Biophysical Journal 12: 552-563 (1972)
© 1972 the Biophysical Society

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Anode Break Excitation in Space-Clamped Squid Axons

ABSTRACT

Strength-duration curves for space-clamped squid axons, using square wave anode breaks as stimuli, established the existence of four distinct regions. For the average experimental axon the intersection of the first two regions, ₁, occurs at about 7 msec. This agrees with computations based on the Hodgkin-Huxley (HH) equations and corresponds to the accommodation time constant found previously for a linearly rising ramp, as given by the HH equations and as found experimentally. The second break in the curve, ₂, at about 200 msec, and the third break, ₃, at 1 sec, are far beyond the range of the HH equations and may be the counterpart in the excitability of the long time constants, which have been apparent from a number of other types of experiments. The regions of the curve before 1 msec and beyond 2 or 3 sec are quite variable and may represent breakdown. Rheobase increases in both experimental and computed axons when temperature is raised. In both experimental and computed axons ₁ descreases slightly when the temperature is raised from 10 to 15°C. At 20 and 25°C, ₁ of the experimental axon increases markedly.