Potential expression mechanisms for LTP. A. LTP could involve presynaptic mechanims due to an increase in p(r), number of release sites (n) or cleft glutamate (glu) concentration. B. Postsynaptic mechanisms could involve a modification of AMPA receptor properties, such as an increase in their mean open-time, probability of opening on binding glutamate p(o), or an increase in their single channel conductance (γ). C. Postsynaptic mechanisms could also involve an increase in the number of AMPA receptors at synapses. D. Synaptic growth is likely to involve both pre and postsynaptic changes.