Motility in the stomach, phases of gastric motility
Gastric Motility
GASTRIC MOTILITY
Motility in the stomach allows for its various functions: the stomach expands to receive and store food, it "churns" to break down food particles and mix them with gastric secretions to form chyme, and, it controls the release of chyme into the duodenum, the first part of the small intestine.
Although we won't show it here, be aware that, even during the fasting state, slow migrating motor complexes form slow peristaltic waves. These waves begin in the fundus and pass through the stomach distally, sweeping food and gastric secretions towards the small intestine.
Filling Phase
The first phase of gastric motility is the filling phase, which relies on the vagovagal reflex (be careful not to confuse this with the vaSovagal reflex!).
When the lower esophageal sphincter opens to release food to the stomach, receptive relaxation promotes reduced pressure and increased volume in the orad region of the stomach. During filling, the stomach can expand several times its empty volume.
Mixing Phase
The next phase is mixing of the stomach contents, which is crucial for digestion and absorption.
Slow tonic contractions of the smooth muscle begin proximally and move distally towards the pylorus; this "churning" action of the stomach breaks food into smaller particles and mixes them with gastric juices to form a semi-liquid called chyme. Recall that the oblique layer adds "twisting" movements to further squeeze the stomach contents.
When food reaches the pylorus, strong contractions in the antrum forcefully push the chyme against the closed pyloric sphincter, which produces retropulsion - food is repelled backwards for increased mixing in the antrum.
Emptying phase
Forceful contractions, sometimes called the "pyloric pump" of the pyloric antrum push small amounts of chyme through the pyloric sphincter and into the duodenum of the small intestine, where further digestion and absorption take place.
Some important notes about the rate of gastric emptying:
Water and other nutrient-poor liquids leave the stomach soon after arriving there; food particles are churned in the antrum until they are about 1mm3 or smaller.
On average, it takes at least a few hours for the stomach to completely empty. As mentioned before, the chemical and physical make-up of the stomach contents determines the emptying rate.
In general, meal ingestion releases hormones that slow gastric emptying, including CCK, leptin, GLP1, glucagon, and insulin. Fatty and/or high protein foods, as well as acidic foods, need longer in the stomach for processing, so they slow the rate of emptying.
During fasting, ghrelin and motilin are released; they increase gastric emptying rate, which keeps the stomach clear of food and digestive juices.
Gastric Motility Regulation: The rate at which food is propelled through the GI tract is controlled via neuronal and hormonal signals that are specific to different types of food. The gastric emptying rate is carefully regulated to ensure sufficient time for digestion and absorption in the stomach and intestines.
Anatomical Structures
The oral cavity is where mastication occurs – the tongue and teeth break down ingested food into smaller particles and mix it with saliva.
The pharynx is a striated muscular tube – we commonly refer to it as the "throat"; it is a shared passageway for air, foods, and liquids.
The esophagus is a posterior continuation of the pharynx; it carries food to the stomach.
The respiratory tract is an anterior continuation of the pharynx; the larynx and trachea deliver air to the lungs.
When we swallow, it's essential that we direct foods and liquids in the pharynx towards the esophagus, and away from the respiratory tract. Show that a cartilaginous leaf-shaped flap, called the epiglottis, covers the opening of the larynx.
Let's see how the epiglottis works to direct the passage of air, food, and liquids in the pharynx.
First, we draw the esophagus and respiratory tract; then, show that the epiglottis remains open to allow air to enter the larynx and trachea.
Then, we re-draw the esophagus and respiratory tract, but show that the epiglottis falls over the opening of the respiratory tract so that foods and liquids are diverted towards the esophagus and blocked from entering the larynx and trachea.
In the stomach, we indicate three anatomical regions: the fundus, body, and pylorus; label the pyloric antrum, specifically.
In addition to anatomical regions, we also need to know the physiological regions; we label the orad region, which comprises the fundus and proximal body (the proximal 1/3 of the stomach), and, the caudad region, which comprises the distal body and the pylorus (the distal 2/3 of the stomach).
Three sphincters regulate the passage of GI contents:
- The Upper Esophageal Sphincter, which regulates the passage of food into the esophagus.
- The Lower Esophageal Sphincter, which regulates the passage of food into the stomach.
- In the stomach, the pyloric sphincter, which regulates the passage of food from the stomach to the small intestine.