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), which is transported to the presynaptic membrane by synaptic vesicles (round). The vesicles bind with calcium channels on the presynaptic membrane (upper orange line), triggering the release of the neurotransmitter into the synaptic cleft. Glutamate binds to NMDA (N-methyl-D-aspartate) receptors on the postsynaptic membrane (lower orange line) allowing calcium ions (yellow) to pass through it, changing the electrical charge within the cell and starting an action potential. For a labelled version of this image see F032/1155.