Voltage Clamp Studies on Internally Perfused Axons

Probably the most striking and surprising observation in internally perfused axons is that full-sized action potentials can be obtained in the face of a very low resting potential when the ionic strength is low In all experiments at normal ionic strength known to the author, the explosive all-or-none responsiveness was lost when the internal and/or external monovalent cations were arranged to give a resting potential of-20 to-30 mv unless the system was strongly hyper-polarized previous to the stimulus. Both Narahashi (1963) and Baker et al. (1964) found that the threshold potential changed along with a resting potential and both papers suggest a shift in the sodium inactivation curve along the voltage axis. Although neither used sodium in the internal perfusing medium, both found that the height of the spike above the resting potential was roughly constant in magnitude. Narahashi, Ulbricht, and I set out to test the hypothesis of the shift in the sodium inactivation by voltage-clamping a perfused axon, using our standard sucrose gap method as shown in Fig. 1 (from Moore, Narahashi, and Ulbricht, 1964). The axon in the potential pool was cut so that it could be continuously perfused. No correction was made in the measured membrane potential when perfusing with the HK solution because the potential across the membrane in the side pool was probably very close to zero. However, when the LK solution was perfused through the axon the interior of the axon in the potential pool is some 40 mv more positive than the outside (Narahashi, 1963). The low conductance of the LK solution and the high membrane conductance made the length constant very short in the potential pool. Therefore, a 40 mv junction correction was added to the observed difference between the central and potential pools in estimating the potential of the interior of the "node." The composition of the perfusing solution is given in Table I. We used a small amount of sodium in the perfusing solution for two reasons: (a) to II The Journal of General Physiology