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It is well known that postsynaptic calcium signal plays a central role in the induction of synaptic plasticity such as long-term potentiation(LTP)but less known why individual synapses perform differently responding to induction of synaptic plasticity.By imaging fluorescent signals of GCaMP6,a highly sensitive calcium indicator,we investigated transient calcium events at individual dendritic spines of cultured hippocampal neurons.We observed a broad distribution of spontaneous synaptic calcium transients occurring in dendritic spines in magnesium free extracellular solution with tetrodotoxin.These calcium events depend on the activation of single or few NMDA receptors by glutamate spontaneously released from either presynaptic terminals or glial cells.During the process of chemically induced long-term potentiation,only spines with calcium events exhibit significant enlargement in spine size,and for many spines,the enlargement happened right after the occurrences of synaptic calcium events within one minutes.These results provide a signaling mechanism underlying the heterogeneous behaviors of individual synapses during the process of plasticity.