Effect of voltage drop within the synaptic cleft on the current and voltage generated at a single synapse.

In a model of a single synapse with a circular contact zone and a single concentric zone containing receptor-gated channels, we studied the dependence of the synaptic current on the synaptic cleft width and on the relative size of the receptor zone. During synaptic excitation, the extracellular current entered the cleft and flowed into the postsynaptic cell through receptor channels distributed homogeneously over the receptor zone. The membrane potential and channel currents were smaller toward the cleft center if compared to the cleft edges. This radial gradient was due to the voltage drop produced by the synaptic current on the cleft resistance. The total synaptic current conducted by the same number of open channels was sensitive to changes in the receptor zone radius and the cleft width. We conclude that synaptic geometry may affect synaptic currents by defining the volume resistor of the cleft. The in-series connection of the resistances of the intracleft medium and the receptor channels plays the role of the synaptic voltage divider. This voltage dividing effect should be taken into account when the conductance of single channels or synaptic contacts is estimated from experimental measurements of voltage-current relationships.