlap in peripheral innervation and spinal nerve roots.

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Key Concepts

  • Maintenance of fecal continence and defecation are complex processes requiring both voluntary and involuntary reflexes that have yet to be fully characterized.
  • Normal continence is dependent on coordination between neuronal reflexes, sensory and motor pathways, the rectum, anal sphincters, and pelvic floor and requires adequate rectal compliance and competence of the anal sphincter.
  • During defecation, sensory mechanisms allow the rectum to stretch to accommodate feces, the pelvic floor muscles relax, and intraabdominal pressure increases. Simultaneously, the puborectalis relaxes and straightens the anorectal angle, the anal canal shortens, and the pelvic floor descends. Finally, the anal sphincters relax and evacuation is initiated.
  • Anatomy and physiology of the rectum and anus are intrinsically related, allowing physiologic testing to be exceedingly useful for diagnosis and management of anorectal pathologies.
  • Disorders of continence can derive from deficits of mental, anatomic, and physiologic functions, including reflexes, sensory and motor nerves, and the muscles of the rectum, sphincters, and pelvic floor.
  • Functional defecatory disorders frequently coexist with urogynecologic conditions likely due to the shared musculature of the pelvic floor and urogenital diaphragm, as well as from the overlap in peripheral innervation and spinal nerve roots.

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Last updated: January 26, 2022