We have found a fantastic resource, part of the IBM Blue Brain Project, that clearly and interactively maps out interactions between neocortical neurons. The data comes from their attempts to simulate a piece of cortex down to the level of biologically-realistic neurons.
Interactive neocortex browser tool here:
https://bbpnmc.epfl.ch/nmc-portal/web/guest/microcircuit
The paper containing the original research on which the website is based is here:
Thanks to Yuwei Cui who posted the link to the Nupic Theory Mailing list.