Balanced synapse-to-synapse short-term plasticity ensures constant transmitter release.

Synaptic strength can vary greatly between synapses. Optical quantal analysis at Drosophila glutamatergic motor neuron synapses shows that short-term plasticity also varies greatly between synapses, even those made by an individual motor neuron. Strong and weak synapses are randomly distributed in the motor neuron nerve terminal, as are facilitating and depressing synapses. Although synapses exhibit highly heterogeneous basal strength at low-action potential firing frequency and undergo varied plasticity when firing frequency increases, the overall distribution of strength across synapses remains remarkably constant due to a balance between the number of synapses that facilitate versus depress and to their degree of plasticity and basal synaptic weight. Constancy in transmitter release can ensure robustness across changing behavioral conditions.