Passive Electrical Properties of Horizontal Cells in the Retina of the Turtle, Pseudemys Scripta Elegans
Carras, Peter Lewis
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Permalink
https://hdl.handle.net/2142/70710
Description
Title
Passive Electrical Properties of Horizontal Cells in the Retina of the Turtle, Pseudemys Scripta Elegans
Author(s)
Carras, Peter Lewis
Issue Date
1983
Department of Study
Physiology and Biophysics
Discipline
Biophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biophysics, General
Abstract
A core conductor analysis was applied to the passive relaxation of the transmembrane potential in response to 20 ms pulses of current in luminosity-type and chromaticity-type horizontal cells. The time course of the relaxation was described by the sum of two exponentially decaying processes. The passive responses were analyzed in terms of a finite cable model; values for equivalent electrotonic length (L) and dendrite:soma conductance ratio ((rho)) were calculated for each cell. Luminosity-type cells could be divided into two populations on the basis of their values for L and (rho). The larger population had mean values for L and (rho) in the dark of 1.4 and 6.8, respectively; these probably represented H1 cell bodies. The smaller population had a mean value for L in the dark of 0.8 and a mean value for (rho) in excess of 20 (the largest measurable value); these probably represented H1 axon terminals. The relatively small values for L and large values for (rho) observed in all luminosity-type cells mean that a post-synaptic signal, introduced at the distal end of a dendrite, can spread passively along the dendrite, cross the soma, and enter the adjacent dendrites with little loss of amplitude. This demonstrates an integrative role for the horizontal cell dendrites that had previously been only inferred.
In both populations of luminosity-type cells illumination with constant white light led to hyperpolarization, an increase in time constants and input resistance, a corresponding decrease in (rho), but no detectable change in L. Two mechanisms are proposed to explain the constancy of L. In the few chromaticity-type cells encountered in this study short wavelength illumination hyperpolarized the cells, increased the time constants and increased input resistance; long wavelength illumination depolarized the cells, decreased the time constants and decreased input resistance. These data support the hypothesis that the depolarizing response of chromaticity-type cells is mediated by a sign-inverting connection between luminosity-type horizontal cells and short wavelength sensitive photoreceptors.
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