Excitation System

This system is also discussed in Chap. 23. The cellular AP consists of a transient, local transsarcolemmal depolarizing current that raises the transmembrane potential from its normal resting value of negative 80 to 90 mV to slightly positive values, followed by a depolarizing current that returns the potential to its resting value4-6 (Fig. 3-4). The AP is initiated within the specialized conduction tissue and is propagated to individual myocytes. It results from a series of coordinated changes in the conductance of specific ionic species through variably gated sarcolemmal channels. The earliest and largest component of membrane depolarization is caused by a rapid, inward Na current. The resting potential is established and maintained by the transsarcolemmal Na-K-ATPase, which uses energy from ATP hydrolysis to pump Na ions out of the cytoplasm.

Myocyte Labeled

Figure 3-4: Phases of cellular AP and major associated currents in ventricular myocyte. Initial phase zero spike (not labeled) and overshoot (1) is caused by rapid inward Na current, the plateau phase (2) by slow inward Ca current through L-type Ca channels, and repolarization (phase 3) by outward K current. Phase 4 resting potential (Na efflux, K influx) is maintained by the Na-K-ATPase. Na-Ca exchanger is mainly responsible for Ca extrusion. In specialized conduction system tissue, there is spontaneous depolarization during phase 4 until the voltage resulting in opening of the Na channel is reached.

Figure 3-4: Phases of cellular AP and major associated currents in ventricular myocyte. Initial phase zero spike (not labeled) and overshoot (1) is caused by rapid inward Na current, the plateau phase (2) by slow inward Ca current through L-type Ca channels, and repolarization (phase 3) by outward K current. Phase 4 resting potential (Na efflux, K influx) is maintained by the Na-K-ATPase. Na-Ca exchanger is mainly responsible for Ca extrusion. In specialized conduction system tissue, there is spontaneous depolarization during phase 4 until the voltage resulting in opening of the Na channel is reached.

With respect to initiation of contraction, the most important component of the AP is a relatively slow, inward Ca current through voltage-sensitive, L-type (for long-lasting) Ca channels5,7,8 (Ca2+ influx in Fig. 3-4). These channels open, and the current begins when transmembrane potential reaches -35 to -20 mV and, because of its slow kinetics, continues well after the Na current has ceased. The Ca current is mainly responsible for the AP plateau phase. It ceases when L-type channels become inactivated, and regenerative currents (mainly K efflux) begin the repolarization process. L-type channels, also termed dihydropyridine (DHP)receptors, are concentrated in invaginations of the sarcolemma called the transverse-tubule system, in close proximity to sarcoplasmic reticulum membrane-associated ryanodine receptor (RyR) Ca release channels (discussed below).

The AP results in a net movement of Ca ions into and a net movement of Na ions out of the cytoplasm. Ionic balance is restored mainly by another sarcolemmal ion-transport mechanism, the Na-Ca exchangerJ-9 11 The exchanger is a shuttle that moves one Ca ion out of the cell against its concentration gradient while using energy from the Na gradient to move one Na ion into the cell. The exchanger also can function in so-called reverse mode, moving a Ca ion into the cytoplasm and a Na ion out.9-11 Normally, the reverse mode does not contribute significantly to inward movement of Ca ions.

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Essentials of Human Physiology

Essentials of Human Physiology

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