Anatomy of Neurons. A. Two connected neurons. Neurons have a soma that contains a nucleus, an axon, and a dendritic tree. A single synapse (red circle) is formed at the point where an axon's neuron (black) connects to another neuron's dendrite, soma, or axon (blue). B. A magnified view of a single synapse. On the arrival of an action potential at the presynaptic terminal, calcium triggers the release of neurotransmitters from the synaptic vesicles into the synaptic cleft. Neurotransmitters diffuse across the synaptic cleft to activate postsynaptic receptors. Contributed by K Aubrey, MD