Starburst cells are radially symmetric interneurons with synaptic outputs and inputs coexisting on neuronal processes, called dendrites. Based on their position in the retina, starburst amacrine cells have been hypothesized to activate directionally selective ganglion cells. Genetic experiments have supported this theory, but how the cells calculate direction was unknown.
Now, the power of these cells to use Ca2+ to report direction is shown. Euler et al. found that, unlike membrane voltage in the soma, dendritic Ca2+ signals are directionally selective. Larger Ca2+ increases were evoked by light moving outward from the cell soma to the end of the dendrites, where synaptic outputs are clustered, than were evoked by the same stimulus moving toward the soma. Thus, a starburst dendrite acts as an independent module to compute stimulus direction. How different Ca2+ responses are generated based on direction is unclear. Possibly, the specific dendrite morphology promotes temporal summation of the signal when the stimulus moves toward the periphery, but not in the reverse direction. ▪