PSDs were originally identified by electron microscopy as an electron-dense region at the membrane of a postsynaptic neuron.

[1] PSDs vary in size and composition among brain regions, and have been studied in great detail at glutamatergic synapses.

The structure and composition of the PSD have been the focus of numerous molecular studies of synaptic plasticity, a cellular model of learning and memory.

Current estimates are greater than several hundred proteins are found at PSDs among brain regions and during different states of development and synaptic activity.

PSDs also contain cell adhesion molecules and a diverse set of other signaling proteins.