The NMDA receptor (NMDAR) does not, in resting or near-resting membrane potential conditions, contribute significant current to the EPSP. Following the presynaptic release of the glutamate that binds to and opens the AMPAR, the NMDAR also binds this glutamate and opens. However, current does not flow through the NMDAR ion channel because it is instantaneously blocked by a magnesium ion (Mg) that binds to a site "inside" the open pore of the NMDAR channel. Magnesium has access to this binding site only when the NMDAR channel is opened by glutamate binding, a so-called open channel block.