Membrane conductance changes in single nodes of ranvier,measured by laser-induced temperature-jump experiments

Temperature-jump experiments on isolated myelinated nerve fibers were done using a pulsed laser system in the Q switched mode. Voltage-clamp and temperature perturbations were used to measure the relaxing ionic conductances of both the Na⁺ and K⁺ systems. It is shown that the T jump can be used to probe the K⁺ and Na⁺ conductances during non-steady state conditions and thereby elicit relaxation times for a variety of initial states. Temperature-induced K⁺ conductance relaxation times were consistent with voltage-clamp measurements. The temperature-perturbation experiments were done as a combination of a temperature step and impulse change due to an adsorption of carbon black particles on the nerve. The experiments support the hypothesis that the relaxation times of the K⁺ system are independent of the previous history of the axon. It is concluded that the K⁺ conductance is at least a second-order system whose relaxation spectrum is composed of two exponential terms the magnitudes of which are markedly dependent on the initial conditions.