Sodium ions enter the postsynaptic cell, resulting in a slight depolarization of that neuron.
Calcium ions enter the cell, resulting in a slight depolarization of the postsynaptic neuron.
The glutamate released from the axon terminal binds to both AMPA and NMDA receptors but only activates the NMDA receptors.
The AMPA receptors depolarize the postsynaptic neuron and drive the Mg^2+ plug out of the NMDA channel.