Effects of norepinephrine and acetylcholine on spontaneous diastolic depolarization (automaticity) in a pacemaker cell for the sinoatrial node. The pacemaker cell discharges spontaneously when the threshold potential (TP) is attained. The rate of spontaneous discharge is determined by the initial slope of the membrane potential and the time required to reach the threshold potential. A. Control recording showing the spontaneous diastolic depolarization. B. The effect of norepi-nephrine is to increase the slope of diastolic depolarization. The frequency of spontaneous discharge is increased. This effect is mediated through the activation of p-adrenoceptors in sino-atrial nodal cells. C. Acetylcholine stimulates muscarinic receptors in sinoatrial nodal cells. There is a decrease in the slope of diastolic depolarization as well as hyperpolarization of the cell. The time to reach the threshold potential is prolonged, with the net effect being a decrease in the rate of spontaneous depolarization.
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