2. During the upstroke of an action potential in squid giant axon, the probability of a sodium channel being open changes. At rest the probability of an open channel is 0.01, while at the peak of an action potential it is 0.2. Consider 0.6 mm of cell membrane. On the average, how many channels change from closed to open as the membrane moves from rest to peak, i.e., during the upstroke of the action potential? BIOELECTRICITY: A QUANTITATIVE APPROACH 85 Table 4.1. Conductance of Sodium and Potassium Channels Preparation Channels (PS) (number/um ) Sodium Squid giant axon 4 330 Frog node 6 8 400-2000 Rat node 14.5 700 Bovine chromaffin 17 1.5-10 Potasshon Squid giant axon 12 30 Frog node 2.7-4.6 570-960 Frog skeletal 15 30 Mammalian BK 130-240