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626 Visual System - Visual System I Peripheral 1 The...

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1 V isual S ystem I : P eripheral 1. The Eyeball: Like a “camera” 2. The Retina: Five cells types 3. Receptors: Phototransduction 4. Retinal Processing: High fidelity 5. Retinal Projections to the Brain Human Neuroanatomy The Eyeball Anatomy of Eye: Optics of the lens focuses & inverts the image on the retina at the back of the eye. The intrinsic muscles (smooth) control the Pupil & Lens , and are innervated by ANS: (accommodation & luminance) The six extrinsic muscles (somatic) of each eye control conjugate sacccades , and are innervated by cranial nerves 3, 4, & 6. (perception) Vision is an active event
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2 The Eyeball The Fundus Choked Disk: ICP The Retina Five Types of Retinal Cells: Receptor > H > Bipolar > A > Ganglion Cells
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3 Receptors Rod and Cone outer segments contain: Photopigment : Rhodopsin = ( Opsin + Retinal ) Phototransduction What is Sensory“Transduction”? Where physical energy is changed to neural energy ( Δ Vm ) Phototransduction: Photoreceptors Hyper polarize to light! (Receptors are depolarized in the dark due to “dark current” [Na+ current], with steady release of glutamate . Photon > cis -trans isomerization of rhodopsin > Activates transducin > Activates phosphodiesterase > Destroys cGMP on Na+ channel > NA+ channel closes > Reduces membrane gNa + > Receptor hyperpolarizes > (less glutamate released) .
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4 Retinal Processing In the Straight Through Path: Receptors ( Hyper polarize ) > Bipolars ( De polarize ) > RGC ( Action Potential ) However-- with the addition of the Horizontal & Amacrine Cells : Contrast is Enhanced through a process called…… Lateral Inhibition Retinal Processing Lateral Inhibition Enhances contrast… of RGC “receptive fields” Example: Illusion of Mach Bands Horizontal cells suppress excitability of adjacent bipolars… & RGCs
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5 What the Eye Tells the Brain RGC output via the Optic Nerve
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