The NMDA receptor is a type of glutamate receptor found in the brain that plays a crucial role in synaptic plasticity, which is essential for learning and memory. It is a ligand-gated ion channel that allows the flow of calcium (Ca²⁺), sodium (Na⁺), and potassium (K⁺) ions when activated by the neurotransmitter glutamate and requires a co-agonist, typically glycine. This receptor is unique because it is voltage-dependent, meaning it only opens when the postsynaptic membrane is sufficiently depolarized. This property helps to prevent excessive neuronal activation and is important for processes like long-term potentiation (LTP), which strengthens synapses and enhances communication between neurons.