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 channel that allows the flow of calcium ions into neurons when activated by the neurotransmitter glutamate and a co-agonist, usually glycine or D-serine. This receptor is unique because it requires both binding of glutamate and a change in voltage across the cell membrane to open. This dual requirement makes the NMDA receptor important for processes like long-term potentiation, which strengthens synapses and enhances communication between neurons.