15.6: Types of Movements in the Digestive Tract

Peristalsis is a wave-like series of muscle contractions and relaxations that moves food and digestive contents along the gastrointestinal tract. When food enters a section of the GI tract, the circular muscles behind the food mass contract, narrowing the tube and pushing the food forward, while the circular muscles in front of the food mass relax, creating a receptive space for it to move into. This coordinated wave-like sequence progresses from one section of the GI tract to the next, with the segment behind the food contracting and the segment ahead relaxing (Figure 6). The longitudinal muscles in front of the food also contract to help shorten the section of the GI tract, assisting in opening up the next area and making it easier for the food to be propelled forward. This peristaltic wave continues along the length of the GI tract, moving food from the esophagus through the stomach, small intestine, and large intestine, eventually leading to the rectum. Besides moving food, peristalsis helps mix digestive contents, aiding in the breakdown and absorption of nutrients, particularly in the intestines. Controlled by the enteric nervous system, peristalsis is also influenced by various hormones and neurotransmitters, which modulate the speed and strength of these contractions based on the body’s needs.

Irritation of the gut's inner lining due to the presence of irritating substances or abnormal conditions can also initiate peristalsis. Nerve impulses from the parasympathetic nervous system play a vital role in initiating peristaltic contractions by stimulating the myenteric plexus.

Anesthetics, especially general anesthesia, can decrease gastrointestinal motility by slowing peristalsis, resulting in delayed gastric emptying and a higher risk of aspiration during surgery. To reduce this risk, patients are often advised to fast before receiving anesthesia.^{1, 3}

​​Figure 6 | Peristalsis |10  This figure shows the sequential muscular contractions that move a food bolus down the esophagus toward the stomach. Arrows indicate the direction of muscle contractions and bolus movement during peristalsis.

Segmentation is a type of rhythmic, alternating contraction of the smooth muscle in the intestines, primarily in the small intestine, that serves to mix and churn food rather than propel it forward. This non-propulsive movement helps to break down food particles and ensures efficient contact with digestive enzymes, enhancing nutrient absorption. Unlike peristalsis, which moves food along the digestive tract, segmentation involves localized contractions in specific segments of the intestine, creating a back-and-forth motion that helps distribute the contents evenly. This process is regulated by the enteric nervous system, with modulation by hormones such as CCK and gastrin, which increase segmentation in response to food intake. The alternating contractions mix the food with digestive juices, optimizing digestion and nutrient absorption through the intestinal wall.

The Migrating Motor Complex (MMC) is a pattern of cyclical contractions that occurs in the GI tract during fasting, playing a key role in maintaining gut motility and digestive health. It consists of four distinct phases: Phase I, which is characterized by minimal or no contractions, and marks a period of quiescence in the stomach and small intestine; Phase II, which involves short, intermittent contractions that increase in frequency and intensity, helping to clear undigested food particles and secretions; Phase III, the most active phase, marked by strong, rhythmic contractions that travel from the stomach to the small intestine to sweep debris and undigested food through the digestive tract; and Phase IV, a brief transition period back to Phase I. The MMC originates in the stomach and moves distally along the small intestine, ensuring the intestines are cleared of waste, bacteria, and secretions in preparation for the next meal. This process is regulated by the hormone motilin, which stimulates contractions during Phase III. The MMC ceases with food intake, and the digestive tract shifts to a continuous pattern of motility associated with digestion. Vagotomy disrupts the gastric MMC, while eating or stomach distension interrupts this process.^{1, 8}