MAPT and Alzheimer disease: Figure 2A–C show a normal state of the NMDA receptor, where the open channel (to which MK-801 can bind) allows calcium ion to flow through. The availability of open ion channels may be increased by the agonist glutamate (which does not cross the BBB) or in pathology (Figure 2D–F show an AD brain where the presence of Aβ plaques and Tau may alter the functional status of the ion channel).