Transmission along and between rods in the tiger salamander retina.

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Abstract
The electrical pathways that couple the rods, and that link the outer segments of the rods to the coupled network, were evaluated. Two separate microelectrodes were inserted into the inner or outer segments of the same or neighboring rods under visual control. Current was passed through 1 electrode, and the resulting potential recorded with the other. The input resistance, measured at the inner or outer segment in a rod in the network, was strongly outward rectifying. It was typically near 40 m.OMEGA. when the membrane was hyperpolarized in 10 mV or more by extrinsic current, less than 10 M.OMEGA. when the membrane was depolarized by 5 mV or more, and near 30 M.OMEGA. at the no-current level. When current was injected into the outer segment, the response in the inner segment was nearly identical with that at the outer segment, suggesting that the resistance coupling the segments was not high relative to the input resistance of the rod in the network. Under voltage clamp the light response current for a rod in the network was of constant magnitude for potential levels between -80 and -20 mV. This suggested that there was little or no measurable light elicited conductance change associated with the response, possibly a consequence of coupling between rods. The rod response increased with increasing diameter of a concentric test flash up to about 200 .mu.m, or about 16 rod diameters. When current was injected into 1 rod, the response in its immediate neighbors was between a quarter and 1/10 that recorded in the injected rod for all potential levels in the injected rod. The membrane time constant, measured in a rod in the network, was proportional to the voltage-dependent input resistance at 0.16 ms/M.OMEGA.. With assumptions about the geometry of the rod network this represents a membrane capacitance of 1.5 .mu.F[farads]/cm2. The data could be approximated by a network model of square array. The model predicted that the outer segment contributes less than half the current for the total rod response, the membrane resistance of an individual rod is greater than twice the measured input resistance for the rod in the network, near 60 M.OMEGA., and the coupling resistance for each arm of the network is about 4 times the individual rod resistance, near 240 M.OMEGA.