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Differences in sodium conductance between end-plate and extrajunctional regions of rat lumbrical muscle fibres were measured by comparing action potential maximum rate of rise (Vmax) in the two regions and by using a vibrating micro-electrode to record steady inward current produced by application of veratridine. In normal Krebs solution, action potential Vmax was significantly greater (by 43%) in the end-plate region than in extrajunctional regions of the fibres. When chloride conductance was greatly reduced by bathing muscles in solutions with low chloride concentration, Vmax was still significantly higher (by 28%) in the end-plate region than in extrajunctional regions. The increased Vmax could be recorded only within a distance of about 150-200 microns of the end-plate. Steady inward current was recorded with a vibrating micro-electrode at the end-plate in response to veratridine; the current persisted when veratridine was introduced in low-chloride Krebs solution, and it was rapidly reversed by tetrodotoxin. The current reflected a 5 mV difference in membrane potential between the end-plate region and extrajunctional regions. The results suggest that sodium conductance is increased in the synaptic region relative to extrajunctional regions of the fibres.

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