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Cephalic Phase of Digestion
The process of digestion is composed of three stages – cephalic, gastric, and intestinal – each with a distinct control center. The cephalic phase is…
Digestion has three overlapping phases — cephalic, gastric, and intestinal — based on the location of their control center.
The cephalic phase begins before food enters the stomach and is controlled by the central nervous system.
Food-related sensory stimuli, including sight and smell, can initiate the cephalic phase.
Additionally, the taste receptors intensify the signals to the brain while eating.
From the cerebral cortex, these stimuli are relayed to the hypothalamus and the medulla.
The neural output is transmitted via the parasympathetic fibers of the vagus nerve, stimulating the mucous, chief, parietal, and G cells in the stomach.
The G cells of gastric glands secrete gastrin, which stimulates the chief cells to release pepsinogen and the parietal cells to release gastric acid.
The mucus from the mucous cells, combined with the secretions of the parietal and the chief cells, forms gastric juice.
Upon entering the stomach, food triggers the stretch receptors in the wall, activating the long reflexes to the medulla.
In turn, the medulla stimulates increased production of gastric juice via the vagus nerve for a few minutes.
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Q1: What triggers the cephalic phase of digestion?
The cephalic phase begins before food enters the stomach and is triggered by food-related sensory stimuli, including sight and smell. These sensory signals travel to the brain, where taste receptors intensify them during eating. The cerebral cortex processes these stimuli and relays them to the hypothalamus and medulla, preparing the stomach for digestion.
Q2: How does the nervous system control the cephalic phase?
The central nervous system controls the cephalic phase through neural pathways. Sensory signals reach the cerebral cortex, hypothalamus, and medulla, which then transmit neural output via parasympathetic fibers of the vagus nerve. This stimulates mucous, chief, parietal, and G cells in the stomach to prepare gastric juice for digestion.
Q3: What stomach cells are activated during the cephalic phase?
The cephalic phase activates four types of stomach cells: G cells secrete gastrin, chief cells release pepsinogen, parietal cells release gastric acid, and mucous cells produce mucus. These secretions combine to form gastric juice, which prepares the stomach for food digestion and can be produced at rates of about 500 mL per hour.
Q4: What is the role of gastrin in the cephalic phase?
Gastrin is a hormone secreted by G cells in response to vagus nerve stimulation during the cephalic phase. It stimulates chief cells to release pepsinogen and parietal cells to release gastric acid. Gastrin essentially amplifies the stomach's digestive preparation, ensuring adequate enzyme and acid production before food arrival.
Q5: How does food entering the stomach affect the cephalic phase?
When food enters the stomach, it triggers stretch receptors in the stomach wall, activating long reflexes to the medulla. The medulla responds by stimulating increased gastric juice production via the vagus nerve for several minutes. This reflex reinforces and extends the digestive preparation initiated during the cephalic phase.
Q6: What is the composition of gastric juice formed during the cephalic phase?
Gastric juice is composed of mucus from mucous cells, pepsinogen from chief cells, and gastric acid from parietal cells. These three components combine to create a protective and digestive fluid. The mucus protects the stomach lining while pepsinogen and acid work together to break down proteins.
Q7: How long does the cephalic phase last and how much gastric juice is produced?
The cephalic phase lasts approximately one to two minutes and results in gastric juice production at rates of about 500 milliliters per hour, equivalent to roughly two cups per hour. This rapid production prepares the stomach for the incoming food and initiates the digestive process before the gastric phase begins.