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NPB+112+11+vision+lecture+notes - NPB 112 Winter 2011...

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1 NPB 112, Winter 2011, lecture notes Vision (Chs. 11, 12) I. The eye (Ch. 11) II. Rods & Cones III. Phototransduction IV. On- and Off-center pathways V. Center-surround organization A. Center-surround antagonism and lateral inhibition B. m- and p-type retinal ganglion cells I. The Eye A. Like a camera. Light passes through aperature (pupil), focused by lens onto back of eyeball, retina. Image inverted. Usually focused on FOVEA, area for high- acuity vision. Fig. 11.1 B. Layer of cells in back called RETINA, consists of photoreceptors (rods, cones) and neurons to which they connect. Figs. 11.5, 11.13b 1. Light passes through several layers of neurons before striking photoreceptors. 2. Distorted image on retina. 3. Brain must sharpen up the image. C. Pupillary light reflex (through midbrain to control pupil diameter). Animation : http://www.sinauer.com/neuroscience4e/animations11.1.html II. Rods & Cones A. Photoreceptors are rods and cones. Fig 11.5 B. RODS: night (scotopic) vision; respond to one or a few photons of light. 1. distributed over retina EXCEPT fovea- no rods C. CONES: daytime (photopic) color vision, high acuity (reading). 1. Discs in outer segment contains photopigments (opsins). Inner segment forms synapse. 2. Releases neurotransmitter (glutamate) when depolarized. 3. fovea: densely packed with cones exclusively 4. 3 main classes of cone: blue (short wavelength sensitivity), green (medium wavelength sensitivity) and red (long wavelength sensitivity)
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2 5. Light travels through many cell layers in retina before striking cones III. Phototransduction: Fig. 11.9. A. Rhodopsin is large protein in disc, 11-cis-retinal embedded in it (FIG 11.9 A) B. When photon of light absorbed by 11-cis-retinal, isomerizes to all-trans form: All-trans retinal splits from rhodopsin. C. Rhodopsin undergoes conformational change to activate G-protein (transducin) in disc membrane 1. Transducin composed of alpha, beta and gamma subunits. 2. alpha subunit binds GTP and dissociates from beta, gamma subunits. D. GTP-activated alpha subunit activates cGMP phosphodiesterase, breaks cytoplasmic cGMP down into 5' GMP. 1. REDUCE concentration of intracellular cGMP. 2. Reduced cGMP cause hyperpolarizes membrane potential. Animation : http://www.sinauer.com/neuroscience4e/animations11.2.html E. DARK CURRENT: Fig. 11.8 1. Photoreceptors DEPOLARIZED in dark; membrane potential -40 mV. a. Depolarization opens voltage-sensitive Ca++ channels in inner segment. b. Neurotransmitter continually released in dark. 2. In dark, high concentration of intracellular cGMP. 3. cGMP binds special Na+ channel keeping it open in dark 4. Na+ influx through channel depolarizes cell. 5. In dark, continual Na+ influx depolarizes outer segment. Membrane pump in inner segment extrudes Na+. Sets up current flow, called DARK CURRENT. 6. Light REDUCES intracellular cGMP (Fig. 11.9) a. cGMP-gated Na+ channels close b. Na+ influx stops c. cell HYPERPOLARIZES.
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