The long-term excitability of myelinated nerve fibres in the transected frog sciatic nerve.

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The long-term excitability and the ionic currents in the nodes of Ranvier were studied in the severed frog sciatic nerve and the unoperated contralateral control nerve. After unilateral nerve section, compound action potentials of the nerve bundles and action potentials of single myelinated nerve fibres remained normal in amplitude and duration in either sciatic nerve for more than 38 days when frogs were kept at 11 degrees C. During this period the resting potentials averaged about -75 mV. Under voltage-clamp conditions Na currents and K currents in transected single myelinated nerve fibres also appeared normal in their kinetics, and in their peak amplitudes. These results indicate that the Na- and K-channel densities are quantitatively unchanged after the nerve transection, up to several weeks. The excitability of the severed sciatic nerve, expectedly, depends strongly on the time course of Wallerian degeneration. When frogs were kept at room temperature, the nerve excitability remained normal for only about 8-10 days, due to the faster Wallerian degeneration; whereas at 4 degrees C it was maintained for more than 84 days, as long as the myelinated nerve fibres did not degenerate. Together, these findings demonstrate that Na channels, K channels, and Na-K pumps are continuously present for several weeks in the transected nerve before nerve degeneration. It is surmised that either, (a) these proteins are extremely stable in the transected myelinated nerve fibres, or (b) they are supplied locally by Schwann cells, an open question recently posed by Chiu, Schrager & Ritchie (1984). In either event, the myelinated nerve fibres do not require cell bodies to provide a significant amount of new channels and pumps in order to retain their long-term excitability.

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