Anode break excitation in space-clamped squid axons

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, tau(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, tau(2), at about 200 msec, and the third break, tau(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 tau(1) descreases slightly when the temperature is raised from 10 to 15 degrees C. At 20 and 25 degrees C, tau(1) of the experimental axon increases markedly.