Diffusion of Sodium in Axoplasm of Myelinated Nerve Fibre. Potential Clamp Analysis

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Abstract
The diffusion of Na+ and Li+ in the axoplasm of myelinated fibers of Xenopus laevis was investigated. The Na+ current associated with pulse steps was measured, and the internal Na+ concentration was calculated from these measurements with the constant field equation. The internal Na+ concentration was increased either by massive pulse trains or by applying a Na+ containing solution to a cut end of the fiber. The time course of the concentration change was measured. Equations were solved numerically for a model axon where Na+ diffused freely in the axoplasm. The experimental results were satisfactorily predicted by this model axon. The calculated diffusion coefficient for Na+ in axoplasm seemed to be somewhat lower than that in water. Similar experiments were performed with Li+. These results agreed with the predictions based on unrestricted diffusion in the axoplasm. There seem to be no major diffusion barriers within the axoplasm.