Abstract
A theory is proposed that biochemical changes at the synapse that occur as a result of stimulation of specific neuronal circuits can lead to long-term changes only if alterations occur in synaptic structures in these circuits. The main synaptic structure that is thought to undergo this alteration is the postsynaptic density (PSD). There are many reports in the literature of overall structural changes at the synapse, including the PSD, resulting from various neuronal stimuli. These structural changes are here envisaged to include those of concentration and conformation of PSD proteins, changes that could alter the neural physiology of dendritic spines and even that of the presynaptic terminal.