Physics The cornea and lens refract light rays coming into the eye. The image projected onto the retina is upside down and backwards. If the focal plane for the lens/cornea is on the retina, the image will be in focus (emmetropia). Fig Fig. 8-4 Ganong

Out-of-focus Images hyperopia –farsightedness –focal plane posterior to the retina myopia –nearsightedness –focal plane anterior to retina astigmatism –no single focal plane irregularities in cornea and lens cataracts –cloudy lens Fig HarperCollins A&P Laserdisc

Retina 3 cell layers –photoreceptor cell layer rods and cones –bipolar cell layer plus interneurons (in “synaptic layers”) –horizontal cells (outer synaptic layer) –amacrine cells (inner synaptic layer) –ganglion cell layer axons of the ganglion cells form the optic nerve fibers Note: The light signal and nerve signal travel in opposite directions. Fig

Photoreceptor Structure outer segment –many membrane infoldings (cones) or discs / cisternae (rods) –photosensitive pigments are transmembrane proteins inner segment –mitochondria cell body –nucleus synaptic region Fig

Photosensitive Pigments transmembrane protein –opsin detachable, vitamin A derivative –retinal In rods the protein / retinal conjugated protein is called rhodopsin (“visual purple”). Fig

Rhodopsin 1-3.activation light received by retinal cis-retinal converted to trans-retinal activated rhodopsin: For a second or two the trans- retinal remains attached to the opsin. 4.inactivation retinal detaches rhodopsin is inactivated or “bleached” 5&6. regeneration retinal returns to cis configuration and reattaches to the opsin, restoring rhodopsin to its resting state resting activated inactivated Fig (bleaching) (regeneration)

Transduction overview –Light activates rhodopsin which results in a hyperpolarization of the cell and a decrease in the release of neurotransmitter. details –Activated rhodopsin activates transducin (a G protein) which activates a phosphodiesterase which catalyzes the breakdown of cGMP. –  [cGMP] cytosol  closure of cGMP-gated Na + channels  hyperpolarization  decreased release of neurotransmitter from photoreceptor to bipolar cells

Fig Fig Ganong Transduction Mechanisms

Amplification by a G-protein System 1 photon 1 activated rhodopsin 500 activated transducins (activated rhodopsin like a pinball in the membrane) 500 activated phosphodiesterases 10 5 cGMP hydrolyzed 250 Na + channels closed 1 million fewer Na + enter 1 mv hyperpolarization Alberts, et al., Molecular Biology of the Cell (The entire amplification cascade lasts about one second.)

Adaptation Vision functions over a wide range of light intensities due to adaptation. –more than a difference between the dimmest light detectable by rods and brightest light detectable by cones (see Fig. 8-27, Ganong) adaptation mechanisms (in increasing order of importance): –pupil diameter –neural circuitry –photoreceptor physiology

Adaptation by Photoreceptor Cells both rods and cones involved light adaptation (decreased sensitivity with exposure to light) –bleaching of photopigment –  [cGMP] cytosol dark adaptation (increased sensitivity with exposure to darkness) –recovery of photopigment –  [Ca ++ ] cytosol  activated guanylate cyclase   [cGMP] cytosol

Adaptation Fig Ganong Sensitivity to light increases during time in the dark.

Color three sets of cones with different absorption maxima –blue (420 nm) –green (531 nm) –“red” (558 nm) Fig

Color color blindness –one or two sets of cones missing Fig

color constancy –Colors are not just interpreted by wavelength, but also by context. Your eye can let you see colors that are not really there. –e.g., When you wear sunglasses, you can still distinguish colors. –e.g., The color differences between fluorescent light and incandescent light are very obvious on film; in visual perception there is very little difference because interpretation by the eye and the brain eliminates most of the differences. ColorColor

Visual Pathways photoreceptor  bipolar cell  ganglion cell (optic nerve fibers) –typically: 100 photoreceptors / optic nerve fiber (e.g. of convergence) –in fovea: 1 cone / optic nerve fiber allowing acute vision With its two neuronal pools (synaptic layers), interpretation begins in the retina. –at least 15 different neurotransmitters –horizontal cells increase contrast by lateral inhibition –amacrine cells phasic response – increase sensitivity to movement Fig

Eye to Brain optic nerve fibers –at optic chiasm medial fibers cross over to opposite side lateral fibers remain ipsilateral –as a result The left side of the brain receives information about the right half of the visual field from both eyes. The right side of the brain receives information about the left half of the visual field from both eyes. For example, cut “C” on the left optic tract prevents information from the right half of the visual field of both eyes from reaching the brain. Fig. 8-4, Ganong

Eye to Brain Fig fibers of optic nerve / optic tract –synapse in thalamus –projection to primary visual area of occipital lobe interpretation of lines / edges and movements –visual association area shapes interpreted 3D perceived –eye and head reflexes via collaterals from optic tracts to superior colliculi –Pupillary reflexes and accommodation via collaterals to pretectal nucleus