What is the contribution of single excitatory synaptic events to the representation of sensory stimuli ? In vitro preparations have provided theoretical limits on single-input coding. However, analysis of stimulus-evoked unitary synaptic activity with physiologically relevant stimuli in vivo has been hampered by compound synaptic responses and poor stimulus control. Taking advantage of cerebellar granule cells as a model system with very few synaptic inputs and a well-controlled quantifiable vestibular stimulus, Arenz et al. (p. 977) explored sensory encoding at single synapses in vivo in real time over a broad range of stimuli. Unitary, direction-sensitive synapses report motion velocity by using a frequency code that is modulated around a tonic rate. The reliability of the synaptic signal ensures that velocity is represented linearly by charge transfer. Only 100 synapses were required for realistic velocity resolution, well within the number of inputs received by many neurons in dedicated sensory processing brain regions. Single-cell computation can thus easily achieve fine-scale reconstruction of sensory stimulus features.
Alexander Arenz, R. Angus Silver, Andreas T. Schaefer, Troy W. Margrie
(Department of Neuroscience, Physiology, and Pharmacology, University College London, University Street, London WC1E 6JJ, UK)