The general function of the small intestine is the absorption of the food we eat. During and after a meal, the intestine normally shows very irregular or unsynchronized contractions.
The contractions move the food content back and forth and mix it with the digestive enzymes that are secreted into the intestine. However, these contractions are not entirely unsynchronized; they move the contents of the intestine slowly towards the large intestine.
It normally takes about 90-120 minutes for the first part of a meal we have eaten to reach the large intestine, and the last portion of the meal may not reach the large intestine for five hours. This pattern of motility is called the "fed (or eating) pattern."
Between meals, the intestine shows cycles of activity that repeat about every 90-120 minutes. The cycle consists, in order, of:
- A short period of no contractions (Phase I)
- A long period of unsynchronized contractions that appear similar to the fed pattern (Phase II)
- A burst of strong, regular contractions that move down the intestine in a peristaltic fashion (Phase III).
- Phase III represents a continuation of the "housekeeper waves" that start in the stomach. Its function is to sweep undigested food particles and bacteria out of the small intestine and into the large intestine.
There are two other kinds of motility seen in the small intestine, but their function is not as well understood.
Discrete clustered contractions are brief bursts of contractions (each burst lasts only a few seconds) which are synchronized (peristaltic). They occur mostly in the upper small intestine and fade out before moving too far down-stream. They occur in most people at infrequent intervals, but in patients with irritable bowel syndrome (IBS) they may be associated with abdominal pain.
The second type of contraction is the giant migrating contraction. This occurs primarily in the lower small intestine (ileum), and it is peristaltic over long distances. It may be part of a defensive reflex that sweeps bacteria and food debris out of the intestine. These giant migrating contractions occur in healthy people and usually cause no sensation, but in some patients, such as those with IBS, they are associated with reports of abdominal pain.
Intestinal dysmotility, intestinal pseudo-obstruction
Abnormal motility patterns in the small intestine can lead to symptoms of intestinal obstruction (blockage). These symptoms are:
Symptoms vary in how severe or how frequent they are, but there are usually periods during which the person is free of symptoms.
These symptoms can result either from weak contractions or from disorganized (unsynchronized) contractions.
Weak contractions of the small intestine are due to abnormalities in the muscle and are usually associated with diseases such as scleroderma. These connective tissue disorders may cause the intestine to balloon out in places so that the contractions of the muscle are not able to move the contents downstream.
Other people have contractions that are strong enough, but they are too disorganized or non-peristaltic to move food along. This type of motility disorder is due to abnormalities in the nerves, which coordinate (synchronize) the contractions of the intestine. This abnormality is easiest to detect by recording the housekeeper waves because these are easily identified peristaltic contractions. In intestinal pseudo-obstruction of the neurological variety, these bursts of contractions occur simultaneously over large parts of the intestine or they may actually move upstream.
Small bowel bacterial overgrowth
This means that there are too many bacteria in the upper part of the small intestine. This leads to symptoms of:
Symptoms occur immediately after eating because the bacteria in the intestine begin to consume the food in the small intestine before it can be absorbed.
This condition is also referred to as small intestinal bacterial overgrowth (SIBO).
These bacteria give off hydrogen and other gases which cause bloating and diarrhea. Small bowel bacterial overgrowth is a result of abnormal motility in the small intestine. When the housekeeper waves do not keep the bacteria swept out because the contractions are too weak or disorganized, the bacteria grow out of control.
Short bowel syndrome
Short bowel syndrome (SBS) is a group of problems related to poor absorption of nutrients that typically occurs in people who have had half or more of their small intestine removed. People with short bowel syndrome cannot absorb enough water, vitamins, and other nutrients from food to sustain life.
Diarrhea is the main symptom of short bowel syndrome and can lead to dehydration, malnutrition, and weight loss. These problems can be severe and can cause death without proper treatment. Other symptoms may include: cramping, bloating, heartburn, weakness and fatigue.
Treatment may involve use of oral rehydration solutions, parenteral nutrition, enteral nutrition, and medications. Having an intestinal transplant may be an option for some patients. Researchers are studying ways to help the small intestine that remains after surgery adapt and function better.
Adapted from IFFGD Publication: Gastrointestinal Motility Disorders of the Small Intestine, Large Intestine, Rectum, and Pelvic Floor by William Whitehead, PhD, Co-Director, Center for Functional GI & Motility Disorders Center Professor of Medicine, Division of Digestive Diseases Professor of Psychology University of North Carolina, Chapel Hill, NC.