has part has part has_part postsynaptic density asymmetric synapse A type of synapse occurring between an axon and a dendritic spine or dendritic shaft. Asymmetric synapses, the most abundant synapse type in the central nervous system, involve axons that contain predominantly spherical vesicles and contain a thickened postsynaptic density. Most or all synapses of this type are excitatory. GO:0032279 Gray's type I synapse cellular_component neuron to neuron synapse