Types of gastric movements
In response to food intake, the tone of the vagus nerves (n. vagus) decreases, leading to receptive relaxation—relaxation of the organ wall to accommodate the volume of food. There are two main types of contractions:
- Peristaltic movements. Necessary for the propulsion of chyme. They are carried out through the coordinated contraction of the circular muscle layer and longitudinal muscles (which contract just ahead of the bolus being moved). The wave is initiated on the greater curvature near the esophagus, where the cardiac pacemaker is located. Most of these waves fade out in the pyloric region. However, the activation of the second pacemaker (in the pyloric zone) causes powerful peristaltic waves and a pressure surge, which triggers evacuation.
- Tonic contractions. These represent a prolonged change in the basal tone of smooth muscles. They reduce the total volume of the stomach and increase intragastric pressure. Their main task is the thorough mixing of food and its impregnation with gastric juice for optimal enzymatic digestion.
Mechanism of chyme evacuation
The passage of contents into the duodenum occurs strictly in portions; the chyme must be liquid or semi-liquid. In an empty stomach, the pyloric sphincter is open, but it closes when food enters. The evacuation cycle depends on the chemical composition of the medium:
- Hydrochloric acid from the stomach causes the sphincter to open.
- A portion of acidic chyme passes into the duodenum.
- In the intestine, hydrochloric acid, carbohydrates, and fats act on local receptors, triggering a reflex closure of the sphincter. This is also facilitated by the hormone enterogastrone.
- The incoming portion is neutralized by bile and pancreatic juice to a neutral or slightly alkaline pH.
- After alkalization, acid from the stomach again triggers the opening of the sphincter for the next portion.
This process is controlled by the enteric nervous system (myenteric plexus — plexus myentericus). The reflex arc includes an afferent link in the stomach, an inhibitory interneuron (to relax the sphincter), an efferent neuron, as well as afferent and excitatory links from the duodenum, which implement the feedback principle to close the valve.
Regulation of motility
Gastric activity is coordinated by neural and humoral factors.
- The parasympathetic nervous system (n. vagus) enhances motility, makes contractions stronger and more frequent, and accelerates emptying.
- Sympathetic innervation, by contrast, weakens the force and slows down the rhythm of movements.
Gastrointestinal hormones play an active role:
- Stimulators: gastrin, serotonin, insulin, and motilin (the latter is synthesized in the duodenal mucosa when the pH of its contents rises).
- Inhibitors: secretin, vasoactive intestinal peptide (VIP), cholecystokinin-pancreozymin, enterogastrone, and bulbogastrone.
The enterogastric reflex also operates: as soon as chyme enters the duodenum, the motor activity of the stomach is reflexively suppressed.
Factors of gastric emptying rate
Liquids are not retained and leave the stomach almost immediately. The evacuation rate of the remaining food is influenced by:
- Chemical composition: carbohydrate food evacuates the fastest. Proteins are retained longer, and fatty food can remain inside for 8–10 hours (fats in the duodenum actively inhibit the opening of the pyloric sphincter).
- Osmotic pressure: hypertonic solutions slow down emptying.
- Degree of filling: gastric distension accelerates the passage of chyme, whereas duodenal distension inhibits it.
In addition, after a meal, protective mechanisms are activated in the stomach to prevent autodigestion: local blood flow increases, mucus is abundantly secreted, and local immunity to metabolites is activated.
Briefly: answers to topical questions
What is the migrating motor complex (MMC) and fasting gastric motility?
Fasting motility is the periodic motor activity of an empty stomach: periodic contractions of the stomach after food evacuation. Its onset coincides with the state when the arteriovenous glucose difference approaches zero. Methodological materials also describe the "empty stomach" theory, according to which the sensation of hunger was associated with impulses from gastric mechanoreceptors during its periodic contractions; however, this theory is cited as untenable. The term "migrating motor complex" is not defined in the provided materials.
What is the mechanism of changes in gastric motility during the vomiting reflex?
The mechanism of changes in gastric motility during the vomiting reflex is triggered as a result of the activation of neurons in the vomiting center of the medulla oblongata, which includes the area postrema. The development of this act represents a chain of the following coordinated motor phenomena:
- Antiperistalsis — enhancement of the antiperistalsis of the gastric wall occurs.
- Muscle contraction — simultaneous contraction of the muscles of the diaphragm and abdominal wall is observed.
- Relaxation — relaxation of the muscles of the cardiac region of the stomach and esophagus occurs.
SIGM — main motility stimulators: Serotonin, Insulin, Gastrin, Motilin.
Class and exam questions
Which food stays in the stomach the longest?
Fatty food. It can remain in the stomach for up to 8–10 hours, as the presence of fats in the duodenum inhibits the opening of the pyloric sphincter.
What causes the pyloric sphincter to open and close?
The sphincter opens under the action of hydrochloric acid from the stomach. When acidic chyme enters the duodenum, the acid, fats, and carbohydrates irritate its receptors, causing a reflex closure.
Why are tonic contractions of the stomach necessary?
They increase intragastric pressure and reduce the overall volume of the organ. This is necessary for the thorough mixing of the food bolus with gastric juice.
What else to study on this topic
- Neuronal regulation: myenteric plexus (plexus myentericus)
- Humoral inhibitors of gastric motility
- Excretory function of the stomach
- Enterogastric reflex
- Protective mechanisms of the gastric mucosa after a meal
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