This section outlines the nervous and hormonal mechanisms that control the secretion of gastric juice and the motility of the stomach.
The stomach produces approximately 2-3 litres of gastric juice daily. Its secretion is tightly regulated by both the nervous system and hormones.
- Nervous Mechanism: Involves signals from the brain and local nerve reflexes.
- Endocrine Mechanism: Involves hormones like gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory polypeptide (GIP).
This phase is initiated by the brain before food enters the stomach. It involves the Neuron→ pathways of the autonomic nervous system.
Stimuli:
- The taste and smell of food (detected by Receptors→)
- Tactile stimulation from chewing and swallowing
- Pleasant thoughts about food
Pathway: These stimuli activate centres in the medulla oblongata.
Action:
- Nerve impulses travel to the stomach mucosa via the vagus nerve.
- The neurotransmitter acetylcholine (ACh) is released.
- Acetylcholine directly stimulates:
- Parietal cells: To secrete hydrochloric acid (HCl)
- Chief cells: To secrete pepsinogen
- G cells (endocrine cells): To secrete the hormone gastrin
This phase is initiated by the arrival of food in the stomach and is primarily mediated by the hormone gastrin.
Primary Stimuli:
- Distention (stretching) of the stomach wall as it fills with food.
- The presence of amino acids and peptides from protein digestion.
Hormone: Gastrin
- Release: Secreted by G cells in the stomach mucosa in response to acetylcholine and the presence of food.
- Transport: Gastrin enters the bloodstream.
- Action: It travels back to the parietal cells, stimulating them to secrete additional gastric juice. This creates a positive feedback loop.
Peristaltic Waves: These are powerful, infrequent contractions that move stomach contents toward the pyloric sphincter.
Function: They force chyme (partially digested food) towards the pyloric sphincter at the stomach's exit.
Pyloric Sphincter:
- This muscular valve remains in a state of mild tonic contraction (partially closed).
- Each strong peristaltic wave forces a small amount of chyme through the sphincter into the duodenum (the first part of the small intestine).