NMDAR magnesium voltage threshold

[wick]

Hi-
I'm an undergraduate studying neuroscience. My question is:

NMDA receptors have that magnesium ion blocking calcium entry even in the event of bound glutamate/glycine. I know that the shifting of the magnesium is "voltage-dependent", but what's the threshold? Is the sodium entering the spine locally via AMPARs enough to push it off? Or does the far-off axon need to depolarize? Or is it even a threshold? That is, is the amount of displaced Mg just sort of function of voltage?

If anybody could shed some light on this, it would be great.
-wick