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 (Narahashi, 1963; Baker, Hodgkin, and Shaw 1962; Baker, Hodgkin, and Meves 1964; Tasaki, Watanabe, and Takenaka 1962). 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 hyperpolarized 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