PR interval corresponds to atrial depolarization/ repolarization

QT interval corresponds to ventricular depolarization/ repolarization

FIGURE 34-1 Action potentials that occur during normal impulse propagation and the time course of the currents that generate them. Hypothetical action potentials from different regions of the heart are shown. The current magnitudes are not to scale. In atrial myocytes, Purkinje fibers, and ventricular myocytes the Na+ current is ordinarily 50 times larger than any other current, although the portion that persists into the plateau (phase 2) is small. Multiple types of Ca2+ current, transient outward current (/TO), and delayed rectifier (/K) have been identified. Each represents a different channel protein, usually associated with ancillary (function-modifying) subunits. 4-AP (4-aminopyridine) is a widely used in vitro blocker of K+ channels. ITO2 may be a Cl- current in some species. Components of Ik have been separated on the basis of how rapidly they activate: slowly (7^), rapidly (I^), or ultrarapidly (I^). For all currents shown here (with the possible exception of /TO2), the genes encoding the major pore-forming proteins have been cloned.

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