If the limiting slope were less steep than we measured, then we should have seen orders of magnitude greater activity at the most negative potentials. For example, a "limiting" slope of e-fold/4 mV (six charges) fits our **data** between -60 and -40 mV. Extending this curve to -80 mV would give a Po of 8 * 10 -5, or 400-fold higher than we actually saw. Since this extra activity was absent, we must conclude that the slope could not have been as low as six effective charges.