Nerve synapse. Artwork of a nerve impulse being passed across the junction, or synapse, between two nerve cells (neurons). The electrical signal has reached the end of the first neuron (upper frame), causing its vesicles (blue spheres) to release their neurotransmitters (red) at the presynaptic membrane. These chemicals travel across the synapse and bind to the protein receptors (holes) on the postsynaptic membrane. This initiates a signal in the second neuron, relaying the message.