Synapse. Illustration of a synapse, the junction between two nerve cells. The green structures in the terminal swelling (bouton) of the pre-synaptic cell are synaptic vesicles, which contain neurotransmitters. When the nerve signal reaches the synapse the vesicles burst through the cell membrane and release neurotransmitters across the synaptic cleft, stimulating the adjacent cell. The folded pink structure is endoplasmic reticulum, studded on the right with ribosomes, where proteins are made. The grey bodies are energy- producing mitochondria, and the yellow fibres are microtubules.