The varying patterns of movements in the GI tract
Three basic patterns of GI smooth muscle contraction exist. These are:
- Peristaltic contractions
- Segmentation contractions also called mixing contractions
- Migrating motor complex
1. Peristaltic contractions
Peristaltic contractions are GI smooth muscle contractions that occur during or immediately after a meal. By way of definition, peristalsis refers to the progressive waves of contraction that move from one section of the GI tract to the next. Peristalsis occurs in all segments of the gastrointestinal tract all the way from the esophagus to the rectum.
Peristalsis is essentially a reflex response (which involves neurons intrinsic to the gut) initiated by stretch of the gut wall. The occurrence of peristalsis is an intrinsic property of GI smooth muscles independent of extrinsic innervation; however, nerve fibers of the autonomic nervous system reaching the gut may increase or decrease the rate of peristaltic contractions.
Basic peristaltic contractions are purely under the control of the enteric nervous system, and this shows clearly the ability of the enteric nervous system to integrate and control gut functions without inputs from the CNS. In other words, peristaltic contractions and the progression of gut contents will occur even if the gut is rid of all extrinsic innervation since the peristaltic reflex does not involve the CNS.
During peristalsis, circular muscles of the muscularis externa contract just behind a bolus or mass of food. The contraction pushes the bolus forward into the next segment along the GI tract. The segment that receives the bolus, called the receiving segment, responds to the stretch by contracting. The contraction continues the forward movement of the bolus. Peristaltic contractions propel a bolus forward along the GI tract at a speed ranging from three (3) to twenty-five (25) cm per second.
Peristalsis occurs in all regions of the GI tract but it is most relevant in the esophagus where it propels food materials through the esophagus to the stomach. In the stomach, peristalsis mixes food with enzymes as the digests.
Peristalsis involves both contraction and relaxation and propels contents of GI lumen aborally.
In the absence of diarrhea and other disorders with bowel motility, peristaltic waves along the intestines is minimal and do not travel far. Movement of substances along the intestines is significantly slow in order to allow maximum time for absorption. Another pattern of smooth muscle contraction predominates in the intestines.
2. Segmentation contractions
A special form of contraction called segmentation contractions predominates in the intestines. In segmental contractions, short segments of the intestines contract and relax alternately. Segmentation contractions occur when chyme is present in the lumen of the gut and persists for as long as chyme remains in the lumen of the gut.
As in the case of peristalsis, the enteric nervous system integrates signals that control segmentation contractions without input from the CNS. Thus, segmentation contraction is an inherent property of the gut controlled by enteric nervous system. Although segmentation contractions occurs independent of central nervous inputs, the CNS can act to modulate both the rhythm and force of the contractions.
Segmentation contractions are responsible for churning and mixing the intestinal contents with digestive juices, since there is no net forward or backward movement of the substances. Moreover, segmentation contractions serve to keep the intestinal contents in contact with the absorptive epithelium of the intestines.
Lastly, segmental contractions move intestinal contents for short distances.
Segmentation contractions slow down net movement of intestinal content along the gut length to allow enough time for digestion and absorption.
In segmental contractions, the contracting segments almost never exceed five (5) centimeters in length. In any segment undergoing segmental contraction, the circular muscles contract whereas longitudinal muscles relax. The pattern of segmental contractions varies, they may occur at regular intervals or randomly along the intestines. Physiologically, segmentation contractions mix intestinal contents over short stretches of the gut.
3. Migrating motor complex
Migrating motor complex is a pattern of contraction that begins in the stomach passing slowly from region to region finally reaching the large intestine. In other words, a migrating motor complex is a cycle of motor activity that migrates from stomach to the distal ileum.
Motilin, a GI polypeptide secreted by enterochromaffin cells and Mo cells in the stomach, small intestine, and large intestine initiates and regulates migrating motor complexes. Enterochromaffin cells and Mo cells secrete more motilin between meals, when the gut is largely empty. Motilin binds to receptors on enteric neurons in stomach, duodenum and colon to cause contraction of smooth muscle in the stomach, small intestine, and large intestine.
Migrating motor complexes fail to occur when the gut is largely filled because various factors act to suppress the secretion of motilin until digestion and absorption is complete. Secretion of motilin ceases once an individual ingests a meal; however, the factors that act to suppress the secretion of motilin remain unclear.
Each migrating motor complex takes about 90 minutes to travel form the stomach to the large intestine.
Migrating motor complexes occur mainly between meals when the tract is mostly empty and serves to “sweep” food remnants and microbes from the upper regions of the GI tract to the intestines clearing the stomach and small intestine of luminal content preparing the gut for the next meal. Therefore, migrating motor complex serves to prepare the GI tract for the next meal. Moreover, migrating motor complex ceases once we eat a meal.
Disorders with GI motility are among the most common gastrointestinal disturbances. These disorders are not major health risks but can be very troublesome. They include diarrhea, constipation, delayed stomach emptying, and esophageal spasms.
Irritable bowel syndrome is another GI disorder associated with GI motility. It is a chronic functional disorder characterized by severe abdominal pains.