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Activity-dependent excitability changes in normal and demyelinated rat spinal root axons.

H Bostock, P Grafe

The Journal of Physiology

|August 1, 1985

Summary

Demyelinated nerve fibers are blocked by impulse trains due to hyperpolarization, not potassium buildup. This hyperpolarization increases nerve fiber threshold, causing conduction failure.

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Area of Science:

Neuroscience
Cellular Electrophysiology

Background:

Demyelination reduces the safety factor for nerve impulse conduction.
Impulse trains can easily block conduction in demyelinated nerve fibers.

Purpose of the Study:

Investigate the mechanisms underlying conduction block in demyelinated nerve fibers during impulse trains.
Clarify the role of membrane potential changes and ion concentrations in impulse conduction failure.

Main Methods:

In vivo and in vitro electrophysiological recordings from rat ventral root fibers.
Experimental manipulation of extracellular ion concentrations (NaCl replaced by LiCl).
Application of a 'threshold clamp' using d.c. polarizing current.

Main Results:

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Normal fibers showed hyperpolarization and increased threshold during impulse trains.

Demyelinated fibers exhibited conduction block preceded by increased threshold and internodal conduction time.

Threshold changes were linked to hyperpolarization, not extracellular potassium accumulation.

The 'threshold clamp' prevented conduction failure and threshold changes.

Conclusions:

Conduction block in demyelinated fibers during impulse trains is caused by membrane hyperpolarization.
This hyperpolarization results from electrogenic sodium pumping, not extracellular potassium accumulation.
Understanding these mechanisms is crucial for addressing neurological disorders involving demyelination.