1. Copyright © 2010 Pearson Education, Inc. THE EYE: PART B

2. Copyright © 2010 Pearson Education, Inc. Light and Optics Refraction-Bending of a light ray Occurs when a light ray meets the surface of a different medium at an oblique angle The greater this angle, the greater the bending

3. Copyright © 2010 Pearson Education, Inc. Focusing Light on the Retina Light is refracted (bent) as it passes through 3 areas of the eye Entering the cornea Entering the lens Leaving the lens Since the lens shape can be changed, it can be used to: Increase refractory power above that of cornea alone Bend light rays so that they converge at a single point (focal point) Fine focus on an image

4. Copyright © 2010 Pearson Education, Inc. The image formed at the focal point is upside- down and reversed right to left Focusing Light on the Retina

5. 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

6. Copyright © 2010 Pearson Education, Inc. Focusing for Distant Vision Light rays from distant objects are parallel and need little refraction, so ciliary muscles are relaxed and the lens is stretched flat by tension in the 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 a close object diverges as it approaches the eye; Three processes must occur to produce clear vision: Accommodation— increases refractory power of lens Ciliary muscles contract Tension in ciliary zonule decreases The lens bulges

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. Focusing for Close Vision Constriction of the pupils— Constriction of pupils prevents the most divergent rays from entering eye Sphincter pupillae muscles of iris decrease pupil size Mediated by parasympathetic NS Convergence—medial rotation of the eyeballs toward the object being viewed

11. Copyright © 2010 Pearson Education, Inc. Problems of Refraction Emmetropia- normal vision Myopia (nearsightedness)—focal point in front of retina Hyperopia (farsightedness)—focal point behind retina

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

13. 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)

14. 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)

15. Copyright © 2010 Pearson Education, Inc. Photoreception and the Retina Structure of the Retina Pigmented Layer-single cell thick, abuts the choroid; stores vitamin A Neural Layer – Innermost layer of retina Extends anteriorly to the ora serrata

16. Copyright © 2010 Pearson Education, Inc. Composed of three layers: Photoreceptors- rods and cones The rods and cones are adjacent to: the pigmented layer Rods Sensitive to dim light Best suited for night and peripheral vision Perceived input is in gray tones only & images are fuzzy and indistinct Outer segment: rod shaped, contains -molecules that change shape as they absorb light (visual pigments ) Inner segment: of each joins the outer segment to the cell body Photoreception and the Retina

17. Copyright © 2010 Pearson Education, Inc. Functional Anatomy of Photoreceptors Cones Operate in bright light Provide high-acuity, color vision

18. 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

19. Copyright © 2010 Pearson Education, Inc. Neurons of the Retina Bipolar Cells Receive signals from photoreceptor cells and do not generate action potentials Link photoreceptors to ganglion cells Ganglion Cells Receive input from bipolar cells Only neurons of retina that generate action potentials

20. Copyright © 2010 Pearson Education, Inc. Visual Pigments and Stimulation of Photoreceptors Retinal - Light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments Synthesized from vitamin A Two isomers: 11-cis-retinal (bent form) and all-trans- retinal (straight form) Conversion of 11-cis-retinal to all-trans-retinal initiates a chain of reactions leading to transmission of electrical impulses in the optic nerve

21. 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.

22. Copyright © 2010 Pearson Education, Inc. Stimulation of Photoreceptors Excitation of Rods The visual pigment of rods is rhodopsin (opsin + 11-cis- retinal) In the dark, rhodopsin forms and accumulates When light is absorbed, (11-cis-retinal is converted to all- trans-retinal) all-trans-retinal separates from opsin and rhodopsin breaks down (bleaching of the pigment) Enzymes slowly convert the all-trans-retinal back to11-cis retinal in the pigmented epithelium Retinal returns to outer segment of rods, rejoins with opsin, forming rhodopsin

23. Copyright © 2010 Pearson Education, Inc. Excitation of Cones There are three types of cones: Named for the colors of light absorbed: blue, green, and red Intermediate hues are perceived by activation of more than one type of cone at the same time 100 x less sensitive than rods (takes brighter light to activate them) Stimulation of Photoreceptors

24. Copyright © 2010 Pearson Education, Inc. Breakdown and regeneration of visual pigments: Light absorption by visual pigments triggers pigment breakdown and then pigments are regenerated in the dark The threshold for activation of cones is: 420 nm (blue), 530 nm (green) and 560 nm (red) Stimulation of Photoreceptors

25. 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

26. 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

27. 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

28. Copyright © 2010 Pearson Education, Inc. Photoreceptor Imbalances Nyctalopia (night blindness) – rod function is seriously hampered Caused by: prolonged vitamin A deficiency (poor nutrition) or retinitis pigmentosa (areas of good nutrition) This leads to the degeneration of rods

29. Copyright © 2010 Pearson Education, Inc. Color blindness Due to lack of: one or more of the cone types; inherited as an X-linked condition More common in males than females Photoreceptor Imbalances

30. Copyright © 2010 Pearson Education, Inc. Figure 15.19a Right eyeLeft eye Fixation point Optic radiation Optic tract Optic chiasma Optic nerve Lateral geniculate nucleus of thalamus Superior colliculus Occipital lobe (primary visual cortex)

31. Copyright © 2010 Pearson Education, Inc. Cortical Processing Optic NerveOptic chiasma fibers from medial region of each eye cross over to the opposite side optic tracts lateral geniculate body of the thalamus optic radiations primary visual cortex in the occipital lobe of the brainconscious perception of the image