Brighter light leads to larger hyperpolarization

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Brighter light leads to larger hyperpolarization Amplification occurs at different points in signal transduction cascade Increases sensitivity (rod sensitive to 1 photon, amplification = close several channels)
Pigmented plate (flat-sheet eye) Photoreceptor layer (retina) + pigment shielding Pigmented eye cup Directionality but poor image Found in many worms Camera eye Pinhole aperture with lens to focus light Vertebrates, some cephalopods Compound eye Annelids, molluscs, arthropods Ommatidium = photoreceptor unit “mosaic eye” sees ~2-3 degrees of visual field (humans – 0.02 degrees of visual field) Resolution with # of ommatidia Fig. 6.33/7.35
Fig. 7-34, Randall et al. 2002 Vesicular/Camera eye vs. the compound eye Fig. 6.33/7.35
Compound eyes Structure Ommatidium Cornea and crystalline lens 8-12 retinular cells – rhabdomeric photoreceptor cells Rhabdom = aggregated microvilli of rhabdomeres of retinal cells Fig. 6.34/7.36
Image formation Apposition eye independent ommatidia Diurnal insects Superposition eye Separation of lens from rhabdom Increases light-gathering power Decreases resolution of image Nocturnal insects Light-induced pigment migration » Night adapted: pigment is withdrawn from clear zone » Day adapted: pigment migrates to the clear zone Fig. 13.23, Hill et al. 2004
Fig. 6.35/7.37 Camera eyes Improved image-forming ability Cornea – refraction, protection Iris – peripheral filter Constricted – circular muscle – parasympathetic control Dilated – radial muscle – sympathetic control Lens – fine tuning of refraction Accommodation (focus adjustments) Move lens (aquatic) Change shape of lens (terrestrial) » ciliary muscles » presbyopia = decreased accommodation with age Fig. 6.36/7.38
Accomodation: Changes in lens position relative to retina: Polychaetes: Change volume of fluid in eye to change distance between lens and retina Most invertebrates: Alter focal length by moving lens forward or backward Changes in lens shape: Lizards, birds, mammals: alter focal length by changing shape of lens polychaete
Accomodation: Lens Shape Lens becomes more curved, bends light more to maintain image on retina Lens shape modified by contraction and relaxation of ciliary muscles Fig. 6.36/7.38 Light rays reflected off nearby object are not parallel Focal length too long
Vertebrate Retina Fig. 6.37/7.39 multiple layers Photoreceptors: Rods and cones back of retina embedded in pigment epithelium Form synapses with layer of bipolar cells Bipolar cells synapse with layer of ganglion cells Axons of ganglion cells run along surface of retina and join to form optic nerve Optic nerve exits retina at optic disk (no photoreceptors)
Organization of the Retina Photoreceptors in deepest retinal layer Light travels through bipolar cells and ganglion cells before reaching photoreceptors

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