## Mg nonlinearity
### Overview
NMDA receptor has voltage-dependent Mg block: at hyper-polarised potentials, magnesium is blocking the channel; depolarisation pushes Mg out of the pore, allowing the current to flow.
In general:
$$m(V) = \frac{\alpha_{\mathrm{scale}}}{1 + \alpha_{\mathrm{slope}}\exp(-\alpha_{\mathrm{sens}}\cdot V)}$$
Default implementation in SPM only considers $\alpha_{\mathrm{sens}}$ as a free parameter.