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Mechanical Processes - Notes 2.docx

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BIOL 153
Paul Hewitt

Mechanical ProcessesGastric Stretching and EmptyingThe initial response of the stomach to filling is to relaxThis is due toReceptive relaxation is a reflex involving the swallowing center of the brainstem and vagus nerve that causes the stomach to relax when food is in the esophagusAdaptive relaxation reflex is activated when the stomach is fillingStretch receptors on the stomach wall respond and cause the stomach to dilatePlasticity of the stomach allows it to respond to stretch without greatly increasing its tension and contracting expulsivelyGastric Contractile ActivityIn the wall of the stomach are pacemaker cells which slowly depolarize and repolarize at a rate of about 3 per minute called the basic electrical rhythm or BEROn their own the depolarizations do not reach threshold and no muscle contraction occursWhen outside stimulus arrives stretch is the most important of these threshold is reached action potentials are fired and muscle contraction occursAll of the smooth muscle cells of the stomach are connected via gap junctions therefore the whole organ contractsThis gives rise to the peristaltic waves which begin near the cardiac sphincterThe waves start out very small but as they descend over the stomach they increase and become more vigorousAs the waves reach the pyloric end of the stomach a small amount of chyme is squeezed out through the pyloric valveThe opening of the valve is very small so only liquid and small particles will pass throughThe wave of the contraction constricts the valve so that only a small amount of chyme will pass through
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