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22.14A: Cephalic Phase

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  • Page ID
    8071
  • The cephalic phase of gastric secretion occurs before food enters the stomach due to neurological signals.

     

    LEARNING OBJECTIVES

     

    Outline the cephalic phase of digestion

     

    KEY TAKEAWAYS

    Key Points

     

    • The cephalic phase of gastric secretion is initiated by the sight, smell, thought or taste of food.
    • Neurological signals originate from the cerebral cortex and in the appetite centers of the amygdala and hypothalamus.
    • This enhanced secretory activity is a conditioned reflex.
    • This phase of secretion normally accounts for about 20 percent of the gastric secretion associated with eating a meal.

     

    Key Terms

     

    • conditioned reflex: A response, to a stimulus, that has been acquired by operant conditioning.
    • cephalic phase: This occurs before food enters the stomach, especially while it is being eaten.

    The cephalic phase of gastric secretion occurs before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food; and the greater the appetite, the more intense is the stimulation.

    Neurogenic signals that initiate the cephalic phase of gastric secretion originate from the cerebral cortex, and in the appetite centers of the amygdala and hypothalamus. They are transmitted through the dorsal motor nuclei of the vagi, and then through the vagus nerve to the stomach.

    This phase of secretion normally accounts for about 20% of the gastric secretions that are associated with eating a meal. Since this enhanced secretory activity is brought on by the thought or sight of food it is a conditioned reflex—it only occurs when we like or want food. When one’s appetite is depressed this part of the cephalic reflex is inhibited.

    The cephalic phase causes ECL cells to secrete histamine and increase HCl acid in the stomach. There will also be an influence on G cells to increase gastrin circulation.

     

    Chain of Events for the Nervous System and Hormone System

     

    1. Thinking of food (i.e., smell, sight) stimulates the cerebral cortex.
    2. The cerebral cortex sends messages to the hypothalamus, the medulla, and the parasympathetic nervous system via the vagus nerve, and to the stomach via the gastric glands in the walls of the fundus and the body of stomach.
    3. The gastric glands secrete gastric juice.
    4. When food enters the stomach, the stomach stretches and activates stretch receptors.
    5. The stretch receptors send a message to the medulla and then back to the stomach via the vagus nerve.
    6. The gastric glands secrete more gastric juice.
    7. Chemical stimuli (i.e., partially digested proteins, caffeine) directly activate G cells (enteroendocrine cells) that are located in the pyloric region of the stomach to secrete gastrin; this in turn stimulates the gastric glands to secrete gastric juice.