Lecture_9_Physiology_of_Vision.pdf

Mechanism of image formation on the retina The visual process is the detection and translation of light into static or dynamic mental images. Photoreceptor cells, rods & cones, within the retina transduce visible light energy into electrical signals that ultimately...

Mechanism of image formation on the retina The visual process is the detection and translation of light into static or dynamic mental images. Photoreceptor cells, rods & cones, within the retina transduce visible light energy into electrical signals that ultimately pass to the visual cortex. The light entering the eyes is altered by changing the size of the pupil and light waves are focussed on retina by changing the shape of the lens (Accommodation) The image projected onto the retina is inverted or upside down. Visual processing in the brain reverses the image  Lens Accommodation  The eye is set for distance vision (over 20 ft away)  Light must be focused to a point on the retina for optimal vision  The lens must change shape to focus for closer objects  Shape of lens changed by ciliary muscle to make light focus on the retina  Parasympathetic ANS promotes lens accommodation for near vision Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Ciliary Body and Ciliary muscles control the shape of the Accomodation of the Lens lens  Relaxation - no accomodation  Contraction – accomodation (to see near objects) Suspensory ligaments (ciliary zonules) attach the muscles to the lens The Lens Focuses light on the retina (neural layer of the eye) Transparent and jelly-like in youth but becomes hard and opaque with age (cataract) Unaccomodated Lens The lens becomes less elastic – leading to Far vision Accomodated Lens presbyopia which makes it difficult to see Near vision nearby objects clearly Retinal layers through which light must pass through before reaching photo receptors Ganglion cell axons form the optic nerve Retinal Anatomy :Photoreceptors in retina : Rods and Cones Light must pass through several layers before reaching photo receptors  Differences between Rods and Cones Black and white vision Colour vision by Cones by rods Rods and cones Dark pigment epithelial layer absorbs stray light & reduces reflection Disks in rods & cones are the site of transduction Transduction process mediated by pigments in the disks  Phototransduction It’s the process of converting light stimuli into electrical stimuli  Receptors generally depolarize on stimulation  But Photoreceptors hyperpolarize on light absorption  Graded potential is generated in Photo receptors  Ganglion cell – Fires action potential A photo pigment consists of 2 parts 1.Opsin – an integral protein seen in the disc plasma membrane 2.Retinal (Vitamin A derivative) – Light absorbing part of the photopigment Photo pigment During Dark Photo pigment During Light 11-cis retinal all –trans retinal  Responses of Photo Pigments in Dark Synaptic terminals with calcium channels  Photo pigment response during dark Photo pigment response during light High concentration of cGMP Activation of Photo pigment (changes its structure) Na+ channels opens in outer segment Activation of Photo pigment (changes its structure) Membrane depolarization Activation of transducin (a G-protein) Opens Ca2+ channels in synaptic terminal Activates CGMP phosphodiesterase Release of inhibitory neurotransmitter Converts CGMP to 5’GMP Decrease In cGMP (Inhibition) Closure of Na+ channels in the outer segments Membrane Hyperpolarization Bipolar cells inhibited Closes Ca2+ channels in the synaptic terminals Decrease release of inhibitory neurotransmitter No action potentials in Ganglionic cells (No inhibition) Bipolar cells are excited No action potentials in Optic nerve Graded potential is generated in bipolar cells Fires an action potential in the Ganglion cells Action potentials in Optic nerve Visual pathway  Visual pathway  The axons of the ganglion cells collect together at the optic disk before passing out of the eye into the brain via the optic nerve.  The optic nerves from each eye meet at the optic chiasm and and passing into the optic tracts  At the optic chiasm, the nerve fibers associated with the nasal half of the retina from each eye cross over while the nerve fibers from the temporal retina travel down ipsilaterally.  At the end of each optic tract, the nerve fibers connect with other visual pathway nerves in the lateral geniculate nucleus (LGN) of the thalamus for primary processing of visual information.  A series of radiating nerve fibers, the optic radiation, convey the information to the visual cortex in the occipital lobe of the brain for final interpreting and processing visual information.