## Role of the Nervous System and Gastrin Hormone on the Secretion of Gastric Juice 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). ## Nervous Control (Cephalic Phase) This phase is initiated by the brain before food enters the stomach. It involves the pathways of the autonomic nervous system. **Stimuli:** - The taste and smell of food (detected by ) - Tactile stimulation from chewing and swallowing - Pleasant thoughts about food **Pathway:** These stimuli activate centres in the **medulla oblongata**. **Action:** 1. Nerve impulses travel to the stomach mucosa via the vagus nerve. 2. The neurotransmitter **acetylcholine (ACh)** is released. 3. Acetylcholine directly stimulates: - **Parietal cells:** To secrete hydrochloric acid (HCl) - **Chief cells:** To secrete pepsinogen - **G cells (endocrine cells):** To secrete the hormone **gastrin** ## Hormonal Control (Gastric Phase) 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. ## Stomach Motility and Emptying **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).

1.4 Role Of The Nervous System And Gastrin Hormone On The Secretion Of Gastric Juice | Digestive System Of Man | Biology 12