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The cephalic phase is the first of three phases associated with gastric secretions in the digestive system. It is the point at which the brain sends a signal through the vagus nerve to the stomach, triggering the production of gastric juices. Specifically, the cephalic phase begins with the sight, smell, or taste of food and ends once the first bite is swallowed. Once food is swallowed and enters the digestive system, the gastric and intestinal phases take over.
In many respects, the cephalic phase is similar to warming up a gas combustion engine. Most gas combustion engines require some type of oil in order to facilitate internal movement. An ignition key or switch signals the oil to start circulating in the motor via an electrical wire. The cephalic phase is similar in that the senses serve as the key, the vagus nerve serves as the electrical wire, and peptic and gastric acids act as the oil.
Anticipation is the key to the cephalic phase. Cephalic, by definition, relates to the head, or in the case of digestion, the brain in particular. Seeing, smelling, or tasting food stimulates the brain with regard to food. Immediately following hunger or initial eating messages, the brain must pass that information to the stomach in order to begin the production of acidic enzymes in preparation for digestion. Gastric enzymes and similar stomach secretions are critical to the digestive process, as they break down food into usable proteins, sugars, and other components.
Without the cephalic stage, the stomach would be unable to produce the necessary digestive enzymes until after food had reached the stomach. While this would not prohibit digestion, it would indeed slow down the process considerably. Distention of the stomach caused by food during the gastric phase would be the only digestive signal the body would receive, cutting the production of gastric juices by as much as 50 percent. Proper digestion requires specific chemicals, released at just the right intervals, in order to provide the body with the most efficient method of nutrient absorption.
Other than a more efficient digestive process, the cephalic phase has a tremendous impact on subsequent digestive phases. During the gastric phase, gastrin and histamine control the pH of stomach contents. Lack of a cephalic phase would have consequences that would negatively affect the existing pH levels at the start of the gastric phase. Absence of initial peptic acids or a drop in initial levels during the cephalic phase could cause an increase or decrease in the pH level during the gastric phase, resulting in poor digestion and a less efficient intestinal phase.