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Replicating Physiological Patterns of Activity with Prosthetic Stimulation

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Annual rept. 11 Jun 2008-10 Jun 2009

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We want to develop more effective methods of neural stimulation in order to improve the clinical outcomes associated with retinal prosthetics. To accomplish this, we are investigating the mechanisms by which different types of retinal neurons respond to electric stimulation. Previous studies have shown that ganglion cells, the output cells of the retina, can be activated directly and exclusively with short duration stimulus pulses1-4. However, the site of spike initiation in ganglion cells e.g. the element with the lowest threshold is not known. Here, we found that the lowest thresholds occurred along the proximal axon, about 40 m from the soma this region of low threshold was spatially coextensive with a band of dense sodium channels also centered about 40 m from the soma. The sodium channel bands formed a homogeneous population for a given type of ganglion cell e.g. alpha, but the properties of the band were different across different types e.g. the lengths and locations varied. As expected from the differences in band properties, the size and location of the low threshold regions were also different for different ganglion cell types. A computational model reveals that the band length and the density of sodium channels in the band have the strongest influence on the response to electric stimulation. Most other neuronal properties, e.g., soma size, dendritic field extent, etc. have only a modest effect on threshold. Temporally, most types respond to stimulation frequencies up to 350 Hz, however some respond to stimulation at frequencies 500 Hz.

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  • Anatomy and Physiology
  • Medicine and Medical Research

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