Anal Physiology: The Physiology of Continence and Defecation
The physiology of the anus and its surrounding structures is the physiology of continence and controlled defecation. This is a physiology of balance and continuous feedback and complex reflexes. Continence requires balance between the pressure inside the
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Abbreviations RAIR SNS FI MR
Rectoanal inhibitory reflex Sacral nerve stimulation Fecal incontinence Magnetic resonance
Key Concepts • The innervation of the anal sphincter complex is a mixed sympathetic and parasympathetic crossed over system that provides redundant safeguards to continence. • Normal continence and defecation require intact sensation and motor control and reflexes to sense, retain, and voluntarily expect the rectal contents at a socially appropriate time and place. • The normal physiology of the anus can be disturbed in a variety of ways resulting in lack of control, inability to expel, or chronic pelvic pain. • The process of childbirth can contribute significantly to alteration in anorectal anatomy and physiology resulting in a variety of disorders of defecation and/or incontinence.
Introduction The physiology of the anus and its surrounding structures is in essence the physiology of continence and controlled defecation. This is a physiology of balance and continuous feedback and complex reflexes. Normal continence requires a balance between the pressure inside the rectum and the combined tone of the internal and external sphincters. Defecation and the controlled passage of gas or stool at socially Electronic supplementary material: The online version of this chapter (doi:10.1007/978-3-319-25970-3_3) contains supplementary material, which is available to authorized users.
appropriate circumstances required very fine sensation and ability to discern the rectal contents. Defecation requires the balance to tip in favor of the rectal pressure and contraction with simultaneous coordinated relaxation of the pelvic floor and internal and external sphincters. Disturbance in any part of this complex balance can result in incontinence either through reduced anal tone, excess rectal contraction, reduced sensation, or the inability to differentiate the consistency of the rectal contents. Alternatively, disorders tipping in the opposite direction may result in inability to properly or completely empty the rectum. Additionally, more proximal conditions resulting in chronic diarrhea or constipation may tip the balance. And forces even higher can contribute to the behavioral and psychosocial aspects of ordered and disordered function of the rectum and anal canal. It is the patient and skilled practitioner who listens to what the patient can teach and tell about how and what they are doing combined with a good working knowledge of anorectal physiology that can effectively intervene in disorders of defecation.
Normal Anatomy and Physiology For a detailed discussion on the anal anatomy, see Chap. 1. Briefly, the musculature of the anus is made up of three concentric cylindrical structures. The internal sphincter is derived as an extension of the involuntary circular smooth muscle of the rectum. The longitudinal muscle is derived from the outer longitudinal smooth muscle of the rectum, and ultimately does extend into the anus and turns medially through the internal sphincter to comprise the muscles of Treitz that sup
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