Search PubMed⌕ Search

Biomedical subjects

P Arhem

Publications and source records attributed to P Arhem.

62 records · Page 4Linked to original sources
PubMed1976

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.

Animals↗
PubMed1975

Steady state current rectification in potential clamped nodes of Ranvier (Xenopus laevis).

The membrane potential in single nodes of Ranvier was changed in rectangular pulse steps while the membrane currents, associated with the potential steps, were measured. Changes were made in the ionic composition of the external and the internal medium. The latter changes were obtained by free diffusion through a cut internode. The steady state currents, described on the basis of potassium and leak permeability, were affected by the solution composition in a characteristic way. Increased inside concentration of sodium and lithium caused a striking rectification of the outward steady state currents at large potential steps. Instantaneous potassium currents in high [K+]o at a second potential step to E approximately equal to minus 80 mV were not affected by [Na+]1. Neither [Na+]o nor [K+]i affected the potential at which this rectification appeared. Increased [K+]o shifted the region for rectification along the potential axis in positive direction. These findings form strict limitations for satisfactory models describing the mechanism for the steady state current in myelinated nerve.

Animals↗
PubMedDate not supplied

Non-stationary fluctuation analysis of the Na current in myelinated nerve fibers of Xenopus laevis: experiments and stochastic simulations.

Na current fluctuations under voltage-clamp conditions during pulse steps in the potential range from -65 to -30 mV were measured in myelinated nerve fibers of Xenopus laevis. The covariance functions for four consecutive 1 ms intervals were calculated. The time courses of the covariance functions were well fitted with monoexponential functions with time constants between 0.5 and 3 ms, larger at the end of the pulse and larger at more positive potentials. To analyze the underlying channel kinetics we simulated current fluctuations at a step to -35 mV of eight published Na channel models and calculated corresponding covariance functions. None of the models did explain the experimental fluctuation results. We therefore developed a new Na channel model that satisfactorily described the results. Features that distinguished this model from the other tested ones were a slower deactivation rate, and an inactivation transition directly from a closed state.

Animals↗
PubMedDate not supplied

Na channel kinetics: developing models from non-stationary current fluctuations by analytic methods.

In a previous study, we analyzed Na current fluctuations in myelinated axons from Xenopus laevis under voltage clamp conditions. The statistical properties were analyzed in terms of covariance functions for consecutive time intervals of varying duration during the pulse step. The underlying channel kinetics was analyzed by performing stochastic simulations of published Na channel models and calculating corresponding covariance functions. None of the models explained the fluctuation results. We therefore developed a novel minimal Na channel model that satisfactorily described the results. In the present paper, we extend the analysis and specify the possible models explaining the experimental data by using analytical methods. We derive general relations between the experimental data, including the covariance functions, and the rate constants of specific one-open-state models. A general feature of these models is that they comprise an inactivation step from the first closed state and a relatively low backward rate from the open state. This is in accordance the minimal model inferred from numerical stochastic calculations in the previous study.

Animals↗