S.A node firing of the heart results from a spontaneous depolarization. Acetycholine is released by parasympathetic nervous system and binds to muscarinic receptor which activates G protein coupled receptors and blocks the CAMP pathway normally activated by noradrenaline in the sympathetic system. The release of noradrenaline in the sympathetic system allows an influx of cations in order to reach the threshold and cause action potential firing of the S.A node. Together, these two mechanisms account for heart rate at rest. Electrical activity is then propagated through cardiac tissue to produce timed contractions of various chambers by slowing down at the S.A and A.V nodes to allow for right atrium contraction and repolarization prior to ventricular contraction.
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