Metaplasticity....

[Nyquist]

Hi there, 

I'm having some difficulty understanding the concept of "metaplasticity."
While I have been attempting to read the literature, the material there is 
something that I must be missing.  Is metaplasticity an event that happens 
subsequent to LTP or other modes of plasticity causing the neuron to act 
function differently if subjected to the same stimulus?

Also, if someone could explain why exactly after inducing LTP can LTD be 
induced?  

Thanks for your attention, 
-Nyquist