Figure 1. Activation maps and bipolar electrograms recorded from a canine left posterior papillary muscle, about 12 x 18 mm, orientated tip (chordae tendinae) to the top, cardiac apex to the bottom. (A) illustrates activation of the Purkinje fiber layer, which was stimulated (*symbol*) at the false tendon attachment close to site A and exhibited rapid activation by parallel isochrones within 4 or 5 ms. (B) shows that the earliest activation of ventricular muscle (VM) occured within 10 ms at site E, the selected Purkinje ventricular-muscle junction (PVJ). Thereafter, the muscle was activated in the opposite direction toward the tip (C) by the sequence of Purkinje (P) and ventricular (V) deflections in electrograms recorded at sites between A and E.