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involving only a couple of synapses. Diffusion is fast enough that the time course of NMDA receptor activation (right) may be similar at the site of release (red) and in adjacent synapses (green). (b) More broadly distributed spillover, in which glutamate released from one synapse reaches a larger number of synapses, though at a much lower concentration. As a result, very few receptors are activated in any adjacent synapse (right). (c) Cooperation between active synapses could enhance activation of receptors at quiescent synapses in between, resulting in a larger postsynaptic response (right).