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Figure 1: Potential modes of classical and nonclassical neuron-NG2 cell synapse transmissions. (a) Classical transmission. The neuron axon or cell body forms close contacts with NG2 cell processes or cell bodies, thereby forming specialized synaptic junctions similar to neuron-neuron synapses. A presynaptic terminal can simultaneously innervate two or more NG2 cells, or an NG2 cell process and a neuron spine (right). Neurotransmitters released from the neuronal terminal can reach the receptors of postsynaptic structures, leading to a cascade of NG2 cell activities. (b) Spillover transmission. Variations on the NG2 cell processes, located beside the neuron-neuron synapse, form an enigmatic communication structure. The neurotransmitters from the synaptic cleft diffuse outside and activate the receptors on the varicosities of NG2 cells. (c) Ectopic transmission. The variations of neuron axons and NG2 cell processes form close contacts, thereby leading to ectopic transmission. The neurotransmitters mainly come from the synaptic-like vesicles of neuron axon varicosities. (d) Diffuse transmission. Receptors on NG2 cell processes are divergent, and the neurotransmitters may have multiple sources. Neurotransmitters must diffuse from a large distance before reaching a target cell; thus, the reaction may be not as strong as that in other transmission modes.