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Abstract

A common belief is that the dendritic tree of neurons whose average length (l) is less that its estimated electronic space constant (λ) is virtually equipotential. As a consequence, the geometry of the dendritic tree just merely reflects the spatial convergence of synaptic inputs. The above reasoning is strictly correct for a single cylindrical cable. In branched structures, even those satisfying the equivalent cylinder condition, the argument can be totally wrong.1,2 When a pulse of current is injected at one of the terminal tips, the observed voltage alternation between the tip and the soma can be orders of magnitude higher than that between the soma and the tip, when the same current is injected at the soma. Therefore, even in a small neuron, synaptic inputs can interact locally in a non-linear way, implementing elementary information processing operations.3,4 In particular, it has been recently suggested that direction selectivity of some retinal ganglion cells is due to a non-linear interaction of excitatory conductance changes and of inhibitory inputs with an equilibrium potenatial near the resting membrane potential (shunting inhibition) on the dendritic membrane of the cell.5