Neural mechanisms in disorders of defaecation.

The anal sphincter is made up of the internal and external sphincter, which are arranged concentrically around the anal canal; they overlap except in the most distal part of the anal canal in the region of the superficial part of the external sphincter. Continuous electromyographic (EMG) activity may be recorded in the external anal sphincter at rest (Floyd and Walls, 1953) but the external sphincter contributes only lo-15% of the resting anal pressure (Bennett and Duthie, 1964; Duthie and Watts, 1965; Frenckner and von Euler, 1975). An important function of the external anal sphincter is to reflexly contract in response to transient rises in intra-abdominal pressure (Taverner and Smiddy, 1959; Parks et al, 1962). This activity is mediated via a spinal reflex (Melzack and Porter, 1964). The external sphincter also contracts under voluntary control, thus allowing defaecation to be delayed until a socially appropriate