Dendritic Computation of Direction in Retinal Neurons
The retina utilizes a variety of dendritic mechanisms to compute direction from image motion. The computation is accomplished by starburst amacrine cells (SBACs) which are GABAergic neurons presynaptic to direction-selective ganglion cells (DSGCs). SBACs are symmetric neurons with several branched dendrites radiating out from the soma. Larger EPSPs are produced in the dendritic tips of SBACs as a stimulus sequentially activates inputs from the base of each dendrite outwards. The directional difference in EPSP amplitude is further amplified near the dendritic tips by voltage-gated channels to produce directional release of GABA. Reciprocal inhibition between adjacent SBACs may also amplify directional release. Directional signals in the independent SBAC branches are preserved because each dendrite makes selective contacts only with DSGCs of the appropriate preferred-direction. Directional signals are further enhanced within the dendritic arbor of the DSGC, which essentially comprises an array of distinct dendritic compartments. Each of these dendritic compartments locally sum excitatory and inhibitory inputs, amplifies them with voltage-gated channels, and generates spikes that propagate to the axon via the soma. Overall, the computation of direction in the retina is performed by several local dendritic mechanisms both presynaptic and postsynaptic, with the result that directional responses are robust over a broad range of stimuli.
KeywordsPrefer Direction Bipolar Cell Directional Signal Dendritic Arbor Reciprocal Inhibition
Thanks to David Vaney for the image of the starburst amacrine cell in Fig. 13.1a. This study was supported by NEI grant EY022070.
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