Dendritic Processors

RESEARCH HIGHLIGHTS

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DENDRITES Dendritic processors

The expression of various voltage- light evoked both large- and small- their characteristics to the dendritic gated ion channels in neuronal amplitude dendritic spikes, but only bursts. Two-photon calcium imag- these studies dendrites allows the initiation of local the former drove action potential ing of somatic and dendritic calcium show that dendritic spikes in response to excita- generation. Thus, they reasoned transients, with simultaneous den- tory synaptic input and the back that the small-amplitude spikes dritic recordings, provided the most active propagation of action potentials. The represent terminal dendrite spikes direct evidence of local dendritic dendritic integration of active dendritic events evoked by bipolar input that, in spiking. processing is a is thought to contribute to neuronal turn, trigger large-amplitude, parent Local hyperpolarization of key component computations, although clear evi- dendritic responses. dendrites more strongly attenuated dence for this has been lacking. Now, To show that the small-amplitude the dendritic bursts than isolated of neuronal two studies show that active dendritic spikes originated in terminal den- back-propagating action potentials, computations spiking has a role in computations in drites, they selectively excited areas confirming the local origin of the in the visual the visual system. of the On-DSGC dendritic tree with bursting events. Cell-wide hyperpo- Direction-selective retinal small light spots. Stimulation of an larization attenuated subthreshold system ganglion cells (DSGCs) fire action area including parent and terminal orientation tuning responses in the potentials when visual stimuli move dendrites evoked both small- and layer 2/3 neurons, indicating that across their receptive field in a large-amplitude spikes, whereas dendritic bursting has a functional particular direction (the ‘preferred’ stimulation of terminal dendrites role in enhancing such responses. direction), but are silent when the alone elicited only small-amplitude Finally, the authors examined stimuli move in the opposing direc- spikes. Importantly, the authors the mechanisms underlying the tion (the ‘null’ direction). Sivyer and also showed that small-amplitude, dendritic bursting. They included Williams examined the contribution terminal dendritic spikes were inhib- an NMDA receptor blocker in the of active dendritic integration to ited by null-direction light stimuli, pipette solution during whole-cell direction selectivity in a subtype of indicating that the engagement and dendritic recordings from layer DSGCs — namely, On-DSGCs — by inhibitory synaptic control of active 2/3 cells, which blocked dendritic recording from the soma and par- dendritic integration underlies the spikes, and during whole-cell somatic ent dendrites (the main branches computation of direction selectivity. recordings, and found that, once the of the dendritic tree) of these cells Working in vivo, Häusser and blocker had diffused throughout the in rabbit retinal preparations that colleagues sought to determine dendritic tree, subthreshold orienta- were exposed to moving light bars. whether dendritic spikes are an tion tuning was disrupted. Thus, Somatic recordings confirmed essential component of synaptic NMDA receptor-mediated currents that On-DSGCs showed direction- integration during cortical visual are crucial for dendritic bursting, and selective responses. Interestingly, processing. They presented lightly these dendritic bursts in turn shape dendritic recordings made in the anaesthetized and awake mice with orientation-tuned output. area of the dendritic tree that was drifting square-wave gratings while Together, these studies show that first activated by light bars moving in recording from the apical dendrites active dendritic processing is a key the preferred direction showed that and somas of layer 2/3 neurons of component of neuronal computations dendritic spikes preceded somatic the mouse primary visual cortex, in the visual system, both in the retina responses, revealing that sensory which exhibit orientation-tuned and in the primary visual cortex. input-induced dendritic spikes can responses to visual stimuli. In Darran Yates drive action-potential firing. response to stimuli in the preferred ORIGINAL RESEARCH PAPERS Sivyer, B. & Light-evoked excitation of orientation, these neurons fired, Williams, S. R. Direction selectivity is computed by DSGCs is mediated by bipolar cells, and their dendrites showed high- active dendritic integration in retinal ganglion cells. Nature Neurosci. http://dx.doi.org/10.1038/ which form excitatory synapses frequency bursts of spikes. Back- nn.3565 (2013) | Smith, S. L. et al. Dendritic spikes onto DSGC terminal dendrites propagating action potentials from enhance stimulus selectivity in cortical neurons in (which branch from the parent the soma could also be detected in vivo. Nature http://dx.doi.org/10.1038/ nature12600 (2013) dendrites). The authors found that the dendrites, but these differed in

NATURE REVIEWS | NEUROSCIENCE VOLUME 14 | DECEMBER 2013