Physiology Of Vision Dr.Spandana Charles MD © 2013 Pearson Education, Inc.

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 © 2013 Pearson Education, Inc.

Light absorption (percent of maximum) Figure 15.10 The electromagnetic spectrum and photoreceptor sensitivities. (109 nm =) 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m Gamma rays X rays UV Infrared Micro- waves Radio waves Visible light Blue cones (420 nm) Green cones (530 nm) Red cones (560 nm) Rods (500 nm) 100 Light absorption (percent of maximum) 50 400 450 500 550 600 650 700 Wavelength (nm) © 2013 Pearson Education, Inc.

Light And Optics: Refraction And Lenses 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 Curved lens can refract light © 2013 Pearson Education, Inc.

Figure 15.11 Refraction. © 2013 Pearson Education, Inc.

Concave lenses diverge light Refraction and Lenses Light passing through convex lens (as in eye) is bent so that rays converge at focal point Image formed at focal point is upside-down and reversed right to left Concave lenses diverge light Prevent light from focusing © 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 three times along pathway Entering cornea Entering lens Leaving lens Majority of refractory power in cornea Change in lens curvature allows for fine focusing © 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.

Sympathetic activation Figure 15.13a Focusing for distant and close vision. 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. © 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.

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.

Emmetropic eye (normal) Figure 15.14 Problems of refraction. (1 of 3) Emmetropic eye (normal) Focal plane Focal point is on retina. © 2013 Pearson Education, Inc.

Myopic eye (nearsighted) Figure 15.14 Problems of refraction. (2 of 3) Myopic eye (nearsighted) Eyeball too long Uncorrected Focal point is in front of retina. Concave lens moves focal point further back. Corrected © 2013 Pearson Education, Inc.

Hyperopic eye (farsighted) Figure 15.14 Problems of refraction. (3 of 3) Hyperopic eye (farsighted) Eyeball too short Uncorrected Focal point is behind retina. Convex lens moves focal point forward. Corrected © 2013 Pearson Education, Inc.

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.

Process of bipolar cell Figure 15.15a Photoreceptors of the retina. Process of bipolar cell Light Light Light Synaptic terminals Inner fibers Rod cell body Rod cell body Nuclei Cone cell body Mitochondria Outer fiber Connecting cilia segment Inner Apical microvillus Outer segment Discs containing visual pigments Pigmented layer Discs being phagocytized The outer segments of rods and cones are embedded in the pigmented layer of the retina. Melanin granules Pigment cell nucleus Basal lamina (border with choroid) © 2013 Pearson Education, Inc.

Functional characteristics 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.

Functional characteristics 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 Retinal isomers: 11-cis-retinal (bent form) and all-trans-retinal (straight form) Bent form  straight form when pigment absorbs light Conversion of bent to straight initiates reactions  electrical impulses along optic nerve © 2013 Pearson Education, Inc.

Phototransduction: Capturing Light Deep purple pigment of rods–rhodopsin 11-cis-retinal + opsin  rhodopsin Three steps of rhodopsin formation and breakdown Pigment synthesis Pigment bleaching Pigment regeneration © 2013 Pearson Education, Inc.

Rhodopsin, the visual pigment in rods, Figure 15.15b Photoreceptors of the retina. Rod discs Visual pigment consists of • Retinal • Opsin Rhodopsin, the visual pigment in rods, is embedded in the membrane that forms discs in the outer segment. © 2013 Pearson Education, Inc.

Phototransduction: Capturing Light Pigment synthesis Rhodopsin forms and accumulates in dark Pigment bleaching When rhodopsin absorbs light, retinal changes to all-trans isomer Retinal and opsin separate (rhodopsin breakdown) Pigment regeneration All-trans retinal converted to 11-cis isomer Rhodopsin regenerated in outer segments © 2013 Pearson Education, Inc.

Phosphodiesterase (PDE) Figure 15.17 Events of phototransduction. Slide 1 Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. 1 Light (1st messenger) Receptor G protein Enzyme 2nd messenger Retinal absorbs light and changes shape. Visual pigment activates. Phosphodiesterase (PDE) Visual pigment All-trans-retinal Light cGMP-gated cation channel open in dark cGMP-gated cation channel closed in light 11-cis-retinal Transducin (a G protein) Visual pigment activates transducin (G protein). 2 Transducin activates phosphodiesterase (PDE). 3 PDE converts cGMP into GMP, causing cGMP levels to fall. 4 As cGMP levels fall, cGMP-gated cation channels close, resulting in hyperpolarization. 5 © 2013 Pearson Education, Inc.

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.

released continuously. Figure 15.18 Signal transmission in the retina (1 of 2). Slide 1 In the dark cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. 1 Na+ Ca2+ Voltage-gated Ca2+ channels open in synaptic terminals. 2 Photoreceptor cell (rod) −40 mV −40 mV Neurotransmitter is released continuously. 3 Ca2+ Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 4 Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release. Bipolar Cell 5 No EPSPs occur in ganglion cell. 6 Ganglion cell No action potentials occur along the optic nerve. 7 © 2013 Pearson Education, Inc.

Below, we look at a tiny column of retina. Figure 15.18 Signal transmission in the retina. (2 of 2). Slide 1 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 Light cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. 1 Light Photoreceptor cell (rod) Voltage-gated Ca2+ channels close in synaptic terminals. 2 −70 mV −70 mV No neurotransmitter is released. 3 Lack of IPSPs in bipolar cell results in depolarization. 4 Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released. 5 Bipolar Cell Ca2+ EPSPs occur in ganglion cell. 6 Action potentials propagate along the optic nerve. 7 Ganglion cell © 2013 Pearson Education, Inc.

Move from darkness into bright light 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 © 2013 Pearson Education, Inc.

Move from bright light into darkness 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 © 2013 Pearson Education, Inc.

Night Blindness Nyctalopia Rod degeneration Commonly caused by vitamin A deficiency If administered early vitamin A supplements restore function Also caused by retinitis pigmentosa Degenerative retinal diseases that destroy rods © 2013 Pearson Education, Inc.

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 © 2013 Pearson Education, Inc.

Fibers from thalamic neurons form optic radiation 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) © 2013 Pearson Education, Inc.

Figure 15.19 Visual pathway to the brain and visual fields, inferior view. Both eyes Fixation point Right eye only Left eye only Right eye Left eye Optic nerve Supra- chiasmatic nucleus Pretectal nucleus Optic chiasma Optic tract Lateral geniculate nucleus Superior colliculus (sectioned) Uncrossed (ipsilateral) fiber Crossed (contralateral) fiber Lateral geniculate nucleus of thalamus Optic radiation Superior colliculus Occipital lobe (primary visual cortex) Corpus callosum 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. Photograph of human brain, with the right side dissected to reveal internal structures. © 2013 Pearson Education, Inc.

Both eyes view same image from slightly different angles Depth Perception Both eyes view same image from slightly different angles Depth perception (three-dimensional vision) results from cortical fusion of slightly different images Requires input from both eyes © 2013 Pearson Education, Inc.

Lateral geniculate nuclei of thalamus Visual Processing Lateral geniculate nuclei of thalamus Process for depth perception, cone input emphasized, contrast sharpened Primary visual cortex Neurons respond to dark and bright edges, and object orientation Provide form, color, motion inputs to visual association areas (prestriate cortices) © 2013 Pearson Education, Inc.