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SENSITIVITY AND DYNAMIC RANGE OF ROD PATHWAYS IN THE
MAMMALIAN RETINA
by
Arvin Cyrus Arman
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(NEUROSCIENCE)
August 2010
Copyright 2010 Arvin Cyrus Arman
Object Description
| Title | Sensitivity and dynamic range of rod pathways in the mammalian retina |
| Author | Arman, Arvin Cyrus |
| Author email | aarman@usc.edu; cyrus@cma-consultants.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Neuroscience |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2010-05-28 |
| Date submitted | 2010 |
| Restricted until | Unrestricted |
| Date published | 2010-08-03 |
| Advisor (committee chair) | Sampath, Alapakkam P. |
| Advisor (committee member) |
Gryzwacz, Norberto Hirsch, Judith Tjan, Bosco Weiland, James |
| Abstract | Nearly all sensory systems must find a way to represent a wide range of input signals and translate them into meaningful neural responses. The human visual system is able to operate effectively from starlight to bright sunlight, a range that spans about twelve orders of magnitude of light intensity. To reliably transmit changes in light stimuli over this range the mammalian retina has evolved several specializations to report changes in the light environment that include: 1) the evolution of two photoreceptor types, the rods and cones, that operate at different light levels, 2) several neural pathways with which to encode the output of these photoreceptors, and 3) adaptive mechanisms at all levels of retinal processing to modulate light sensitivity based on light history. To address questions regarding the sensitivity and the operating range of rod pathways in the mammalian retina, I will present data from two projects designed to investigate how rod-generated signals traverse the retinal circuitry under various lighting conditions.; In the first study titled “OFF cone bipolar cells do not contribute to OFF signals near visual threshold”, I examined how signals traverse the retinal circuitry under fully dark-adapted conditions. Under these conditions a specialized retinal pathway conserved across mammalian species, called the Rod Bipolar Pathway, carries visual signals near absolute threshold. In this pathway rod signals converge downstream on depolarizing (AII) amacrine cells, but the route for signal flow from AII amacrine cells to OFF ganglion cells near visual threshold remains unclear; signals can be relayed directly to OFF ganglion cells, or to OFF ganglion cells through OFF cone bipolar cell synaptic terminals. I show in physiological recordings from dark-adapted retinas that glycinergic synapses between AII amacrine cells and OFF ganglion cells conveyed sensitive rod signals, whose response thresholds were elevated by strychnine. Strychnine, surprisingly, did not affect response threshold in any subtype of OFF cone bipolar cell, but produced a mild depolarization of the resting membrane potential, indicating a tonic glycinergic inhibition under dark-adapted conditions. These studies indicate that near visual threshold that rod signals are conveyed to OFF ganglion cells from AII amacrine cells, without contributions from OFF cone bipolar cells. This argues against the notion that the rod pathways must ultimately impinge on traditional cone pathways before reaching ganglion cells.; In the second study, titled ”Bleached rod photoresponses persist and traverse the retina”, alongside Dr. Kiyoharu Joshua Miyagishima, I examined the mechanism by which rods remain responsive despite the bleaching of a majority of their visual pigment. Exposure to bright bleaching light results in adaptation in a manner similar to exposures to background light that desensitize the cell. In rods, which are more sensitive, this adaptation is long-lasting since they are slow to regain their sensitivity. The loss in sensitivity can be explained by two phenomena; the first resulting from the loss in available photopigment for photon absorption, and the second resulting from residual catalytic activity of the photoproducts of bleaching. We measured the steady-state sensitivity following defined extents of pigment bleaching in the mouse retina. In single cell recordings from bleached retina we show that alternative rod pathways preserve and pool rod signals to improve overall sensitivity in the mesopic range.; The experiments described in this dissertation focus on the functional properties of rod vision by using transgenic mice where cone photoreceptors are unable to generate light activated responses (Gnat2-/-). These mice allow the study of the lower and upper limits of rod vision. Studying scotopic signaling in dark-adapted retinas from Gnat2-/- mice has implications for understanding specific forms of nyctalopia, such as: congenital stationary night blindness and retinitis pigmentosa (RP), which are classified as deficiencies in seeing under dim lighting conditions. The experiments described in Chapter 2 have contributed to our understanding how visual signals generated by the rod photoreceptors traverse the retinal pathways en route to more central structures within the brain. These pathways are likely to be the therapeutic targets for nyctalopic diseases.; Conversely studying the functional properties of light-adapted retinas using the same animal models can give insight into hemeralopia, which is defined as any deficiency in seeing under very bright lighting conditions. Hemeralopic individuals typically present with achromatopsia and an inability to form visual signals interpretable by the nervous system under bright lighting conditions. The experiments in Chapter 3 have contributed to our understanding of how individuals with no functional cones (rod monochromats) are able functionally navigate under bright lighting conditions. |
| Keyword | neuroscience; retina; scotopic; photoreceptors; circuitry |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m3259 |
| Rights | Arman, Arvin Cyrus |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | http://www.usc.edu/isd/libraries/services/ask_a_librarian/email/ |
| Filename | etd-Arman-3734 |
| Archival file | uscthesesreloadpub_Volume17/etd-Arman-3734.pdf |
Description
| Title | Page 1 |
| Full text | SENSITIVITY AND DYNAMIC RANGE OF ROD PATHWAYS IN THE MAMMALIAN RETINA by Arvin Cyrus Arman A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (NEUROSCIENCE) August 2010 Copyright 2010 Arvin Cyrus Arman |
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