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Visual system

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Western University
Physiology 3120
Tom Stavraky

Human Physiology Wednesday, November 4, 2009 “Neuro VI” The Visual System • Cell types in the retina  Rods/cones: receptors  Bipolar cell  Ganglion cell  Horizontal cell  Acts on axon terminal of rods/cones  Amacrine cells • Functional characteristics of the rod & cone systems  Rods  Night vision  Peripheral (few in fovea)  Sensitive to faint light (single photon) • Due to higher photopigment & convergence  Cones  Day vision/bright light/colour vision  Concentrated in the fovea • Most sensitive part of the eye • Must constantly move the eye to focus objects of interest on the fovea  Greater acuity (less convergence) • Flow diagram of the steps in the retina by which light is transduced to action potentials  Dark  Sodium channels open, so inward sodium current  Depolarization of rods & cones  Release of inhibitory NT  Hyperpolarization of on-centre bipolar cells  Light  Activated pigment closes sodium channels  Hyperpolarization of rods & cones proportional to amount of light  Decrease in inhibitory transmitter  Depolarization of on-centre bipolar cells, leading to depolarization & action potentials in ganglion cells (the first cells in the pathway that generate action potentials) • Bipolar cells
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