How does the bowel work?

Authors: SCIRE Community Team | Reviewer: Bonnie Nybo | Published: 20 March 2018 | Updated: 23 October 2024

To better understand how bowel function changes after a spinal cord injury, it is helpful to first understand how the bowel works when the spinal cord is not injured.

The gastrointestinal tract (GI tract or digestive tract) is a long tube that runs from the mouth to the anus. It is a part of the digestive system, which is responsible for extracting energy and nutrients from food and getting rid of waste.

When food is taken in through the mouth, it travels down a tube called the esophagus into the stomach, where mechanical forces and acidic stomach juices begin breaking it down. It then travels into the intestines (also called the bowel) where it is broken down further to form a stool.

The bowel extends from the bottom of the stomach to the anus.¹

The bowel (intestine or “gut”) is the segment of the gastrointestinal tract that runs from just below the stomach to the anus. It has two main parts, the small intestine and the large intestine. The bowel is where nutrients are extracted from food and indigestible waste products are formed into a stool (also called feces, pronounced “fee-sees”).

The stool is moved through the bowel by a wave-like movement of the intestinal walls called peristalsis. The last part of the bowel is called the rectum. When stool reaches the end of the bowel and stretches the walls of the rectum, signals are sent through the spinal cord to the brain, which is felt as an urge to empty.

Bowel emptying is controlled by two muscles called anal sphincters, which surround the opening of the anus:

• The internal anal sphincter is smooth muscle that is controlled automatically by a reflex.

• The external anal sphincter is skeletal muscle that is controlled consciously by the brain.

These muscles tighten to close the anus and “hold in” stool and relax to allow the bowel to empty.

The pelvic area also has pelvic floor muscles, which are a group of muscles that support the internal organs from below. These muscles also help to control bowel movements by contracting and relaxing in a coordinated fashion.

Part of bowel emptying occurs because of reflexes in the spinal cord. When a large amount of stool enters the rectum, it triggers emptying reflexes in the spinal cord. However, because the brain can consciously control tightening and relaxing of the external anal sphincter, the stool can be “held” until an appropriate time.

When ready to empty the bowel, these reflexes cause the internal anal sphincter to relax so that stool can leave through the anus and triggers movement in the walls of the rectum to push the stool out.