Recording of Intracellular Electrical Activity with the Voltage-Clamp Technique with Double Sucrose Gap
In most nerve fibers, when depolarization of the membrane once reaches the critical firing level, the action potential is produced in an explosive manner. According to the ionic theory, this is due to dependence of the Na conductance on the membrane potential; i.e., the Na conductance is increased by depolarization allowing inward Na current and this inward movement of positive ions in turn depolarizes the membrane further, the process thus being regenerative. Therefore, in order to fully analyze the relationship between the membrane potential and ionic conductances, it is necessary to prevent the membrane potential from moving and to clamp it at constant selected values with a special device. This “voltage-clamp” method provides another advantage. Since the membrane has resistive and capacitive components in parallel, the current flowing across the membrane is divided into resistive (ionic) current and capacitive current whenever changes in the membrane potential occur. However, under the ideal voltage-clamp condition, the capacitive current can be eliminated, except in a transient phase of the potential step since this current is proportional to the rate of potential change, and thus measurements of ionic current become possible.
KeywordsMembrane Potential Voltage Clamp Membrane Conductance Giant Axon Capacitive Current
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