Effect of low sodium, tetrodotoxin, and temperature variation upon excitation.

The lowering of external sodium raised both the constant quantity threshold, Q(o), and the rheobase, I(o), in both real space-clamped squid axons and the theoretical axon as computed on the basis of the standard Hodgkin-Huxley equations. In both real and theoretical axons the minimum intensity for excitability for short pulses, which occurs at about 15 degrees C, was still present when low sodium replaced seawater. Low sodium did not affect the temperature dependence of the strength-duration relationship in the range, 5 degrees to 25 degrees C. The excitability of tetrodotoxin-treated real axons was found to be more temperature-dependent than that of normal real axons. Also the data on dosage-response to TTX of real axons fit the dose-response relationship of a hypothetical system in which one TTX ion binds reversibly to its receptor to produce a fraction of the inhibitory effect, the curve being identical to a simple adsorption isotherm. The Hodgkin-Huxley equations describe the broad outline of events occurring during excitation quite well.