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 rone-and a single co ncentric zone containing receptor-gated channels, we studied the dependence of the synaptic current on the synaptic cleft width and on the relative si ze of the receptor zone, During synaptic excitation, the extracellular curr ent entered the cleft and flowed into the postsynaptic cell through recepto r channels distributed homogeneously over the receptor zone. The membrane p otential and channel currents were smaller toward the deft center if compar ed to the cleft edges. This radial gradient was due to the voltage drop pro duced by the synaptic current on the cleft resistance. The total synaptic c urrent:conducted by the same number of open channels was sensitive to chang es in the receptor zone radius and the cleft width. We conclude that synapt ic geometry may affect synaptic currents by defining the volume resistor of the cleft.:The in-series connection of the resistances of the intracleft m edium and the receptor channels plays the role of the synaptic voltage divi der. This voltage dividing effect should be taken into account when the con ductance of single channels or synaptic contacts is estimated from experime ntal measurements of voltage-current relationships.