Laminar profile of resistivity in frog retina

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.