Place the events of a sinoatrial node cell generating a pacemaker potential in order. Start when the membrane potential is most negative (-60 mV), and end when the membrane potential becomes most negative again, immediately prior to the next pacemaker potential. Membrane potential is -60 mV Membrane potential returns to -60 mV Answer Bank Voltage-gated sodium channels open. Voltage-gated calcium channels open. Voltage-gated potassium channels open. The membrane slowly depolarizes. The membrane quickly depolarizes. The membrane repolarizes.
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The pacemaker action potentials in the heart a. are due to opposing actions of norepinephrine and acetylcholine. b. are generated by the bundle of His. c. depend on the gap junctions between the cells that make up the atria and those that make up the ventricles. d. are due to spontaneous depolarization of the plasma membranes of modified cardiac muscle cells. e. reflect large depolarizations of membrane potential due to opening of voltage-gated $\mathrm{Na}^{+}$ channels.
The rapid repolarization of the membrane in an action potential is caused by none of the above opening of voltage-gated K+ channels in response to peak depolarization closing of the slow inactivation gates on Na+ channels opening of voltage-gated K+ channels in response to the initial threshold depolarization opening of voltage gated Ca2+ channels in the axon terminal
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