The movement of sodium ions across the membrane of the squid giant axon was measured by the use of Na22 and Na24. Unidirectional fluxes were measured at rest and when the nerve was stimulated. The difference was considered the extra flux association with nerve impulses. The extra influx in intact axons at room temperature was 5.5 p mole/cm2 impulse. At 6° C the extra influx was 6.5 p mole/cm2 impulse giving a Q10 of 1/1.2. In perfused axons a Q10 of 1/1.6 was obtained for the extra sodium influx in bracketed experiments on individual axons. The Q10 of the extra sodium efflux associated with nerve impulses was found to be 1/1.2 in intact axons. Hodgkin & Huxley had predicted a much larger temperature dependence for the extra fluxes. If this difference between prediction and experiment does not result from some experimental error, then the class of models for the ion fluxes suggested by Hodgkin & Huxley may be inapplicable.
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