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

Abstract

SUMMARY1. 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.2. 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 'C. During this period the resting potentials averaged about -75 mV.3. Under voltage-clamp conditions Na currents and K currents in transacted 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 transaction, up to several weeks.4. The excitability of the severed sciatic nerve, expectedly, depends strongly on the time course ofWallerian 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 'C it was maintained for more than 84 days, as long as the myelinated nerve fibres did not degenerate.5. Together, these findings demonstrate that Na channels, K channels, and Na-K pumps are continuously present for several weeks in the transacted nerve before nerve degeneration. It is surmised that either, (a) these proteins are extremely stable in the transacted 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.

Characteristics of Na+ current in Schwann cells cultured from frog sciatic nerve

“…An attractive but highly speculative hypothesis that Schwann cells might act as a local factory of ionic channels for axon was proposed (Bevan et al, 1985;Shrager et al, 1985;Waxman and Ritchie, 1985). Indirect evidence (Ritchie, 1988;Tytell and Lasek, 1984;Wang, 1985) supports this hypothesis. However, it is not directly applicable in the frog peripheral nervous system for the following reason (see also Barres et al, 1990).…”

Sodium currents in axon‐associated Schwann cells from adult rabbits.

“…In the central nervous system, protrusions of oligodendrocyte cytoplasm from the inner tongue of the myelin sheath into the axonal cytoplasm can be observed (Novotny, 1982). Relatedly, Wang (1985) has recently demonstrated that sodium channels are continuously present for several weeks in the nodal membranes of transected myelinated fibres in frog, a result that is consistent with either an unusually slow turnover rate for sodium channels or a non-neuronal source of sodium channels. …”

Temperature Modulation Reveals Three Distinct Stages of Wallerian Degeneration

“…The effect of cool external temperatures slowing Wallerian degeneration in vivo is well known (Gamble et al, 1957;Gamble and Jha, 1958;Usherwood et al, 1968;Wang, 1985;Sea et al, 1995). In rats, Sea and colleagues (1995) showed that the time course for myelinated axons to degenerate after axotomy was 3 d at 32°C and 6 d at 23°C.…”

“…Experiments on Wallerian degeneration in mammalian, lower vertebrate, and invertebrate axons have all demonstrated that cold temperatures can slow the rate of progression (Gamble et al, 1957;Gamble and Jha, 1958;Usherwood et al, 1968;Wang, 1985;Bittner, 1988;Sea et al, 1995). This effect has been attributed variously to an alteration in the activity of degradative enzymes (changing the Q 10 by threefold) or the rate of axoplasmic transport (Cancalon, 1982(Cancalon, , 1985.…”