First phase: complex reflex (cephalic)
This stage, also known as the cerebral phase, serves as the trigger. Its leading mechanism is neural impulses, and juice secretion begins even before the food bolus is swallowed.
- Conditioned reflex pathway: a response to the sight, smell of food, sounds, and surrounding environment. I.P. Pavlov called the secretion produced in this way the "appetite juice".
- Unconditioned reflex pathway: occurs when food irritates the receptors of the oral cavity and pharynx.
The existence of this phase was proven by I.P. Pavlov in experiments with sham feeding in dogs with a transected esophagus (esophagotomy). The food fell out without entering the stomach, but 5–10 minutes later, the "starter juice" began to be secreted in large amounts. It differs by its extremely high enzymatic activity, as it contains enzymes accumulated by the glands during the resting period.
The transmission of stimulating signals to the stomach proceeds along a neural pathway via the vagus nerve (n. vagus). Humorally, the first phase is maintained by the hormone gastrin, which is secreted in the pyloric region. If the vagus nerve is transected or the pylorus is removed, complex reflex secretion disappears.
Second phase: gastric
The gastric phase is triggered upon direct contact of food with the mucous membrane. Its defining mechanism is a neurohumoral response to irritation.
The humoral nature of this stage was proven by I.P. Razenkov: when blood was transfused from a fed dog (at the peak of digestion) to a starving one, active juice secretion began in the latter.
Mechanism of development of the gastric phase:
- Food mechanically stretches the pyloric part.
- This activates G-cells, which release gastrin into the blood.
- Gastrin acts on the glands, stimulating their activity.
- Simultaneously, histamine enters the blood, causing parietal cells to secrete hydrochloric acid (HCl).
In addition to gastrin, secretion is stimulated by acetylcholine, meat and vegetable extractive substances, protein breakdown products, and bombesin. Secretin and cholecystokinin-pancreozymin (CCK-PZ) play a special role—they have a dual effect: they suppress hydrochloric acid synthesis, but enhance the release of pepsinogens. Inhibitors (brakes) of the second phase include fat breakdown products, VIP, serotonin, gastrone, and enterogastrone.
Third phase: intestinal
The final stage of digestion in the stomach is regulated by signals from the duodenum and jejunum. Both neural pathways (from mechanical and chemoreceptors of the intestine) and humoral factors are involved.
The entry of chyme into the duodenum stimulates the production of local regulators under the influence of extractive substances:
- Enterogastrin — maintains and enhances gastric secretion.
- Enterogastrone — inhibits it.
Stimulation at this stage is provided predominantly by protein hydrolysis products. However, over time, the inhibitory influence increases. There is a strict hierarchy of substances that inhibit secretion. Fat hydrolysis products exert the strongest inhibitory effect. They are followed by polypeptides, amino acids, and starch breakdown products. Also, hydrogen ions sharply block juice production when the pH drops below 3.
Dependence of secretion on the type of food
Analysis of juice from an isolated stomach showed that the dynamics and severity of the phases change depending on the macronutrient composition of the meal.
| Type of food | Intensity of excitation | Features of the phases |
|---|---|---|
| Meat (proteins) | Strong stimulant | A sharp rise in secretion is observed; all three phases are clearly and fully expressed. |
| Bread (carbohydrates) | Moderate stimulant | Secretion lasts up to 10 hours. The gastric phase is completely absent because bread contains few chemical stimulants (extractive substances). |
| Milk (fats) | Fats are a strong inhibitor | The first phase is weakly expressed. The secretion peak is heavily shifted and occurs only by the end of the 3rd hour (during the gastric phase). |
Briefly: answers to topical questions
Which neural centers (nuclei) provide the transmission of impulses to the stomach in the first phase?
The transmission of neural impulses to the gastric glands in the complex reflex phase is provided by the gastric juice secretion center localized in the medulla oblongata.
- Dorsal nucleus of the vagus nerve (Nucleus dorsalis nervi vagi) gives rise to parasympathetic visceromotor fibers innervating the abdominal organs, including the stomach.
From this center, efferent impulses travel along the vagus nerve fibers directly to the gastric glands, triggering secretory activity.
Which cells of the gastric fundic glands secrete pepsinogens?
Pepsinogens in the glands of the fundus of the stomach are secreted by chief cells — chief glandulocytes.
Cellular composition of the gastric fundic glands:
- Chief glandulocytes (chief cells) — produce pepsinogens.
- Parietal glandulocytes (parietal cells) — synthesize and secrete hydrochloric acid; parietal cells also produce intrinsic factor (Castle's factor).
- Mucocytes (accessory cells) — secrete mucoid substance.
The juice of the fundic region contains pepsins and a large amount of hydrochloric acid; it is of paramount importance in gastric digestion.
How do acetylcholine, gastrin, and histamine interact at the parietal cell level?
Acetylcholine, gastrin, and histamine activate specific receptors on the basolateral membrane of the parietal cell, triggering intracellular cascades that converge on a single final step.
- Acetylcholine — activates $M_3$-cholinergic receptors, triggering the calcium pathway.
- Gastrin — activates gastrin receptors (CCK₂ receptors), also triggering the calcium pathway.
- Histamine — activates $H_2$ histamine receptors, triggering the adenylate cyclase pathway (increasing cAMP).
All these signaling pathways together lead to the activation of the $H^+/K^+$-ATPase (proton pump), which ensures the transport of hydrogen ions into the gastric lumen. Additionally, there is indirect stimulation: acetylcholine stimulates the production of gastrin and histamine, which secondarily activate the parietal cell.
For quick memorization of the three phases of secretion in order, use the phrase: «My Capital Stomach» — Cerebral (complex reflex), Gastric, Intestinal.
Class and exam questions
What did I.P. Pavlov's sham feeding experiment prove?
It confirmed the presence of the first (complex reflex) phase of secretion. Although food fell out of the transected esophagus and did not enter the stomach, juice still began to be secreted abundantly due to neural impulses.
Why is the second phase absent during the digestion of carbohydrates (bread)?
Bread contains practically no extractive substances and chemical stimulants, which are necessary for the full initiation of the humoral mechanisms of the gastric phase.
How does fatty food affect stomach function?
Fat breakdown products are potent inhibitors. When consuming milk, the complex reflex phase is suppressed, and maximum juice secretion is delayed until the third hour.
What else to study on this topic
- Mechanism of G-cell activation and gastrin production
- Dual action of secretin and cholecystokinin
- The role of parietal cells in hydrochloric acid (HCl) secretion
- Humoral inhibitors: gastrone, enterogastrone, VIP, and serotonin
- I.P. Pavlov's surgical techniques: the isolated pouch
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