Types of Gastric Movements
In response to food intake, vagal tone (n. vagus) decreases, leading to receptive relaxation—the relaxation of the organ wall to accommodate the volume of food. There are two main types of contractions:
- Peristaltic movements. Necessary for propelling 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 starts on the greater curvature near the esophagus, where the cardiac pacemaker is located. Most of these waves fade in the pyloric region. However, activation of the second pacemaker (in the pyloric zone) triggers powerful peristaltic waves and a pressure rise, initiating evacuation.
- Tonic contractions. These represent prolonged changes in the basal tone of smooth muscle. They decrease the overall volume of the stomach and increase intragastric pressure. Their main task is thorough food mixing and soaking with gastric juice for optimal enzymatic digestion.
Mechanism of Chyme Evacuation
The transfer of contents into the duodenum occurs strictly in portions; chyme must be liquid or semi-liquid. In an empty stomach, the pyloric sphincter is open, but it closes upon food entry. 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 duodenum, hydrochloric acid, carbohydrates, and fats act on local receptors, triggering the reflex closure of the sphincter. The hormone enterogastrone also promotes this.
- The incoming portion is neutralized by bile and pancreatic juice to a neutral or slightly alkaline pH.
- Following alkalinization, acid from the stomach once again prompts the sphincter to open for the next portion.
This process is controlled by the enteric nervous system (myenteric plexus — plexus myentericus). The reflex arc includes an afferent limb in the stomach, an inhibitory interneuron (to relax the sphincter), an efferent neuron, and afferent and excitatory limbs from the duodenum that implement a negative feedback principle to close the valve.
Regulation of Motility
Gastric function is coordinated by neural and humoral factors.
- Parasympathetic nervous system (n. vagus) enhances motility, makes contractions stronger and more frequent, and accelerates emptying.
- Sympathetic innervation, conversely, weakens the force and slows 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.
Additionally, the enterogastric reflex operates: as soon as chyme enters the duodenum, gastric motor activity is reflexively suppressed.
Factors Affecting Emptying Rate
Liquids do not linger and leave the stomach almost immediately. The emptying rate of remaining food depends on:
- Chemical composition: carbohydrate food empties the fastest. Proteins take longer, and fatty foods can remain inside for 8–10 hours (fats in the duodenum actively inhibit the opening of the pyloric sphincter).
- Osmotic pressure: hypertonic solutions delay emptying.
- Degree of distension: gastric distension accelerates chyme passage, whereas duodenal distension inhibits it.
Furthermore, protective mechanisms are activated in the stomach postprandial to prevent auto-digestion: local blood supply increases, mucus is abundantly secreted, and local immunity to metabolites is activated.