A three-dimensional finite element method (FEM) simulation of a neuron (retinal ganglion cell) interfaced to a high-density CMOS-based microelectrode array (MEA) is performed and compared to recordings. The adopted multiscale simulation approach accurately reproduces the signals recorded by the patch clamp and the MEA sensors. The simulation leads us to identify bendings of the axon initial segment of a few neurons embedded in tissue, which are reported here for the first time.