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EYES! l.m. of embryonic eye Vitreous humour forming Choroid (pigmented) layer forming conjunctiva cornea Retina forming lens Iris forming Embryonic eye development Spherical lens retina cornea External anatomy of the eye Pigmented iris – circular and longitudin al muscles sclerotic Pupil – diameter controlled by iris muscles Curved transparent cornea – responsible for refraction of light Figure 6.2 Cross section of the vertebrate eye Note how an object in the visual field produces an inverted image on the retina. Label the following: j i h a b c d g e f Figure 6.18 An illustration of lateral inhibition Do you see dark diamonds at the “crossroads”? • Dark & Light Adaptation • Adaptation - process by which the eye becomes • more or less sensitive to light Cones and Colour Colour Vision Do objects possess colour? Is a lemon “yellow”? NO! Light has no colour Is a chili pepper “red”? Trichromatic Theory of Colour Vision Human eye has 3 types of cone receptors sensitive to different wavelengths of light. Short Helmholtz 1852 Medium Long People see colours because the eye does its own “colour mixing” by varying ratio of cone neural activity Bleaching • Bleaching occurs when you have looked at a red picture too long the red iodopsin has being bleached so when you look at white paper the red iodopsin is temporally out of order. Transduction Both Rods and Cones contain photopigments (chemicals that release energy when struck by light) 11-cis-retinal is transformed into all-trans-retinal in light conditions this results in hyperpolarization of the photoreceptor the normal message from the photoreceptor is inhibitory… Light inhibits the inhibitory photoreceptors and results in depolarization of bipolar and ganglion cells • Retina – Several layers of cells in inner surface of choroid – Contains photoreceptors - Rods & Cones Rods Cones More abundant Less abundant Periphery of retina Center of retina Black & White Color Poor definition High resolution Night Vision Daytime Rods & Cones: Distibution • Rod density high away from the center – The more sensitive rods (~100_rods-1_neuron map) help track peripheral image motion – ~120 million rods in retina • Cone density high near the center – The 0.3 mm dia fovea has only high density of cones (1_cone1_neuron map) helps form sharp brilliantly colored images – ~6-7 million cones in retina A rod cell (upper) and a cone cell From which direction would light come? The Photo-receptors: Rods & Cones • Cones – – – – Phototopic Chromatic Fast Foveal • Rods – – – – Scotopic Achromatic Slow Peripheral vision A rod cell Figure 6.4 Visual path within the eyeball The receptors send their messages to bipolar and horizontal cells, which in turn send messages to the amacrine and ganglion cells. The axons of the ganglion cells loop together to exit the eye at the blind spot. They form the optic nerve, which continues to the brain. Rods & Cones: Fovea & Blind Spot • Fovea a 0.3 mm spot with cone-only distribution: highest acuity and color rendition • Blind spot where optic nerve leaves the retina retina Rod cells B-P Cells Gcells LIGHT Retinal signal processing • Integrator neurons – – – – Horizontal cells Bipolar cells Amacrine cells Ganglion cells • Cones – Cone > Bipolar cell > Ganglion cell • Rods – Rod > Bipolar cell > Amacrine cell > Ganglion cell Rods & Cones • Photosensitive protein is rhodopsin, membrane protein, that modulates membrane ion conductivity via a biochemical cascade once it absorbs a photon, with the cell getting hyperpolarized as a function of light • Different amino-acid sequences in the ‘opsin’ segments of rhodopsin give the different color sensitivities of rods & cones Bipolar Cells • Many Rod cells are connected to one bipolar cell which means that when only one of the Rod cells are activated an impulse is sent to the brain. • One Cone cells is connected to one bipolar cell which means that the light needs activate each Cone cell to send an impulse. This is why the Cone cells have a higher acuity and why they cant function in the dark. Link to brain: Primary pathway • Optic nerve • Optic chiasm • Lateral geniculate body • Optic radiation • Visual cortex http://www.brother.com/usa/printer/advanced/lcv/light1.html