Isopotentiality and an optical determination of series resistance in Limulus ventral photoreceptors.

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Abstract
Photoreceptor somas in the ventral rudimentary eye of L. polyphemus were impaled with 3 micropipettes. Two micropipettes were connected in a voltage-clamp circuit and the cells were stimulated by brief flashes. The 3rd micropipette did not measure any significant deviations from the clamped voltage during responses to the flashes, in several geometries of electrode placement, even for very bright flashes. Using the described techniques there is no evidence for spatial non-uniformity of intracellular voltage in the soma of these photoreceptors. A voltage-sensitive dye was used to monitor light-induced changes in membrane voltate while intracellular voltage was held clamped by a feedback circuit. With a known series resistance connected between the bath and ground the dye recorded a light-induced change in membrane voltage. When there was no added series resistance, the light-induced change was smaller and often undetectable. The naturally occurring series resistance was calculated as .ltoreq. 30 k.OMEGA.. From these measurements and calculations for a model spherical cell, membrane potential can be controlled to within 2 mV using micropipette point clamp methods for all but the brightest stimuli.