Body systems

Digestive system

People with spinal cord injury may experience changes to their bowel function. The large intestine in our digestive system uses muscular, wave-like contractions to propel faecal mass towards the rectum. The rectum holds faecal bulk ready for evacuation. The anus releases faeces during defaecation.

The defaecation process occurs when there is communication between the brain and the external anal sphincter via the spinal cord at S2-4. When sufficient stool enters the rectum, stretch receptors send messages to the brain via afferent pathways to nerves that make us aware of the urge to defaecate.

At the same time, the internal sphincter relaxes, and the anal canal establishes if the contents within it are solid, liquid or gas. The urge and recognition of the contents allows us to choose whether to defaecate or put it off until a later time.

When we do choose to follow through with defaecation at the time, added abdominal pressure aids elimination. When innervation is intact (to the lower thoracic cord T6-12), the valsalva maneuver or abdominal straining increase in intraabdominal pressure. As the external anal sphincter relaxes, stool from the rectum is forced out allowing for defaecation. We can restimulate this urge by abdominal straining, but this is not as effective as using the urge created by the natural reflex. If we choose not to defaecate, the external sphincter remains contracted through motor impulses sent down efferent pathways. The rectum relaxes more to further accommodate more stool and the urge to defaecate reduces and disappears.

Brain communicates the urge to defaecate with the external anal sphincter via spinal cord at S2-4

Spinal cord injury

A spinal cord injury causes changes to the nerves supplying the muscles that line the large intestine, rectum, and anus. Changes to these nerves reduce peristalsis through the bowel, resulting in faeces taking longer to reach the rectum. As faeces pass through the large intestine, it reabsorbs water from the faecal mass through the intestinal wall. The longer it takes, the more water is absorbed out of the faeces and the harder the stool will get. Changes to the nerves supplying the muscles in the rectum and anus can result in an inability to predict or control bowel movements. Thus, people with spinal cord injury may experience constipation and/or unplanned bowel movement.