A premature atrial contraction occurs when a focus in the atrium (not the sinoatrial node) generates an action potential before the next scheduled SA node action potential. There are four main ...
A premature ventricular contraction occurs when a focus in the ventricle generates an action potential before the next scheduled sinoatrial nodal action potential. Premature, occurring earlier than ...
Muscle contraction is the tightening, shortening, or lengthening of muscles when you do some activity. It can happen when you hold or pick up something, or when you stretch or exercise with weights.
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