Intro to Nutrition #52: THE DIGESTIVE SYSTEM Anatomy: Digestion Starts in the Mouth. BIOLOGY LECTURE

Intro to Nutrition #52: THE DIGESTIVE SYSTEM Anatomy: Digestion Starts in the Mouth. BIOLOGY LECTURE


As food enters the mouth, its mechanical breakdown
begins with mastication. Mastication is partly voluntary and partly reflexive. We voluntarily
put food in our mouths and contract the muscles that close our jaws. The pattern and rhythm
of continued jaw movements are controlled mainly by stretch reflexes in response to
pressure inputs from receptors in the cheeks, gums, and tongue. To send food on its way
from the mouth, it’s first compacted by the tongue into a bolus and then swallowed.
Swallowing is a complicated process that involves coordinated activity of some 50 pairs of muscles
and has two major phases: the buccal and the pharyngeal-esophageal. The buccal phase occurs
in the mouth and is voluntary. In the buccal phase, we place the tip of the tongue against
the hard palate, and then contract the tongue to force the bolus into the oropharynx. As
food enters the pharynx and stimulates tactile receptors, it passes out of our control and
into involuntary reflex activity. Once food enters the pharynx, respiration is momentarily
inhibited and all paths into the digestive tract are blocked off. The tongue blocks the
mouth. The soft palate rises to close off the nasopharynx. The larynx rises so that
the epiglottis covers its opening into the respiratory pathways, and the upper esophageal
sphincter relaxes. Wavelike peristaltic contractions create pressure waves that propel food through
the pharynx and into the esophagus. Just before the peristaltic wave reaches the esophagus,
the gastroesophageal sphincter relaxes to allow food to enter the stomach.

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