Dynamics of the rectum and anus

Abstract

The anorectum is a continuously active area with integration of smooth and striated muscle sphincters. Their responses to the entry of material from the colon into the rectum are usually at a subconscious level but voluntary control can be exerted. In the accommodation response the rectum adapts to the presence of faeces and the rise in rectal pressure caused by their entry into the rectum passes off in a few minutes. The sampling response consists of transient relaxation of the upper part of the internal anal sphincter which allows the rectal content to come in contact with the somatic sensory area in the anal canal to assess the nature of the content. Defaecation is the most complex response. Controlled defaecation occurs when a moderate stimulus from the full rectum reaches consciousness by release of cortical inhibition. When the rectal stimulus is greater and combined with stimulation of the anal canal urgent defaecation takes place.