Maybe you've had this scenario described to you by a customer: About an hour after dinner she begins to notice an uncomfortable sensation in her abdomen. She says the pain is cramplike, builds in intensity and shoots back toward her right shoulder blade. Nausea follows. She takes an antacid but wonders whether she has symptoms of appendicitis, food poisoning or an ulcer. A visit to her doctor reveals that she is actually suffering from complications of cholelithiasis, also known as gallstones.
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The action potentials generated by the depolarization of SA node cells propagate from cell to cell throughout the atria, producing the atrial contraction. The atrial depolarization also propagates to the AV node, which then transmits the electric impulse to the ventricles through specialized conduction bundles of the interventricular septum (the bundle of His) and then to the Purkinje fibers of the ventricle walls, causing a ventricular contraction. (Atrial contraction precedes ventricular contraction so that blood fills the ventricles before ventricular contraction.)