Mechanisms controlling normal defecation and the potential effects of spinal cord injury

Spinal cord injury frequently leads to bowel dysfunction with the result that emptying the bowel can occupy a significant part of the day and reduce the quality of life. This chapter contains an overview of the function and morphology of the normal distal gut in the human, and of gut behaviour in normal defecation. In humans, this can be monitored and is described, but knowledge of the mechanisms controlling it is limited. Work on animals has shown that the intrinsic activity of the smooth muscles and their interactions with the enteric nervous system can program the activity that is necessary to expel waste material, but the external anal sphincter is controlled through somatic nerves. The gut however also receives input from the central nervous system through autonomic nerves, and a spinal reflex centre exists. Voluntary effort to induce defecation can influence all the control mechanisms, but the precise importance of each is not understood. The behaviour and properties of the individual muscles in the normal human rectum and anal canal are described, including their responses to intrinsic nerve stimulation and adrenergic and cholinergic agonists. The effects of established spinal cord injury are then considered. For convenience, supraconal and conal/cauda equina lesions are considered as two categories. Prolongation of transit times and disordered defecation are common problems. Supraconal lesions result in reduced resting anal pressures and increased risk of fecal incontinence. The acute effects of spinal cord injury are described, with injury causing ileus (prolonged total gastrointestinal transit times), constipation (prolonged colonic transit times) and fecal incontinence (passive leakage).