Neural plasticity.
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   Activity in neurons can strengthen or weaken signaling at a synapse.
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<Li> High  activity  of  neuron  N[1] leads to recruitment of more axon
terminals  from the same neuron. Lack of activity at N[2] leads to
loss of synapses.
<Li> If two synapses are often active at the same time, the strength of
the  postsynaptic response may increase at both synapses, mediated
by long-term potentiation.