Neurons can form synapses with just one postsynaptic target or with hundreds, and an individual neuron can make single or multiple connections with each postsynaptic cell. To add to this diversity, connections also vary in strength. These multiple variables are difficult to measure electrophysiologically. On page 1188 of this issue, Guerrero et al. report a new optical approach to compare transmission from different connections of an individual neuron in the neuromuscular junction of the fruitfly Drosophila.

The authors created an innovative genetically encoded calcium reporter called SynapCam to selectively study calcium influx through glutamate receptors (which in Drosophila are permeable to Ca2+). They were able to target SynapCam postsynaptically, and image single boutons. They report that calcium signals through glutamate receptors were uniform within a given connection but varied considerably among different connections made by a single axon at the same neuromuscular junction. Moreover, they observed a gradient of transmission strength along axonal branches, with the strongest synapses at the terminal end of an axonal branch (picture).