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A. C. Lasker, J. F. Rizzo, A. Koizumi, S. I. Fried; The Size and Location of the Spike Initiation Zone is Variable in Rabbit Retinal Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3030.
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As part of our efforts to create a functional retinal-prosthetic our lab wants to better understand which part or parts of retinal ganglion cells are activated by electric stimulation. Previous findings from our lab indicate that the minimum threshold for action potential initiation is found in a small region adjacent to the soma; most likely this (physiological) region corresponds to the anatomical region known as the axon initial segment (IS). However, we observed substantial variability in the exact location of the physiological region raising the possibility that there is variability in the location of the anatomical IS. Here, we have investigated the size and location of the IS in retinal ganglion cells.
In the flat-mount rabbit retina, ganglion cell initial segments were located by immunohistochemically labeling sodium channels and/or AnkyrinG. To determine the relative position of an individual initial segment, the ganglion cell was filled with fluorescent dye. After immunohistochemistry was complete, cells were imaged with a confocal microscope. Morphology and/or light response was used to classify cells into known subtypes.
We analyzed the initial segments of 60 ganglion cells and found considerable variability in the length of the IS (13 - 46µm) as well as the distance between the soma and the proximal edge of the IS (0 - 39µm). We classified each ganglion cell into one of the known sub-types (G1 - G11) and found that the IS properties were consistent within a single sub-type (e.g. G7) but IS properties of individual sub-types were different from those of other sub-types (e.g. G7 vs. G4). At least one sub-type (G11/alpha cell), had a much wider distribution of IS properties. We found that the (anatomical) IS was spatially aligned with the physiological region of low threshold.
Our findings suggest that IS properties are different for different ganglion cell sub-types. This raises the question of whether the differences in IS properties contribute to the different signaling patterns (spiking) used by each sub-type. The wider range of IS properties in the G11 suggest that this sub-type may contain multiple sub-groups. The precise alignment between the IS and the physiological region of low threshold suggests that the IS is the target of electric stimulation.
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