The Long Duration of the Cardiac Action Potential Guarantees a Period of Relaxation (and Refilling) Between Heartbeats

Na* channels become inactivated al the peak of the cardiac action potential. Na* cannot pass through an inactivated channel; therefore, as long as the Na’ channels remain inactivated, another action potential cannot occur.

While the Na^t channels are inactivated, the cell is refractory (resistant) with regard to the formation of an action potential. The time after the beginning of one action potential during which another action potential cannot be initiated is called the absolute refractory period (or refractory period). Because Na⁺ inactivation lasts until the membrane potential returns to its resting level, the refractory period lasts about as long as an action potential. Thus the refractory period in a nerve or skeletal muscle cell lasts about I or 2 msec, whereas the refractory period in a cardiac muscle cell lasts IOO to 250 msec (see Figure 19-4).

The long refractory period in cardiac muscle guarantees a period of relaxation (and cardiac refilling) between cardiac contractions. Figure 19-6 (top) depicts the quickest possible succession of three action potentials in a cardiac muscle cell: the second action potential begins immediately after the conclusion of the refractory period for the first action potential. Likewise, the third action potential begins immediately after the con­clusion of the refractory period for the second. The bottom graph shows the pattern of muscle contraction that results from the three action potentials. Note that contractile strength reaches a peak late in the plateau phase of each action potential, and that the contractile strength decreases (the muscle begins to relax) during the repolarization phase of each action potential.

The pattern of changes in muscle tension depicted in the bottom of Figure 19-6 corresponds closely to the changes in the cytosolic Ca^{2 4} concentration. This makes sense, considering that the increase in cytosolic Ca^2, concentration during an action potential initiates muscle contraction, and the sub­sequent removal of Ca^2f from the cytosol permits the muscle to relax. Contractile tension decreases almost to its resting level during the repolarization phase of the action potential, because cytosolic Ca²⁺ concentration is reduced almost to its resting level by the time the action potential is over. In other words, during the repolarization phase of the action potential, active transport pumps move most of the free, cytosolic Ca²' back into the sarcoplasmic reticulum or out into the extracellular fluid.

In skeletal muscle cells, an action potential lasts only 1 to 2 msec. The membrane is repolarized (and the refractory period is over) even before the release of Ca^2k from the sarcoplasmic reticulum is finished, and many milliseconds before the released Ca²⁺ is pumped back into the sarcoplasmic reticulum. As a result, the cytosolic Ca²* concentration reaches its peak level after the action potential is over, and the contractile tension resulting from the action potential also reaches its peak after the action potential is over. Because a contractile twitch lasts much longer than the refractory period in skeletal muscle, several action potentials can occur during the time of a single con­tractile twitch. Multiple action potentials in quick succession cause cytosolic Ca²^ concentration to build to a high level and stay there. The resulting contractile tension is stronger than the tension that results from a single action potential, and it is sustained for a longer time. In effect, the muscle twitches caused by successive action potentials “fuse” together. This phenom­enon is called temporal summation. Fusion and temporal sum­mation are the mechanisms that permit graded and prolonged tension development in skeletal muscle. In contrast, the long refractory period in cardiac muscle cells prevents the fusion and summation of cardiac contractions. Each contraction of the heart (each heart beat) is followed immediately by a relaxation.