The pharynx is the cavity at the rear of the throat and links the mouth with the esophagus. It serves as a common passageway for the respiratory and digestive systems. The swallowing reflex takes place largely in the pharynx. This is an example of an all-or-none reflex in which, once the process has begun, it cannot stop. Swallowing may be initiated voluntarily when the tongue pushes a bolus of food toward the back of the mouth and into the pharynx. The stimulation of pressure receptors in the pharynx results in transmission of nerve impulses to the swallowing center in the medulla of the brainstem. This elicits a coordinated involuntary reflex that involves contraction of muscles in the appropriate sequence. A wave of contraction sweeps down the constrictor muscles of the pharynx. The epiglottis moves downward over the larynx to seal off the trachea and the upper esophageal sphincter relaxes, allowing the bolus of food to enter the esophagus. Once the food bolus enters the esophagus, the upper esophageal sphincter closes in order to prevent the swallowing of air. This phase of the swallowing reflex is referred to as the pharyngeal stage and lasts approximately 1 sec.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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