Swallowing Mechanism

Swallowing reflexes can be divided into three stages. In the first stage, which is voluntary, food is chewed and mixed with saliva. Then, the tongue rolls this mixture into a mass (bolus) and forces it into the pharynx. The second stage of swallowing begins as food reaches the pharynx and stimulates sensory receptors around the pharyngeal opening. This triggers the swallowing reflexes, illustrated in figure 17.14, which includes the following actions:

1. The soft palate raises, preventing food from entering the nasal cavity.

2. The hyoid bone and the larynx are elevated; the epiglottis closes off the top of the trachea so that food is less likely to enter.

3. The tongue is pressed against the soft palate, sealing off the oral cavity from the pharynx.

4. The longitudinal muscles in the pharyngeal wall contract, pulling the pharynx upward toward the food.

5. The lower portion of the inferior constrictor muscles relaxes, opening the esophagus.

6. The superior constrictor muscles contract, stimulating a peristaltic wave to begin in other pharyngeal muscles. This wave forces the food into the esophagus.

As the swallowing reflexes occur, breathing is momentarily inhibited. Then, during the third stage of swallowing, peristalsis transports the food in the esophagus to the stomach.

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