How do nervous and hormonal mechanisms regulate the volume and content of gastric secretions?

SouravNovember 9, 2024

Answer

The regulation of gastric secretions involves a complex interplay of nervous and hormonal mechanisms that control both the volume and content of these secretions. Here’s a detailed overview of how these systems work together to manage gastric function:

Nervous Mechanisms

1. Cephalic Phase:
   • The cephalic phase is triggered by the sight, smell, taste, or even thought of food. This sensory input activates the brain, particularly the medulla oblongata, which sends signals via the vagus nerve to stimulate gastric secretions even before food enters the stomach. This anticipatory response prepares the digestive system for incoming food.
2. Gastric Phase:
   • When food enters the stomach, it causes distension (stretching) of the stomach walls. Stretch receptors in the stomach wall detect this distension and send signals to the central nervous system (CNS), which in turn enhances gastric secretion.
   • The vagus nerve releases neurotransmitters such as acetylcholine (ACh), which directly stimulates parietal cells to secrete hydrochloric acid (HCl) and activates G cells to release gastrin, further promoting acid secretion.
3. Enteric Nervous System:
   • The enteric nervous system (ENS) operates independently within the gastrointestinal tract and coordinates local reflexes. It contains a network of neurons that respond to local stimuli (like food presence) and regulate gastric motility and secretion through short reflex arcs.

Hormonal Mechanisms

1. Gastrin:
   • Gastrin is the primary hormone involved in stimulating gastric acid secretion. It is secreted by G cells located in the gastric antrum in response to protein-rich foods, stretching of the stomach, and neural stimulation from vagal activity.
   • Gastrin acts on parietal cells to promote HCl secretion and stimulates enterochromaffin-like (ECL) cells to release histamine, which further enhances acid secretion by acting on H2 receptors on parietal cells.
2. Inhibitory Hormones:
   • As gastric acidity increases, negative feedback mechanisms come into play. Hormones such as secretin and somatostatin inhibit gastrin release when the pH drops too low. Secretin is released from S cells in the duodenum when acidic chyme enters, signaling a reduction in gastric acid secretion to protect the intestinal lining.
3. Cholecystokinin (CCK):
   • Released from I cells in the duodenum in response to fatty acids and amino acids, CCK not only stimulates pancreatic enzyme secretion but also inhibits gastric emptying and reduces gastric acid secretion, helping coordinate digestive processes between the stomach and intestines.

Integration of Nervous and Hormonal Control

• The regulation of gastric secretions is a finely tuned process involving both excitatory and inhibitory signals:
  • Excitatory Signals: Neural inputs (via ACh from vagus nerve) and hormonal signals (like gastrin) promote gastric juice secretion.
  • Inhibitory Signals: As digestion progresses, hormones like secretin and somatostatin inhibit further acid production when necessary, preventing excessive acidity that could damage the gastrointestinal tract.
• Feedback Loops: The interplay between these mechanisms creates feedback loops that ensure appropriate levels of gastric secretions based on food intake and digestion status.