 The hippocampus is a particularly favorable site for studying LTP because of its densely packed and sharply defined layers of neurons, but similar types of activity-dependent synaptic change have also been observed in many other brain areas.The best-studied form of LTP has been seen in CA1 of the hippocampus and occurs at synapses that terminate on dendritic spines and use the neurotransmitter glutamate.The synaptic changes depend on a special type of glutamate receptor, the "N"-methyl-D-aspartate (NMDA) receptor, a cell surface receptor which has the special property of allowing calcium to enter the postsynaptic spine only when presynaptic activation and postsynaptic depolarization occur at the same time.