Diffusion of sodium in axoplasm of myelinated nerve fibre. Potential clamp analysis

The diffusion of sodium and lithium in the axoplasm of myelinated fibres of Xenopus laevis was investigated. The sodium current associated with pulse steps was measured and the internal sodium concentration was calculated from these measurements with the constant field equation. The internal sodium concentration was increased either (i) by massive pulse trains or (ii) by applying a sodium containing solution to a cut end of the fibre. The time course of the concentration change was measured. Equations were solved numerically for a model axon where sodium diffused freely in the axoplasm. It was found that the experimental results were satisfactorily predicted by this model axon. The calculated diffusion coefficient for sodium in axoplasm seemed to be somewhat lower than that in water. Similar experiments were performed with lithium. These results agreed with the predictions based on unrestricted diffusion in the axoplasm. It is concluded that no major diffusion barriers were present within the axoplasm.