The resistance in series with the membrane capacitance in the giant axon of the squid Loligo pealei was measured using potentiometric probes that exhibit absorbance changes proportional to the voltage across the plasma membrane proper. The method relies upon the fact that a voltage drop across the series resistance produces a deviation in the true transmembrane voltage from that imposed by a voltage clamp. Optical measurement of the true transmembrane potential, together with electrical measurement of the ionic current, permits the immediate determination of the series resistance by Ohm's law. An alternative method monitored the amount of electronic series resistance compensation required to force the optical signal to match the shape of the reference potential. The value of the series resistance measured in artificial seawater was 3.78 +/- 0.95 omega X cm2. The estimated value of the contribution of the Schwann cell layer to the series resistance was 2.57 +/- 0.89 omega X cm2.

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