visual_physiology_lfc (2).ppt

1 VISUAL PHYSIOLOGY Dr. Samantha Solecki, DC, MS Instructor, Biology Thinker . Learner . Motivator . Lover of Anatomy & Physiology [email protected] © 2019 Pearson Education, Inc. 2 Learning Objectives *Acquired from the Human Anatomy and Physiology Society (HAPS) with personal additions...

1 VISUAL PHYSIOLOGY Dr. Samantha Solecki, DC, MS Instructor, Biology Thinker . Learner . Motivator . Lover of Anatomy & Physiology [email protected] © 2019 Pearson Education, Inc. 2 Learning Objectives *Acquired from the Human Anatomy and Physiology Society (HAPS) with personal additions • Trace the path of light as it passes through the eye to the retina and the path of nerve • • • • • • impulses from the retina to various parts of the brain. Describe the structure of the retina and the cells that compose it. Describe how light activates the photoreceptors. Explain how the optical system of the eye creates an image on the retina. Compare and contrast the function of rods and cones in vision. Explain the process of light and dark adaptation. Relate changes in the anatomy of the eye to changes in vision. Figure 15.4a Internal structure of the eye (sagittal section). 3 Ora serrata Ciliary body Sclera Ciliary zonule (suspensory ligament) Choroid Cornea Iris Pupil Anterior pole Anterior segment (contains aqueous humor) Lens Scleral venous sinus Posterior segment (contains vitreous humor) Retina Macula lutea Fovea centralis Posterior pole Optic nerve Central artery and vein of the retina Optic disc (blind spot) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball. 4 OPTICS 5 Light And Optics: Wavelength And Color • Eyes respond to visible light • Small portion of electromagnetic spectrum • Wavelengths of 400-700 nm • Light • Packets of energy (photons or quanta) that travel in wavelike fashion at high speeds • Color of light objects reflect determines color eye perceives • When visible light passes through spectrum, it is broken up into bands of colors (rainbow) • Red wavelengths are longest and have lowest energy, and violet are shortest and have most energy • Color that eye perceives is a reflection of that wavelength • Grass is green because it absorbs all colors except green • White reflects all colors, and black absorbs all colors 6 Focusing Light on The Retina • Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, photoreceptors • Light refracted three times along pathway • Entering cornea • Entering lens • Leaving lens • Change in lens curvature allows for fine focusing • Refraction: bending of light rays • Due to change in speed of light when it passes from one transparent medium to another and path of light is at an oblique angle • Example: from liquid to air 7 Focusing for Distant and Close Vision Figure 15.13a Focusing for distant and close vision. 8 Light & Optics • Refraction and lenses Convex lenses bend light passing through it, so that rays converge at focal point • Image formed at focal point is upside-down and reversed from left to right • Concave lenses disperse light, preventing light from being focused 9 Focusing For Distant Vision • Eyes best adapted for distant vision • Far point of vision • Distance beyond which no change in lens shape needed for focusing • 20 feet for emmetropic (normal) eye • Cornea and lens focus light precisely on retina • Ciliary muscles relaxed • Lens stretched flat by tension in ciliary zonule Figure 15.13a Focusing for distant and close vision. 10 Sympathetic activation Nearly parallel rays from distant object Lens Ciliary zonule Ciliary muscle Inverted image Lens flattens for distant vision. Sympathetic input relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens. 11 Focusing For Close Vision • Light from close objects (<6 m) diverges as approaches eye • Requires eye to make active adjustments using three simultaneous processes • Accommodation of lenses • Constriction of pupils • Convergence of eyeballs 12 Focusing For Close Vision • Accommodation of the lenses • Changing lens shape to increase refraction • Near point of vision • Closest point on which the eye can focus • Presbyopia: loss of accommodation over age 50 • Constriction of the pupils • Accommodation pupillary reflex involves constriction of pupils to prevent most divergent light rays from entering eye • Mediated by parasympathetic nervous system • Convergence of the eyeballs • Medial rotation of eyeballs causes convergence of eyes toward object being viewed • Controlled by somatic motor neuron innervation on medial rectus muscles Figure 15.5 Pupil constriction and dilation, anterior view. Sympathetic + Parasympathetic + Sphincter pupillae muscle contracts: Pupil size decreases. 13 Iris (two muscles) • Sphincter pupillae • Dilator pupillae Dilator pupillae muscle contracts: Pupil size increases. Figure 15.13b Focusing for distant and close vision. Divergent rays from close object 14 Parasympathetic activation Inverted image Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge. 15 CLINICAL CORRELATES • Problems associated with refraction related to eyeball shape: • Myopia (nearsightedness) • Eyeball is too long, so focal point is in front of retina • Corrected with a concave lens • Hyperopia (farsightedness) • Eyeball is too short, so focal point is behind retina • Corrected with a convex lens • Astigmatism • Unequal curvatures in different parts of cornea or lens • Corrected with cylindrically ground lenses or laser procedures 16 Problems of Refraction Figure 15.14-1 Problems of refraction. 17 Problems of Refraction Figure 15.14-2 Problems of refraction. 18 PHOTOTRANSDUCTION 19 Inner Layer: Retina • Delicate two-layered membrane • Outer Pigmented layer • Single-cell-thick lining • Absorbs light and prevents its scattering • Phagocytize photoreceptor cell fragments • Stores vitamin A • Inner Neural layer • Transparent • Composed of three main types of neurons • Photoreceptors, bipolar cells, ganglion cells • Signals spread from photoreceptors to bipolar cells to ganglion cells • Ganglion cell axons exit eye as optic nerve Figure 15.6a Microscopic anatomy of the retina. 20 Neural layer of retina Pigmented layer of retina Choroid Pathway of light Sclera Optic disc Central artery and vein of retina Optic nerve Posterior aspect of the eyeball Figure 15.6b Microscopic anatomy of the retina. Ganglion cells Axons of ganglion cells Bipolar cells 21 Photoreceptors • Rod • Cone Amacrine cell Horizontal cell Pathway of signal output Pathway of light Pigmented layer of retina Cells of the neural layer of the retina 22 Photoreceptors • Rods • Dim light, peripheral vision receptors • More numerous, more sensitive to light than cones • No color vision or sharp images • Numbers greatest at periphery • Cones • Vision receptors for bright light • High-resolution color vision • Macula lutea exactly at posterior pole • Mostly cones • Fovea centralis • Tiny pit in center of macula with all cones; best vision 23 Functional Anatomy Of Photoreceptors • Rods and cones • Modified neurons • Receptive regions called outer segments • Contain visual pigments (photopigments) • Molecules change shape as absorb light • Inner segment of each joins cell body Figure 15.15a Photoreceptors of the retina. Synaptic terminals Rod cell body Inner fibers Rod cell body Nuclei Cone cell body Mitochondria Pigmented layer Inner Outer segment segment Outer fiber The outer segments of rods and cones are embedded in the pigmented layer of the retina. Light Lig ht Process of bipolar cell Ligh t 24 Melanin granules Connecting cilia Apical microvillus Discs containing visual pigments Discs being phagocytized Pigment cell nucleus Basal lamina (border with choroid) 25 Photoreceptor Cells • Vulnerable to damage • Degenerate if retina detached • Destroyed by intense light • Outer segment renewed every 24 hours • Tips fragment off and are phagocytized 26 Rods • Functional characteristics • Very sensitive to light • Best suited for night vision and peripheral vision • Contain single pigment • Perceived input in gray tones only • Pathways converge, causing fuzzy, indistinct images 27 Cones • Functional characteristics • Need bright light for activation (have low sensitivity) • React more quickly • Have one of three pigments for colored view • Nonconverging pathways result in detailed, high-resolution vision • Color blindness–lack of one or more cone pigments • Most common type is red-green color blindness • X-linked condition 28 Visual Pigments • Retinal: key light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments • Synthesized from vitamin A • Four opsins are rhodopsin (found in rods only), and three found in cones: green, blue, red (depending on wavelength of light they absorb) • Cone wavelengths do overlap, so same wavelength may trigger more than one cone, enabling us to see variety of hues of colors • Example: yellow light stimulates red and green cones, but if more red are triggered, we see orange 29 Visual Pigments • Retinal isomers are different 3-D forms • Retinal is in a bent form in dark, but when pigment absorbs light, it straightens out • Bent form called 11-cis-retinal • Straight form called all-trans-retinal • Conversion of bent to straight initiates reactions that lead to electrical impulses along optic nerve 30 Photoreceptors of the Retina Figure 15.15b Photoreceptors of the retina. 31 Phototransduction • Phototransduction: process by which pigment captures photon of light energy, which is converted into a graded receptor potential • Capturing light • Deep purple pigment of rods is rhodopsin • Arranged in rod’s outer segment • Three steps of rhodopsin formation and breakdown: • Pigment synthesis, pigment bleaching, and pigment regeneration • Similar process in cones, but different types of opsins and cones require more intense light 32 Phototransduction • Capturing light (cont.) • Pigment synthesis • Opsin and 11-cis retinal combine to form rhodopsin in dark • Pigment bleaching • When rhodopsin absorbs light, 11-cis isomer of retinal changes to all-trans isomer • Retinal and opsin separate (rhodopsin breakdown) • Pigment regeneration • All-trans retinal converted back to 11-cis isomer • Rhodopsin is regenerated in outer segments 33 The Formation and Breakdown of Rhodopsin Figure 15.16 The formation and breakdown of rhodopsin. 34 The Formation and Breakdown of Rhodopsin Figure 15.16 The formation and breakdown of rhodopsin. 35 The Formation and Breakdown of Rhodopsin Figure 15.16 The formation and breakdown of rhodopsin. 36 Phototransduction • Light transduction reactions • Light-activated rhodopsin activates G protein transducin • Transducin activates PDE, which breaks down cyclic GMP (cGMP) • In dark, cGMP holds cation channels of outer segment open • Na+ and Ca2+ enter and depolarize cell • In light cGMP breaks down, channels close, cell hyperpolarizes • Hyperpolarization is signal for vision! 37 Events of Phototransduction Figure 15.17 Events of phototransduction. (1 of 5) 38 Events of Phototransduction Figure 15.17 Events of phototransduction. (2 of 5) 39 Events of Phototransduction Figure 15.17 Events of phototransduction. (3 of 5) 40 Events of Phototransduction Figure 15.17 Events of phototransduction. (4 of 5) 41 Events of Phototransduction Figure 15.17 Events of phototransduction. (5 of 5) 42 Information Processing In The Retina • Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs) • When light hyperpolarizes photoreceptor cells: • Stop releasing neurotransmitter glutamate • Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells • Ganglion cells generate APs transmitted in optic nerve to brain 43 Signal Transmission in the Retina Figure 15.18-1 Signal transmission in the retina. 44 Signal Transmission in the Retina Figure 15.18-2 Signal transmission in the retina. 45 Light Adaptation • Move from darkness into bright light • Both rods and cones strongly stimulated • Pupils constrict • Large amounts of pigments broken down instantaneously, producing glare • Visual acuity improves over 5–10 minutes as: • Rod system turns off • Retinal sensitivity decreases • Cones and neurons rapidly adapt 46 Dark Adaptation • Move from bright light into darkness • Cones stop functioning in low-intensity light • Rod pigments bleached; system turned off • Rhodopsin accumulates in dark • Transducin returns to outer segments • Retinal sensitivity increases within 20–30 minutes • Pupils dilate 47 CLINICAL CORRELATES • Nyctalopia (night blindness): condition in which rod function is seriously hampered • Ability to drive safely at night is impaired • Due to rod degeneration, commonly caused by prolonged vitamin A deficiency • If administered early, vitamin A supplements restore function • Can also be caused by retinitis pigmentosa • Degenerative retinal diseases that destroy rods • Tips of rods are not replaced when they slough off 48 NEUROPROCESSING OF VISUAL INFORMATION 49 Visual Pathway To The Brain • Axons of retinal ganglion cells form optic nerve • Medial fibers of optic nerve decussate at optic chiasma • Most fibers of optic tracts continue to lateral geniculate body of thalamus • Fibers from thalamic neurons form optic radiation and project to primary visual cortex in occipital lobes 50 Visual Pathway • Fibers from thalamic neurons form optic radiation • Optic radiation fibers connect to primary visual cortex in occipital lobes • Other optic tract fibers send branches to midbrain, ending in superior colliculi (initiating visual reflexes) 51 Visual Pathway • A small subset of ganglion cells in retina contain melanopsin (circadian pigment), which projects to: • Pretectal nuclei (involved with pupillary reflexes) • Suprachiasmatic nucleus of hypothalamus, timer for daily biorhythms Figure 15.19 Visual pathway to the brain and visual fields, inferior view. 52 Both eyes onl Righ t e ye e ye t Lef only Fixation point y Right eye Suprachiasmatic nucleus Left eye Optic nerve Pretectal nucleus Lateral geniculate nucleus of thalamus Superior colliculus Optic chiasma Optic tract Lateral geniculate nucleus Superior colliculus (sectioned) Uncrossed (ipsilateral) fiber Crossed (contralateral) fiber Optic radiation Occipital lobe (primary visual cortex) The visual fields of the two eyes overlap considerably. Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma. Corpus callosum Photograph of human brain, with the right side dissected to reveal internal structures. 53 Cortical Processing • Occipital lobe centers (anterior prestriate cortices) continue processing of form, color, and movement • Complex visual processing extends to other regions • "What" processing identifies objects in visual field • Ventral temporal lobe • "Where" processing assesses spatial location of objects • Parietal cortex to postcentral gyrus • Output from both passes to frontal cortex • Directs movements 54 Depth Perception • Both eyes view same image from slightly different angles • Visual cortex fuses these slightly different images, resulting in a three-dimensional image, which leads to depth perception • Requires input from both eyes