Laminar profile of resistivity in frog retina

Abstract

Measurements of absolute transretinal resistance and of the relative resistance of the various retinal layers were obtained in the frog. The resistance of clamped sections of isolated retina was 66 omega . cm2, which results in an average resistivity (rho) between inner and outer limiting membranes of 5,050 omega . cm. In eyecups, relative resistances (obtained by passing constant currents across the retina) were assigned to specific layers of the retina with the aid of physiological criteria (e.g., depths of light-evoked field potentials, changes in extracellular K+ concentration, base-line noise level, and resistance). These relative resistances were then converted to absolute values, a calculation feasible because the region between inner and outer limiting membranes, which has the same structure in both isolated and eyecup retinas, could be specified during experiments. Resistivities (in omega . cm) for the retinal layers include 1) subretinal space, 970; 2) inner and outer nuclear layers, 6,800; and 3) inner plexiform layer, 1,750. The ganglion cell and optic nerve fiber layers were too thin to resolve individually, but rho of the two layers combined was 7,900. The outer plexiform layer was also too thin to reliably resolve, but its rho is likely the same as the inner plexiform layer. The extracellular space volume fraction (alpha) of the retinal layers was estimated from these rho s, and the following values were obtained: 1) subretinal space, 0.12; 2) outer and inner nuclear layers, 0.03; 3) inner and outer plexiform layers, 0.11; and 4) ganglion cell and optic nerve fiber layers, 0.02. The decreased rho and increased alpha of the inner plexiform layer and the subretinal space, compared with that of the nuclear layers, are expected from their anatomy. A consideration of these inhomogeneities is required in analyses of field potentials and of changes in extracellular ionic concentrations.

Current source density analysis of linear and non‐linear components of the primate electroretinogram.

“…The possibility of significant tissue damage from the microelectrode deserves some consideration (Karwoski, Frambach & Proenza, 1985). For example, the microelectrode might tear an irreparable hole in the retinal pigment epithelium, destroying the 'R-membrane' after the first penetration.…”

Strength–Duration Relationship for Extracellular Neural Stimulation: Numerical and Analytical Models

“…Detailed expressions and numerical values of the parameters for this kinetic model are listed in APPENDIX C. Here, as in the previous section, a spherical cell was composed of 20 segments. The cell radius was taken as a ϭ 5 m and external resistivity e was 7,900 ⍀·cm, in accordance with measurements of the RGC layer resistivity in the literature (Karwoski et al 1985). Solving these RGC equations on a laptop PC took Յ10 min.…”

“…Figure 10 shows spike latency as a function of stimulus intensity for a spherical RGC in the retina (resistivity of RGC layer ϭ 7,900 ⍀·cm; Karwoski et al 1985) and for a spherical HH cell in saline ( ϭ 70 ⍀·cm). For the RGC (left curve, ϭ 0.1 ms) the latency first decreases rapidly as the stimulus increases above the threshold, as a result of more intense activation of the sodium channels.…”

“…These data were obtained using a large, 0.5 mm diam- eter electrode that provides uniform current flow around the cell. The HH model curve was again computed assuming the radius a ϭ 5 m and the RGC layer resistivity e ϭ 7,900 ⍀·m (Karwoski et al 1985). The corresponding cell polarization time for the HH soma stimulation model was p ϭ Ca[( i ϩ e )/2] ϭ 2 s and thus the potassium-dominated regime is absent in this case.…”

Control of Extracellular Potassium Levels by Retinal Glial Cell K + Siphoning

“…Location (in percent retinal depth) and extracellular volume fraction (in percent, indicated in parentheses) of retinal layers: endfoot, 0 to 4 (100); optic fiber, 5 to 10 (2.4); ganglion cell, 11 to 19 (2.4); inner plexiform, 20 to 40 (13.3); inner nuclear, 41 to 57 (3); outer plexiform, 58 to 61 (13.8); and outer nuclear, 62 to 70 (3 percent). Volume fraction values from (13).…”