a cellular model of learning and memory. In mammals, PSDs are on the order of 250 to 500 nanometres in diameter and 25 to 50 nanometres in thickness, depending on the activity state of the synapse. During synaptic plasticity, the total size of the PSD is increased along with an increase in synaptic size and strength after inducing long-term potentiation at single synapses.

In invertebrates, synapses are often are polyadic,meaning they have multiple