Excitatory synapse, illustration. A synapse is a junction between two neurons (nerve cells). An excitatory synapse increases the likelihood of an action potential firing in the postsynaptic cell. The main excitatory neurotransmitter in the central nervous system is glutamate (red, white and blue molecules). Glutamate transporters (1) move the neurotransmitter into synaptic vesicles (round), with the help of ATPase (5). Vesicle movement to the presynaptic membrane (upper orange line) is regulated by the proteins synapsin (2) and calcium/calmodulin-dependent protein kinase II (CaMKII, 6). Phosphorylation of synapsin by protein kinase A (PKA, 3 and 4) transports the vesicle to the presynaptic membrane. The vesicle binds with a calcium channel (8) on the presynaptic membrane, facilitated by SNARE proteins (7), triggering the release of the neurotransmitter into the synaptic cleft. Glutamate binds to NMDA (N-methyl-D-aspartate, 11) and AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, 12) receptors on the postsynaptic membrane (lower orange line) allowing calcium ions (yellow) to pass through them, changing the electrical charge within the cell and starting an action potential. The cell adhesion proteins neurexin (9) and neuroligin (10) connect the two neurons at the synapse. Actin filaments (13) are also seen in the postsynaptic cell. For an unlabelled version of this image see F032/1154.