This paper deals with a detailed analysis of the junction that develops whenever a neuron is cultured directly on the top of a planar substrate microtransducer consisting of an insulated field effect transistor ( FET). The equivalent circuit description for a patch of excitable membrane sealed to the insulating layer of a FET is provided. The patch is described according to the Hodgkin-Huxley model. Various ranges of capacitance and resistance values have been considered, under physico-chemical assumptions concerning the junction local microenvironment. Different recorded signals of neuronal activity have been simulated as a function of these values. The signals well reproduce the experimental data reported in the literature, thus suggesting the appropriateness of the analysis for the characterization of hybrid neuro-electronic systems.