94 Motility
Learning Objectives
After reading this section you should be able to-
- Compare and contrast the following: peristalsis, mixing waves, segmentation, and mass movement.
- Explain how volume, chemical composition, and osmolarity of chyme affect motility in the stomach and in the duodenum.
Types of motility
As mentioned in the previous section, digestion includes includes six activities, one of which is propulsion, or motility. There are four types of motility- peristalsis, mixing waves, segmentation, and mass movement. Peristalsis, mixing waves, segmentation, and mass movement are distinct types of muscular contractions in the digestive system, each playing unique roles in processing and moving food.
Peristalsis involves rhythmic, wave-like contractions of smooth muscle that propel food along the digestive tract, from the esophagus to the stomach, and through the intestines. These involuntary contractions ensure the continuous movement of ingested material, even against gravity.
Mixing waves occur in the stomach, where they serve to churn and blend food with gastric juices, converting it into a semi-liquid mixture called chyme. These waves are essential for breaking down food particles and initiating chemical digestion through enzyme action.
Segmentation is a process mainly observed in the small intestine, characterized by localized contractions of circular muscle that segment the intestine into smaller sections. This back-and-forth motion mixes the chyme thoroughly with digestive enzymes and facilitates nutrient absorption by increasing contact with the intestinal walls.
Mass movements are powerful, wave-like contractions in the large intestine that propel fecal material toward the rectum. These movements occur a few times daily, typically following meals, and are crucial for moving waste toward excretion.
Factors affecting motility
Volume of Chyme: The amount of chyme in the stomach and duodenum directly impacts motility. A larger volume of chyme stretches the stomach wall, activating stretch receptors that trigger the enteric nervous system to enhance gastric motility. This increased motility helps to mix and propel the chyme into the duodenum. In the duodenum, an increased volume of chyme similarly stimulates stretch receptors, promoting intestinal contractions that aid in mixing and moving the chyme forward. However, excessive volume can slow gastric emptying to prevent overloading the duodenum and ensure efficient digestion and absorption.
Chemical Composition: The chemical makeup of chyme, including its nutrient content, significantly affects motility. Proteins and fats, for instance, slow gastric emptying. Fats are particularly potent in this regard, triggering the release of hormones like cholecystokinin (CCK) from the duodenum, which inhibits gastric motility and prolongs digestion time in the stomach. This slowdown allows for the thorough breakdown of these macronutrients. In contrast, carbohydrates tend to pass through the stomach more quickly, as they require less digestion time.
Osmolarity of Chyme: The osmolarity, or concentration of solutes in chyme, also regulates motility. Hyperosmolar chyme (high solute concentration) in the duodenum draws water into the lumen, diluting the chyme but also potentially causing distension and discomfort. This osmotic effect slows gastric emptying and duodenal motility to allow time for osmotic balance to be restored and for enzymes to act on the nutrients. Conversely, hypoosmolar chyme (low solute concentration) is processed and moved along more swiftly.
Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 23.
movement of food through the digestive tract, including swallowing and peristalsis
sequential, alternating waves of contraction and relaxation of alimentary smooth muscles that push food along the gastrointestinal tract
type of peristalsis occurring in the stomach that blends food with gastric juices to create chyme
process occurring in the small intestine during which circular smooth muscle of the muscularis layer contracts to break up and mix digestive contents with digestive juices and facilitate absorption
powerful wave-like contractions occurring in the large intestine responsible for propelling fecal material toward the rectum
digestive hormone responsible for stimulating the secretion of pancreatic enzymes and bile from the liver for fat and protein digestion
