Modulation of conduction at points of axonal bifurcation by applied electric fields

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Abstract
This study investigated how weak electric fields, on the order of 100 mV/cm, modulate action potential conduction through points of axonal bifurcation in leech touch sensory neurons. Axonal branch points in neurons are ubiquitous structures, and they are sites of low safety-factor for action potential propagation. In this study calibrated electric fields were applied around excised ganglia from the leech central nervous system. The electric fields were generated by 500 ms constant current square waves applied to the bath containing the tissue. Microelectrode penetration of the neurons was used to: 1) record transmembrane potential changes in the cell body of the neuron that resulted from the external field; 2) monitor conduction block when action potentials, evoked in the periphery, propagated into the ganglion; 3) inject current directly into the cell in an experimental analysis of the mechanism by which the externally applied field produced block. Conduction block was reliably induced by electric fields too weak to reach threshold for firing action potentials. In an experimental analysis where block was produced by the direct intracellular injection of negative current, a reversed polarity field relieved it. This indicates that when the external field induces block, it does so by membrane hyperpolarization at the branch point.

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