if you mean upregulatye in the terms of cns transmitting faster and more effeciently, then yes you can alter this mechanism. But this takes a certain kind of training.
if you mean upregulate and make the CNS system more suceptable to stopping output or increasing the sensitivety of the golgi appartatus then yes this is possible as weel, Training to failure all the time will lead to this...
try to be a little more clear, and I think more people will respond