A simple discretization of the various levels of signal at each input/output, a discretization to handle time-of-propagation (which is almost surely part of the computation just because it _can be_ and nature probably hijacks all mechanisms), and a further discretization to handle the various serum levels in the brain, which are either inputs, outputs, or probably both.

Just add a factor 2^D transistors for each original "brain transistor" and re-run your hardware. Hope field effects don't count, and cross your fingers that neurons are idempotent!

Easy! /s

Modelling an analog system in digital will always have a combinatorial curse of dimensionality. Modelling a biological system is so insanely complex I can't even begin to think about it.