The pre-depolarization phase of the SA node Action potential is due to
Correct Answer: Ca++ entry
Description: Effect of Potassium and Calcium Ions on Hea Function In the discussion of membrane potentials in Chapter 5, it was pointed out that potassium ions have a marked effect on membrane potentials, and in Chapter 6 it was noted that calcium ions play an especially impoant role in acti- vating the muscle contractile process. Therefore, it is to be expected that the concentration of each of these two ions in the extracellular fluids should also have impoant effects on cardiac pumping. Effect of Potassium Ions. Excess potassium in the extracellular fluids causes the hea to become dilated and flaccid and also slows the hea rate. Large quantities also can block conduction of the cardiac impulse from the atria to the ventricles through the A-V bundle. Elevation of potassium concentration to only 8 to 12 mEq/L--two to three times the normal value--can cause such weakness of the hea and abnormal rhythm that death occurs. These effects result paially from the fact that a high potassium concentration in the extracellular flu- ids decreases the resting membrane potential in the car- diac muscle fibers, as explained in Chapter 5. That is, high extracellular fluid potassium concentration paially depo- larizes the cell membrane, causing the membrane potential to be less negative. As the membrane potential decreases, the intensity of the action potential also decreases, which makes contraction of the hea progressively weaker. Effect of Calcium Ions. An excess of calcium ions causes effects almost exactly opposite to those of potas- sium ions, causing the hea to go toward spastic con- traction. This is caused by a direct effect of calcium ions to initiate the cardiac contractile process, as explained earlier in the chapter. Conversely, deficiency of calcium ions causes car- diac flaccidity, similar to the effect of high potassium. Founately, calcium ion levels in the blood normally are regulated within a very narrow range. Therefore, cardiac effects of abnormal calcium concentrations are seldom of clinical concern. Ref guyton and hall textbook of medical physiology Edition 12 pg 122
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