The impedance change during the spike

An important landmark in the development of theories about the mechanism of conduction was the demonstration by Cole and Curtis in 1939 that the passage of an impulse in the squid giant axon was accompanied by a substantial drop in the electrical impedance of its membrane. The axon was mounted in a trough between two plate electrodes connected in one arm of a Wheatstone bridge circuit (Figure 4.1) for the measurement of resistance and capacitance in parallel. The output of the bridge was displayed on a cathode-ray oscilloscope, and Rv and Cv were adjusted to give a balance, and therefore zero output, with the axon at rest. When the axon was stimulated at one end, the bridge went briefly out of balance (Figure 4.2) with a time course very similar to that of the action potential. The change was shown to be due entirely to a reduction in the resistance of the membrane from a resting value of about 1000 Ω cm2 to an active one in the neighbourhood of 25 Ω cm2. The membrane capacitance of about 1 µF/cm2 did not alter measurably.

The sodium hypothesis

Cole and Curtis's results were not wholly unexpected, because it had long been supposed that there was some kind of collapse in the selectivity of the membrane towards K+ ions during the impulse.