An improved voltage clamp for gating current recording from the squid giant axon
This chapter deals with three aspects of cylindrical voltage-clamp designs, namely, dynamic response, noise performance, and system linearity. The clamp dynamic performance affects both the speed and accuracy of membrane potential control. Also, because the membrane potential may be treated as an electrical driving function that initiates the membrane-bound gating charge movements, the clamp dynamic characteristics influence the accuracy and intrinsic bandwidth of the recorded gating currents. A random noise, superimposed on the gating current signal at the current-to-voltage converter output, arises from two sources: intrinsic—from the preparation itself, specifically resistor (R s); and extrinsic—from the voltage clamp, recording chamber, and electrodes. Based on equivalent noise resistances, an evaluation of the clamp noise performance can then be made to identify which components contribute significantly to the overall noise. The linearity of the complete signal pathway between electrodes and data acquisition hardware is necessary for accurate registration of gating currents, particularly during their early time course. Since the currents of interest result from the subtraction of two signals that can be up to 1000 times larger, any instrumentation non-linearities will not be readily distinguished from the gating currents themselves. The results of the dynamic and noise analyses have facilitated the realization of a cylindrical axon voltage clamp having a predictable performance compatible with the requirements for high-resolution gating current measurements. These improvements to the clamp design and realization have enabled high-resolution recordings of fast intermediate, and slow, charge movements associated with voltage-dependent processes in the squid giant axon.
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