1. Copyright © 2010 Pearson Education, Inc. Light Eyes respond to visible light Light: packets of energy called photons that travel wavelike Rods & cones respond to diff wavelengths of visible spectrum

2. Copyright © 2010 Pearson Education, Inc. Figure 15.10 Wavelength (nm) Visible light (b) (a) Blue cones (420 nm) Rods (500 nm) Green cones (530 nm) Red cones (560 nm) X raysUVInfrared Micro- waves Radio waves Gamma rays Light absorption (pervent of maximum)

3. Copyright © 2010 Pearson Education, Inc. Refraction and Lenses Refraction Bending light - change in speed when light passes from one transparent medium to another Passing through convex lens bent so rays converge at focal point Image formed at focal point - upside-down and reversed right to left

4. Copyright © 2010 Pearson Education, Inc. Figure 15.12 Point sources (a) Focusing of two points of light. (b) The image is inverted—upside down and reversed. Focal points

5. Copyright © 2010 Pearson Education, Inc. Focusing Light on the Retina Pathway of light entering eye: ? Light is refracted At cornea Entering lens Leaving lens Change in lens curvature allow fine focusing of image

6. Copyright © 2010 Pearson Education, Inc. Focusing for Distant Vision Light from distant objects nearly parallel, need little refraction beyond what occurs at-rest Far point: distance beyond which no change in lens shape needed for focusing; 20 ft for emmetropic (normal) eye Ciliary muscles relaxed Lens stretched flat by ciliary zonule

7. Copyright © 2010 Pearson Education, Inc. Figure 15.13a Lens Inverted image Ciliary zonule Ciliary muscle Nearly parallel rays from distant object (a) Lens is flattened for distant vision. Sympathetic input relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens. Sympathetic activation

8. Copyright © 2010 Pearson Education, Inc. Focusing for Close Vision Light from close object diverges as it approaches eye; requires that eye make active adjustments Accommodation—changing lens shape to increase refractory power Near point determined by max bulge lens can achieve Presbyopia—loss of accommodation over age 50

9. Copyright © 2010 Pearson Education, Inc. Figure 15.13b Divergent rays from close object (b) Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge. Inverted image Parasympathetic activation

10. Copyright © 2010 Pearson Education, Inc. Problems of Refraction Myopia (nearsightedness)—focal point in front of retina, e.g. longer than normal eye Corrected - concave lens Hyperopia (farsightedness)—focal point behind retina, e.g. shorter than normal eyeball Corrected with a convex lens Astigmatism—unequal curvatures in diff parts of cornea or lens

11. Copyright © 2010 Pearson Education, Inc. Figure 15.14 (1 of 3) Focal plane Focal point is on retina. Emmetropic eye (normal)

12. Copyright © 2010 Pearson Education, Inc. Figure 15.14 (2 of 3) Concave lens moves focal point further back. Eyeball too long Uncorrected Focal point is in front of retina. Corrected Myopic eye (nearsighted)

13. Copyright © 2010 Pearson Education, Inc. Figure 15.14 (3 of 3) Eyeball too short Uncorrected Focal point is behind retina. Corrected Convex lens moves focal point forward. Hyperopic eye (farsighted)

14. Copyright © 2010 Pearson Education, Inc. Functional Anatomy of Photoreceptors Rods and cones Outer segment contains visual pigments (photopigments)—molecules change shape as they absorb light Inner segment joins cell body

15. Copyright © 2010 Pearson Education, Inc. Figure 15.15a Process of bipolar cell Outer fiber Apical microvillus Discs containing visual pigments Melanin granules Discs being phagocytized Pigment cell nucleus Inner fibers Rod cell body Cone cell body Synaptic terminals Rod cell body Nuclei Mitochondria Connecting cilia Basal lamina (border with choroid) The outer segments of rods and cones are embedded in the pigmented layer of the retina. Pigmented layer Outer segment Inner segment

16. Copyright © 2010 Pearson Education, Inc. Rods Sensitive to dim light Suited for night and peripheral vision Perceived gray tones Fuzzy and indistinct images

17. Copyright © 2010 Pearson Education, Inc. Cones Bright light for activation (low sensitivity) One of three pigments for colored view Detailed, high-resolution vision

18. Copyright © 2010 Pearson Education, Inc. Chemistry of Visual Pigments Retinal Light-absorbing protein (opsin) form visual pigments Synthesized from vit A Conversion of 11-cis-retinal to all-trans-retinal initiates chain reaction leading to electrical impulses in optic nerve

19. Copyright © 2010 Pearson Education, Inc. Figure 15.15b Rod discs Visual pigment consists of Retinal Opsin (b) Rhodopsin, the visual pigment in rods, is embedded in the membrane that forms discs in the outer segment.

20. Copyright © 2010 Pearson Education, Inc. Excitation of Rods Visual pigment of rods is rhodopsin In dark, rhodopsin forms and accumulates Formed from vit A When light absorbed, rhodopsin breaks down Retinal and opsin separate (bleaching of the pigment)

21. Copyright © 2010 Pearson Education, Inc. Figure 15.16 11-cis-retinal Bleaching of the pigment: Light absorption by rhodopsin triggers a rapid series of steps in which retinal changes shape (11-cis to all-trans) and eventually releases from opsin. 1 Rhodopsin Opsin and Regeneration of the pigment: Enzymes slowly convert all-trans retinal to its 11-cis form in the pigmented epithelium; requires ATP. DarkLight All-trans-retinal Oxidation 2H + Reduction Vitamin A 2 11-cis-retinal All-trans-retinal

22. Copyright © 2010 Pearson Education, Inc. Excitation of Cones Three types of cones, named for color absorbed: blue, green, and red Intermediate hues perceived by activation of more than one type of cone at same time Color blindness due to congenital lack of one or more cone types

23. Copyright © 2010 Pearson Education, Inc. Figure 15.18 (1 of 2) 1 cGMP-gated channels open, allowing cation influx; the photoreceptor depolarizes. Voltage-gated Ca 2+ channels open in synaptic terminals. 2 Neurotransmitter is released continuously. 3 4 Hyperpolarization closes voltage-gated Ca 2+ channels, inhibiting neurotransmitter release. 5 No EPSPs occur in ganglion cell. 6 No action potentials occur along the optic nerve. 7 Neurotransmitter causes IPSPs in bipolar cell; hyperpolarization results. Na + Ca 2+ Photoreceptor cell (rod) Bipolar cell Ganglion cell In the dark

24. Copyright © 2010 Pearson Education, Inc. Figure 15.18 (2 of 2) 1 cGMP-gated channels are closed, so cation influx stops; the photoreceptor hyperpolarizes. Voltage-gated Ca 2+ channels close in synaptic terminals. 2 No neurotransmitter is released. 3 Lack of IPSPs in bipolar cell results in depolarization. 4 Depolarization opens voltage-gated Ca 2+ channels; neurotransmitter is released. 5 EPSPs occur in ganglion cell. 6 Action potentials propagate along the optic nerve. 7 Photoreceptor cell (rod) Bipolar cell Ganglion cell Light Ca 2+ In the light

25. Copyright © 2010 Pearson Education, Inc. Light Adaptation Dark to bright Large amounts of pigments broken down instantaneously, producing glare Pupils constrict Rod function ceases Cones adapt Visual acuity improves over 5–10 min

26. Copyright © 2010 Pearson Education, Inc. Dark Adaptation Bright to dark Reverse of light adaptation Cones stop functioning in low light Pupils dilate Rhodopsin accumulates in dark and retinal sensitivity increases within 20–30 min

27. Copyright © 2010 Pearson Education, Inc. Visual Pathway Axons of retinal ganglion form optic nerve Medial fibers decussate at optic chiasma Optic tracts continue thalamus Fibers connect to primary visual cortex in occipital lobe

28. Copyright © 2010 Pearson Education, Inc. Figure 15.19a Pretectal nucleus Right eyeLeft eye Fixation point Optic radiation Optic tract Optic chiasma Uncrossed (ipsilateral) fiber Crossed (contralateral) fiber Optic nerve Lateral geniculate nucleus of thalamus Superior colliculus 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. Suprachiasmatic nucleus

29. Copyright © 2010 Pearson Education, Inc. Depth Perception Both eyes view same image from slightly different angles Depth perception (three-dimensional vision) results from cortical fusion of slightly diff images