James P. is 23 years old. He is a graduate student at Northern Arizona University. He enjoys living in Flagstaff because it gives him the opportunity to hike in and camp in the mountains. Best of all he likes to snowboard. In the fall he began to notice it was difficult to drive at night. He thinks maybe the windshield on his Jeep is pitted and needs to be replaced. But his girlfriend Maya says it seems ok to her.One evening James and Maya were driving home from visiting friends. After stopping at an intersection James pulled forward right into the path of a Suburban barreling down the road from the right.The driver of the Suburban slammed on her brakes and swerved. She missed James’ Jeep by inches. Then she rolled down her window and let loose a stream of four letter words you just wouldn’t normally associate with a soccer mom. The kids in the Suburban were hootingwith delight.James saw a family medicine doctor who referred him to an ophthalmologist (eye doctor).The doctor noticed pigment deposits (bone spiculepattern) in the periphery of James’ retina.Abnormalities were noticed in a follow-up electroretinogram test. The doctor thinks James has a degree of night blindness as well as defective peripheral vision•How does an understanding of cell physiology help us to understand the defect in James’ vision?•How does learning about James’ vision problem help us understand cell physiology?The Typical CellThe Typical CellWhich cells in the human body are photosensitive?Photoreceptors - rods-conesA very small fraction of retinal ganglion cells - detect light/dark in a way that cues circadian rhythmCornea & tear filmAqueous humorLensCilary musclesScleraChoroidVitreous humorRetinaOptic nerveThe ophthalmologist who saw James could tell there was no apparent defect in transparency of the cornea or lens.Could there be an abnormality of the retina, or perhaps the parts of the brain responsible for vision?The visual pathway extends into the brainRetinal signals are relayed to specific parts of the brainLGN layersThe optic nerve leads from the eye to the lateral geniculate nucleus (LGN).Neuronal layers in LGN correspond to left and right eyesRodsvery sensitiveto lightBlack & white visionConesless sensitive to lightColor visionRetinal photoreceptors are the 1st stepThe retina has multiple layers.Photoreceptors detect photons. Other neurons process the signal to detect motion, edges etc..Distribution of rod and conesRods are far more numerous than cones.Rods are predominant in peripheral retinaCones are predominant in the central retinaRetina - terminologyMacula (or macular lutea) - central area of the retina capable of clearest, most distinct visionFovea -central part of macular where all the photoreceptors are conesBlind spot –optic nerve head where photoreceptors are absent“Bone spicule” appearance of peripheral retina-abnormal, pigment containing cells-not actual boneRemember ..1. The doctor noticed pigment deposits (bone spicule pattern) in the periphery of James’ retina.Remember .2. The doctor thought James’ electroretinogram was slightly unusual.Remember ..3. The doctor thinks James has a degree of night blindness as well as defective peripheral visionCould James have a photoreceptor defect?If so, what photoreceptor is likely abnormal? Rod, cone or both?RecapAll tissues of the eye are made of cells.The cells are highly specializedThe specialization is different in different tissuesThe specializations are apparent at the molecular and organelle levelFor each different tissue, the challenge of homeostasis is differentEye Specializations•Transparency•Focus•Ability to turn photons into a cell signal•MovementLearning objectives1. Know the basic structures of the eye2. Learn the transparent tissues and be able to name structures photons pass through to reach the photoreceptor3. Know rods and cones are distributed differently in the peripheral and central retina4. Know the retina connects via the optic nerve to the brain (LGN) and visual pathway leads to striate