Neural plasticity.
Activity in neurons can strengthen or weaken signaling at a synapse.
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.
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.