Physics

1. 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. 16.30 Fig. 8-4 Ganong

2. 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. 16.33 HarperCollins A&P Laserdisc

3. Fig. 16.34 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.

4. 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. 16.35

5. Photosensitive Pigments • transmembrane protein • opsin • detachable, vitamin A derivative • retinal • In rods the protein / retinal conjugated protein is called rhodopsin (“visual purple”). Fig. 16.36

6. Rhodopsin resting 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 (regeneration) activated inactivated (bleaching) Fig. 16.37

7. 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

8. Transduction Mechanisms Fig. 8-18 Ganong Fig. 16.38

9. Amplification by a G-protein System (The entire amplification cascade lasts about one second.) • 1 photon • 1 activated rhodopsin • 500 activated transducins (activated rhodopsin like a pinball in the membrane) • 500 activated phosphodiesterases • 105 cGMP hydrolyzed • 250 Na+ channels closed • 1 million fewer Na+ enter • 1 mv hyperpolarization Alberts, et al., Molecular Biology of the Cell

10. Adaptation • Vision functions over a wide range of light intensities due to adaptation. • more than a 1012 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

11. 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

12. Adaptation Sensitivity to light increases during time in the dark. Fig. 8-28 Ganong

13. Color Fig. 16.40 • three sets of cones with different absorption maxima • blue (420 nm) • green (531 nm) • “red” (558 nm)

14. color blindness one or two sets of cones missing Color Fig. 16.41

15. Color • 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. http://dragon.uml.edu/psych/colors1.html http://www.uni-mannheim.de/fakul/psycho/irtel/color/kodak.html

16. 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. 16.34 Visual Pathways

17. 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

18. Eye to Brain • 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 Fig. 16.43