Final answer:
Excitatory neurotransmitters like glutamic acid cause depolarization of the postsynaptic neurons by activating NMDA receptors, allowing cations to enter the neuron and depolarize its membrane.
Step-by-step explanation:
Excitatory neurotransmitters like glutamic acid depolarize postsynaptic neurons by activating NMDA receptors, which are a type of ionotropic glutamate receptor (iGluR). These receptors, upon binding to glutamate, open an intrinsic ion channel that allows cations such as sodium (Na+) to flow into the neuron. The influx of these positively charged ions depolarizes the membrane potential of the postsynaptic cell. This process is crucial for normal excitatory neurotransmission in the central nervous system (CNS) and plays a significant role in synaptic plasticity.