Gastric Motility Overview

Study Notes

Gastric Motility

The stomach serves as a reservoir for food, mixes it with gastric secretions, and empties its contents into the duodenum at a controlled rate.
The empty stomach has a volume of approximately 50 ml and contracts in the empty state to form rugae, which are folds in the mucosa and submucosa.
During receptive relaxation, the stomach's smooth muscle cells relax, enabling it to increase volume up to 1.5 liters without significant pressure increase.
Receptive relaxation is mediated by both the vagus nerve and the stomach's enteric nerve plexuses (enteric neurons).
The vagus nerve coordinates with the enteric neurons, which release nitric oxide (NO) and serotonin, causing relaxation of the muscularis mucosae smooth muscle cells.
The orad region of the stomach, which includes the cardia, fundus, and body, has thin muscle layers leading to weak contractions.
Food can stay in the orad region relatively undisturbed for up to an hour.
The caudad region (antrum and pyloric sphincter) has thicker muscle layers, allowing for strong contractions.
Gastric contractions, or peristaltic waves, start in the body of the stomach and travel towards the antrum.
As the wave reaches the antrum, it causes a forceful contraction, while the pyloric sphincter closes.
This leads to retropulsion, pushing the contents of the antrum back into the body of the stomach.
The muscularis layer consists of two layers of smooth muscle: an inner circular layer and an outer longitudinal layer.
Peristaltic waves are achieved through coordinated contractions and relaxations of both circular and longitudinal layers of smooth muscle in the muscularis externa.
In addition to circular and longitudinal smooth muscle layers, the muscularis of the stomach has an inner oblique smooth muscle layer.
This allows the stomach to contract in three directions, facilitating the churning and grinding of food with digestive juices.
Normal gastrointestinal motility relies on coordinated contractions of smooth muscle cells.
These contractions are derived from two basic patterns of electrical activity across smooth muscle cell membranes: slow waves and action potentials.
Smooth muscle cells undergo spontaneous cycles of depolarization and repolarization known as basic electrical rhythm or slow waves.
Slow wave frequency varies by region of the GI tract, with gastric slow waves occurring at approximately 3 waves per minute.
The rhythm of gastric slow waves is generated in the pacemaker zone, which is located in the stomach.
Slow waves do not directly elicit contractions but rather coordinate them by controlling the appearance of action potentials.
Excitatory neurotransmitters and hormones further depolarize the membrane of smooth muscle cells, increasing the amplitude of the slow wave.
If the peak of a slow wave surpasses a certain threshold, the cell fires one or more action potentials.
The intensity of the stimulus, such as the concentration of a hormone or neurotransmitter, determines the number of action potentials fired at the peak of the slow wave.
Action potentials trigger smooth muscle contraction, and the number of action potentials at the peak of a slow wave determines the strength of the contraction.
Gastric emptying is achieved through coordinated contractile activity of the stomach, pylorus, and proximal small intestine.
Regulation of gastric emptying optimizes the digestion and absorption of ingested materials.
The pyloric sphincter, located between the gastric antrum and the duodenum, regulates the emptying of gastric contents at a rate consistent with the duodenum's processing capacity.
The pyloric sphincter also prevents backflow of duodenal contents into the stomach.
The autonomic nervous system controls the pyloric sphincter: sympathetic nerves constrict the sphincter, while parasympathetic (inhibitory vagal) fibers relax the sphincter.
Excitatory vagal fibers contract the sphincter.
Hormones like gastrin, cholecystokinin (CCK), gastric inhibitory peptide (GIP), and secretin constrict the sphincter.
Gastric emptying is regulated in response to the nature of the duodenal contents, with materials high in fat digestion products, high acidity, or hypertonicity slowing gastric emptying.
Receptors on the duodenal and jejunal mucosa sense acidity, osmotic pressure, specific fats, amino acids, and peptides, triggering the release of intestinal hormones.
These intestinal hormones inhibit antral contractions and/or constrict the pyloric sphincter, decreasing gastric emptying.