The Special Senses: Part B

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 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.

Point sources Focal points 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 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 © 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.

Parasympathetic activation Figure 15.13b Focusing for distant and close vision. Parasympathetic activation Divergent rays from close object Inverted 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.

View Ciliary muscle Lens Figure 15.13c Focusing for distant and close vision. View Ciliary muscle Lens Ciliary zonule (suspensory ligament) The ciliary muscle and ciliary zonule are arranged sphincterlike around the lens. As a result, contraction loosens the ciliary zonule fibers and relaxation tightens them. © 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 © 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.

Degenerate if retina detached Destroyed by intense light 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.

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.

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 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.

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.

Retinal cells split input into channels Visual Processing Retinal cells split input into channels Color, brightness, angle, direction, speed of movement of edges (sudden changes of brightness or color) Lateral inhibition decodes "edge" information Job of amacrine and horizontal cells © 2013 Pearson Education, Inc.

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 © 2013 Pearson Education, Inc.