This 3D reconstruction of the presynaptic terminal show the nuts and bolts of intercellular communication in the nervous system. They were generated by Siksou et al, from serial electron micrographs of neurons from the rat hippocampus.
The blue spheres are synaptic vesicles containing neurotransmitter molecules. They are docked at the presynaptic membrane (white). The arrival of an action potential (or nervous impulse) at the nerve terminal leads to an influx of calcium ions, which causes the vesicles to fuse with the membrane and release their contents into the synapse.
The gold spheres are also synaptic vesicles, which are tethered by protein filament (pink) to the membrane. These are in place to dock at the membrane once the vesicles that are already docked have been used.
This process takes place in all neurons, although we now know that some neurons in the autonomic nervous system can release neurotransmitters without generating action potentials.
(Via Pure Pedantry)