HUMAN MEDICAL BIOLOGY, ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedonyuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

CHAPTER 7. NUTRITION PROCESS AND ITS PATHOLOGY

ANATOMY AND PHYSIOLOGY OF THE DIGESTIVE ORGANS

STOMACH

2. DIGESTION IN THE STOMACH

2.2. REGULATION OF GASTRIC JUICE SECRETION

Gastric secretion is regulated by neural and humoral mechanisms.

There are three phases of secretion: cephalic, gastric, and intestinal.

The cephalic phase is a complex reflex process, meaning that gastric juice is secreted via Conditioned and Unconditioned Reflexes. This phase is triggered by conditioned reflexes caused by the sight, smell, or even the thought of food. Subsequently, when food enters the Oral Cavity, unconditioned reflex mechanisms are activated: food stimulates the oral receptors, impulses are transmitted to the Medulla Oblongata, and from there via the Vagus nerve to The Stomach, stimulating gastric juice secretion. I.P. Pavlov referred to this juice as "appetitive" or "psychic".

The gastric phase of secretion begins the moment the food bolus enters the stomach and is driven by neural and humoral mechanisms.

As the chyme fills the stomach, it stimulates the mechanoreceptors of the mucosa. Signals via the afferent fibers of the vagus nerve are transmitted to the secretory center in the medulla oblongata and then back via the efferent fibers of the same vagus nerve to the gastric glands, stimulating their activity.

During the gastric phase, humoral mechanisms join the neural ones. The humoral stimuli include gastrin and acetylcholine. Gastrin is produced by the gastric mucosal Cells and becomes active only in an acidic environment. From the stomach, this hormone is absorbed into the bloodstream and acts upon the glandular apparatus of the stomach, activating it. Gastrin release is enhanced by Protein Hydrolysis products as well as meat and vegetable extractives.

Histamine, produced in the gastric mucosa, also plays a stimulating role in gastric secretion. Its release is intensified in the presence of acetylcholine and gastrin. The action of histamine is likewise mediated through the bloodstream via humoral pathways.

The intestinal phase of gastric secretion begins when the first portion of acidic chyme enters the duodenum. Both stimulatory and inhibitory effects from the intestine on gastric secretion are mediated by neural and humoral mechanisms. Neural signals from the mechanoreceptors and chemoreceptors of the intestine reflexively enhance gastric secretion via the secretory vagus nerve, aiding in the Digestion of the remaining chyme in the stomach.

However, The most significant role in this phase belongs not to reflex mechanisms, but to humoral mechanisms regulated by Gastrointestinal Hormones.

Thus, when acidic gastric contents enter the duodenum, the pH drops (shifting from mildly alkaline to acidic). Under these conditions, the intestinal mucosa secretes secretin and cholecystokinin-pancreozymin, which are absorbed into the bloodstream and, upon reaching the gastric glands, inhibit their secretion while stimulating the release of pepsinogen.

Products resulting from fat hydrolysis have The ability to increase the release of cholecystokinin-pancreozymin, which also inhibits gastric secretion during the intestinal phase.

An inhibitory effect is likewise exerted by hormones such as enterogastrone, serotonin, bulbogastrone, and Somatostatin. Conversely, the absorption of protein hydrolysis products into the Blood stimulates gastric secretion.

Inhibition of gastric juice secretion is also observed during negative emotions, exposure to painful stimuli, and intense physical exertion. These effects are mediated through the sympathetic Nervous system.