How to understand M(t) in W2D3_Tutorial4

[XiaoqianSun0104]

Hi,

I'm still a bit of confusing about how to understand Mt(t) mentioned in here. What M(t) is modeling?
For P_i(t) I understand that it models the change of metabotropic receptor on the postsynaptic membrane triggered by presynaptic spike, but not for M(t).
Can someone explain that to me. Appreciate it a lot!