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Ch. 15 Special Senses: Vision Slides mostly © Marieb & Hoehn 9th ed. Other slides by WCR Light And Optics: Wavelength And Color • Light – Packets of electromagnetic radiation (energy) – Light waves have different wavelengths • Visible light – The (small) range of electromagnetic wavelengths which our eyes can detect: 400-700 nm – Different objects reflect different wavelengths, which we perceive as different colors © 2013 Pearson Education, Inc. Figure 15.10 The electromagnetic spectrum and photoreceptor sensitivities. 10–5 nm 10–3 Gamma rays 103 nm 1 nm X rays UV nm 106 Infrared (109 nm =) nm 1m 103 m Micro- Radio waves waves Light absorption (percent of maximum) Visible light Blue cones (420 nm) 100 50 0 400 © 2013 Pearson Education, Inc. Green Red Rods cones cones (500 nm) (530 nm) (560 nm) 450 500 550 600 Wavelength (nm) 650 700 Light And Optics: Refraction And Lenses Refraction • Bending of light rays – Due to change in speed when light passes from one transparent medium to another – Occurs when light meets surface of different medium at an oblique angle • Convexly-curved lens refract light to bring it to a focus • Image formed at focal point is upside-down and left-right reversed © 2013 Pearson Education, Inc. Figure 15.11 Refraction. © 2013 Pearson Education, Inc. Figure 15.12 Light is focused by a convex lens. Point sources Focal points Focusing of two points of light. The image is inverted—upside down and reversed. © 2013 Pearson Education, Inc. Focusing Light on The Retina • Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, photoreceptors • Light refracted at boundaries along pathway – Air to cornea/aqueous humor – Aqueous humor to lens – Lens to vitreous humor • Most bending happens at air-cornea boundary • Lens curvature is the “fine adjustment” © 2013 Pearson Education, Inc. 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 © 2013 Pearson Education, Inc. Figure 15.13a Focusing for distant and close vision. Nearly parallel rays from distant object Sympathetic activation 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. © 2013 Pearson Education, Inc. 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 © 2013 Pearson Education, Inc. Focusing For Close Vision • Accommodation – 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 – Accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye • Convergence – Medial rotation of eyeballs toward object being viewed © 2013 Pearson Education, Inc. Figure 15.13b Focusing for distant and close vision. Parasympathetic activation Divergent rays Inverted from close object image Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge. © 2013 Pearson Education, Inc. Problems Of Refraction • Myopia (nearsightedness) – Focal point in front of retina, e.g., eyeball too long – Corrected with a concave lens • Hyperopia (farsightedness) – Focal point behind retina, e.g., eyeball too short – Corrected with a convex lens • Astigmatism – Unequal curvatures in different parts of cornea or lens – Corrected with cylindrically ground lenses or laser procedures © 2013 Pearson Education, Inc. Figure 15.14 Problems of refraction. (1 of 3) Emmetropic eye (normal) Focal plane Focal point is on retina. © 2013 Pearson Education, Inc. Figure 15.14 Problems of refraction. (2 of 3) Myopic eye (nearsighted) Eyeball too long Uncorrected Focal point is in front of retina. Corrected © 2013 Pearson Education, Inc. Concave lens moves focal point further back. Figure 15.14 Problems of refraction. (3 of 3) Hyperopic eye (farsighted) Eyeball too short Uncorrected Focal point is behind retina. Corrected © 2013 Pearson Education, Inc. Convex lens moves focal point forward. 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 © 2013 Pearson Education, Inc. Figure 15.15a Photoreceptors of the retina. Process of bipolar cell Synaptic terminals Rod cell body Inner fibers Rod cell body Nuclei Cone cell body Mitochondria The outer segments of rods and cones are embedded in the pigmented layer of the retina. © 2013 Pearson Education, Inc. Pigmented layer Inner Outer segment segment Outer fiber Melanin granules Connecting cilia Apical microvillus Discs containing visual pigments Discs being phagocytized Pigment cell nucleus Basal lamina (border with choroid) 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 © 2013 Pearson Education, Inc. 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 © 2013 Pearson Education, Inc. 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 © 2013 Pearson Education, Inc. Table 15.1 Comparison of Rods and Cones © 2013 Pearson Education, Inc. Chemistry Of Visual Pigments Retinal • Light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments • Synthesized from vitamin A • Isomers: cis- (bent) and trans- (straight) – Absorbing a photon causes bent-to-straight (cis –totrans) shape change – Change from bent-to-straight initiates reactions electrical impulses along optic nerve Rhodopsin = cis-retinal (bent retinal) + opsin © 2013 Pearson Education, Inc. Figure 15.15b Photoreceptors of the retina. Rod discs Visual pigment consists of • Retinal • Opsin © 2013 Pearson Education, Inc. Rhodopsin, the visual pigment in rods, is embedded in the membrane that forms discs in the outer segment. Phototransduction: Capturing Light • Pigment synthesis – Rhodopsin forms and accumulates in dark • Pigment bleaching – Light absorption causes retinal to change to trans isomer – Retinal and opsin separate (rhodopsin breakdown) • Pigment regeneration – trans retinal converted to cis – Cis-retinal rejoins opsin to form rhodopsin © 2013 Pearson Education, Inc. Figure 15.16 The formation and breakdown of rhodopsin. 11-cis-retinal 2H+ 1 Pigment synthesis: Oxidation 11-cis-retinal, derived from vitamin A, is Vitamin A 11-cis-retinal Rhodopsin combined with opsin to form rhodopsin. Reduction 2H+ 3 Pigment regeneration: Enzymes slowly convert all-trans-retinal to its 11cis form in cells of the pigmented layer; requires ATP. Dark Light 2 Pigment bleaching: Light absorption by rhodopsin triggers a rapid series of steps in which retinal changes shape (11-cis to alltrans) and eventually releases from opsin. Opsin and All-transretinal O All-trans-retinal © 2013 Pearson Education, Inc. Figure 15.17 Events of phototransduction. Slide 1 Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. 2nd Light Receptor G protein Enzyme messenger (1st messenger) 1 Retinal absorbs light and changes shape. Visual pigment activates. Phosphodiesterase (PDE) Visual pigment All-trans-retinal Light cGMP-gated cation channel open in dark 11-cis-retinal Transducin (a G protein) 2 Visual pigment activates transducin (G protein). © 2013 Pearson Education, Inc. 3 Transducin activates phosphodiesteras e (PDE). 4 PDE converts cGMP into GMP, causing cGMP levels to fall. cGMP-gated cation channel closed in light 5 As cGMP levels fall, cGMP-gated cation channels close, resulting in hyperpolarization. Phototransduction In Cones • Similar as process in rods • Cones far less sensitive to light – Takes higher-intensity light to activate cones © 2013 Pearson Education, Inc. Light Transduction Reactions • Light-activated rhodopsin activates G protein transducin • Transducin activates PDE, which breaks down cyclic GMP (cGMP) • In dark, cGMP holds channels of outer segment open Na+ and Ca2+ depolarize cell • In light cGMP breaks down, channels close, cell hyperpolarizes – Hyperpolarization is signal! © 2013 Pearson Education, Inc. Information Processing In The Retina • Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs) • When light hyperpolarizes photoreceptor cells – Stop releasing inhibitory neurotransmitter glutamate – Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells – Ganglion cells generate APs transmitted in optic nerve to brain © 2013 Pearson Education, Inc. Figure 15.18 Signal transmission in the retina (1 of 2). Slide 1 In the dark 1 cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. Na+ Ca2+ 2 Voltage-gated Ca2+ channels open in synaptic terminals. Photoreceptor cell (rod) −40 mV 3 Neurotransmitter is released continuously. Ca2+ 4 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 5 Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release. Bipolar Cell 6 No EPSPs occur in ganglion cell. 7 No action potentials occur along the optic nerve. © 2013 Pearson Education, Inc. Ganglion cell Figure 15.18 Signal transmission in the retina. (2 of 2). Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light 1 cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. Light Light Photoreceptor cell (rod) −70 mV 2 Voltage-gated Ca2+ channels close in synaptic terminals. 3 No neurotransmitter is released. 4 Lack of IPSPs in bipolar cell results in depolarization. 5 Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released. Bipolar Cell Ca2+ Ganglion cell © 2013 Pearson Education, Inc. 6 EPSPs occur in ganglion cell. 7 Action potentials propagate along the optic nerve. Slide 8 Visual Pathway To The Brain • Axons of retinal ganglion cells form optic nerve • Half of the fibers (medial half) of each optic nerve cross over at optic chiasm; optic tracts exit • Most optic tract fibers go to lateral geniculate nucleus of thalamus • Fibers from thalamic (LGN) neurons form optic radiation and project to primary visual cortex in occipital lobes • Other optic tract fibers go to superior colliculi in midbrain (initiating visual reflexes) • A few ganglion cells contain melanopsin and project to other brain areas – Regulate pupil diameter, daily rhythms © 2013 Pearson Education, Inc. Figure 15.19 Visual pathway to the brain and visual fields, inferior view. Both eyes Fixation point Right eye Suprachiasmatic nucleus Pretectal nucleus Lateral geniculate nucleus of thalamus Superior colliculus Left eye Optic nerve 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. © 2013 Pearson Education, Inc. Corpus callosum Photograph of human brain, with the right side dissected to reveal internal structures. Depth Perception • Eyes see world from slightly different angles • Depth perception (three-dimensional vision) results from detection of the small differences between R & L eye images • Requires input from both eyes © 2013 Pearson Education, Inc. Visual Processing • Retinal cells – Color, brightness, edge detection (by amacrine and horizontal cells) • Lateral geniculate nuclei of thalamus – Process for depth perception, cone input emphasized, contrast sharpened • Primary visual cortex (striate cortex) – Neurons detect edges, object orientation, movement – Provide form, color, motion inputs to visual association areas (prestriate cortex) © 2013 Pearson Education, Inc. Cortical Processing • Occipital lobe centers (prestriate cortex) continues processing form, color, movement • Complex visual processing extends to other regions – "What" processing identifies objects in visual field – "Where" processing assesses spatial location of objects – Output from both passes to frontal cortex © 2013 Pearson Education, Inc.