4. When the resting transmembrane potential changes, the excitability of nerve and muscle also changes.a. When the resting membrane depolarizes (e.g., with hyperkalemia), the resting potential shifts closer to threshold, and there is an increased likelihood of spontaneous action potentials. EXCITABILITY (IRRITABILITY) INCREASES. In the heart, there are PVCs and, with severe hyperkalemia, ventricular fibrillation.b. Cardioplegic solution used during coronary artery bypass graft surgery has a high concentration of K+, and the cardiac cells depolarize to a level between threshold and 0 mV. As the cardiac cells depolarize to and beyond threshold, an action potential is elicited and a corresponding contraction occurs. Thereafter, however, there is no electrical activity. The sodium gates are shut in the inactivated state until normal K+ is restored.c. When the resting membrane hyperpolarizes, the membrane potential moves farther away from threshold. It becomes more difficult to depolarize the cell to threshold and elicit an action potential. EXCITABILITY (IRRITABILITY) IS DECREASED. Hypokalemia leads to a decrease in excitability of cardiac ventricular cells as well as of nerve, skeletal muscle, and smooth muscle cells. In the heart, the sinoatrial and atrioventricular nodes may become completely depressed. There are, however, ventricular Purkinje cells that more readily depolarize to threshold and PVCs occur.

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