Axon voltage-clamp simulations. III. Postsynaptic region.

This is the third in a series of four papers in which we present the numerical simulations of the application of the voltage clamp technique to excitable cells. In this paper we discuss the problem of voltage clamping a region of a cylindrical cell using microelectrodes for current injection and voltage recording. A recently developed technique (Llinás et al., 1974) of internal application of oil drops to electrically insulate a short length of the postsynaptic region of the squid giant synapse is evaluated by simulation of the voltage clamp of an excitable cylindrical cell of finite length with variable placement of the current and voltage electrodes. Our results show that ENa can be determined quite accurately with feasible oil gap lengths but that the determination of the reversal potential for the synaptic conductance, ES, can be considerably in error. The error in the determination of ES dependp, and especially the membrane resistance at the time the synaptic conductance occurs. It is shown that the application of tetraethylammonium chloride to block the active potassium conductance very significantly reduces the error in the determination of ES. In addition we discuss the effects of cable length and electrode position on the apparent amplitude and time course of the syn aptic conductance change. These results are particularly relevant to the application of the voltage clamp technique to cells with nonsomatic synapses. The method of simulation presented here provides a tool for evaluation of voltage clamp analysis of synaptic transmission for any cell with known membrane parameters and geometry.