Axon voltage-clamp simulations. A multicellular preparation.

In this paper we extend the simulation of the voltage clamp of a single nerve fiber to a bundle of axons. These simulations included not only the description of the voltage clamp circuit and a single unidimensional cable to represent the preparation in the "node" region of a double sucrose gap used previously but also a series resistance and a shunt pathway. The output of the voltage control amplifier is applied across the membrane plus the series resistance, producing a voltage drop across the series resistance due to the current generated by the membrane in response to a depolarizing voltage step. Since the membrane current has an inward and an outward phase, voltage drops of opposite sign are produced across the series resistance. During the transient current and at all points along an axon, the potential deviation produced by the series resistance is opposite to the deviation produced by the longitudinal gradient. Only at a command potential equal to the sodium equilibrium potential, the membrane potential transiently matches the command potential. For the attempted voltage clamp of an axon, values of series resistance larger than 50 omega-cm2 allowed propagated action potentials in the membrane. In spite of the presence of propagated action potentials at the calbe membrane, the recorded current does not show "notches" and it has a phase of inward current and a phase of outward current. It is concluded that, in a multicellular preparation with series resistance, the recording of a square voltage pulse does not indicate voltage control of the transmembrane potential. The presence of a shunt pathway produces inaccurate values of current density. Neither series or shunt resistance produce "notches" in the current records.