Effects of Variations in the Dense Sodium Channel Band in Retinal Ganglion Cells

Abstract

Purpose: : In response to electric stimulation of retinal ganglion cells, we have recently shown that the lowest thresholds arise when the stimulating electrode is positioned near a dense band of sodium channels located in the proximal portion of the axon. We also found that both the sodium channel bands and absolute thresholds are different for different types of ganglion cells. Taken together, these findings suggest that differences in the properties of the sodium channel band underlie the differences in thresholds. Here, we want to understand how each property of the sodium channel band influences threshold.Methodology: We developed a multi-compartment model of ganglion cells with separate Hodgkin Huxley kinetics in each compartment. The stimulating electrode (point source) was translated in 6 µm increments at a height of 25 µm above the cell; at each step, threshold was calculated using a binary search. Threshold was examined as each of five parameters was varied: sodium channel conductance (G_Na), length and location of the band, soma size, and axon thickness.

Results: : Most parameters influenced threshold. 1) Longer tapers resulted in lower stimulation thresholds. 2) Increasing the distance between the band and the soma resulted in lower thresholds; however, increases beyond 60 µm had little additional effect. 3) The absolute minimum threshold was unaffected by soma size. However, the slope of threshold increase (between the band and the soma) increased more steeply for larger somas. 4) Higher G_Na in the sodium channel band lowered threshold. 5) Within realistic ranges, axon thickness had little effect on threshold.

Keywords: electroretinography: non-clinical • retina: distal (photoreceptors, horizontal cells, bipolar cells)