Excitation of Cardiac Muscle Cells

was used to deliver rectangular pulses for extracellular field stimulation of individual heart cells. We found that single frog and guinea pig ventricular myocytes are excitable with rectangular pulse field stimulation over a wide range of pulse durations, ranging from 10 msec to as little as 20 ,usec. The excitation field strength varies inversely with pulse duration as described by the Weiss-Lapicque equation, although the frog myocytes show a significant "notch" at pulse durations of -1-2 msec, and the guinea pig myocytes are more excitable than predicted for pulse durations of <0.2 msec. Every myocyte tested was more excitable when the long axis of the cell was oriented parallel to the stimulating field than when perpendicular to the field. For 2-msec pulses, the difference in field strength was a factor of 5.8±2.0 (n=30) for frog and 2.6+0.5 (n=23) for guinea pig myocytes. Complete excitation strength-duration curves