Abstract
In the wake of cortical spreading depression (CSD) it has been suggested that noxious substances diffuse through the dura with resulting firing of epidural nerves. In my view this is unlikely because there are good reasons to suggest that there must be a dura-brain barrier. Alternatively collateral branches from the trigeminal nerve to the pia and the dura may signal what is happening with ions and substances on the brain surface during CSD to the epidural space.