1. Human Anatomy and Physiology
Tenth Edition
Chapter 15 Part B
The Special Senses
Copyright © 2016 Pearson Education, Inc. All Rights Reserved

2. 15.2 Focusing and Light Overview: Light and Optics
Wavelength and color
Electromagnetic radiation: all energy waves, from long radio waves to short X rays; visible light occupies a small portion in the middle of the spectrum
Light has wavelengths between 400 and 700 nm
Eyes respond only to visible light

3. Figure 15.10a the Electromagnetic Spectrum and Photoreceptor Sensitivities

4. Overview: Light and Optics (1 of 3)
Wavelength and color
Light: packets of energy (photons or quanta) that travel in wavelike fashion at high speeds
When visible light passes through spectrum, it is broken up into bands of colors (rainbow)
Red wavelengths are longest and have lowest energy, and violet are shortest and have most energy
Color that eye perceives is a reflection of that wavelength
Grass is green because it absorbs all colors except green
White reflects all colors, and black absorbs all colors

5. Figure 15.10b the Electromagnetic Spectrum and Photoreceptor Sensitivities

6. Overview: Light and Optics (2 of 3)
Refraction and lenses
Refraction: bending of light rays
Due to change in speed of light when it passes from one transparent medium to another and path of light is at an oblique angle
Example: from liquid to air
Lenses of eyes can also refract light because they are curved on both sides
Convex: thicker in center than at edges
Concave: thicker at edges than in center

7. Figure Refraction

8. Overview: Light and Optics (3 of 3)
Refraction and lenses
Convex lenses bend light passing through it, so that rays converge at focal point
Image formed at focal point is upside-down and reversed from left to right
Concave lenses disperse light, preventing light from being focused

9. Figure 15.12 Light Is Focused by a Convex Lens

10. Focusing Light on the Retina (1 of 5)
Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, and finally photoreceptors
Light is refracted three times along path: (1) entering cornea, (2) entering lens, and (3) leaving lens
Majority of refractory power is in cornea; however, it is constant and cannot change focus

11. Focusing Light on the Retina (2 of 5)
Lens is able to adjust its curvature to allow for fine focusing
Can focus for distant vision and for close vision

12. Figure 15.13a Focusing for Distant and Close Vision

13. Focusing Light on the Retina (3 of 5)
Focusing for distant vision
Eyes are best adapted for distant vision
Far point of vision: distance beyond which no change in lens shape is needed for focusing
20 feet for emmetropic (normal) eye
Cornea and lens focus light precisely on retina at this distance
Ciliary muscles are completely relaxed in distance vision, which causes a pull on ciliary zonule; as a result, lenses are stretched flat

14. Figure 15.13b Focusing for Distant and Close Vision

15. Focusing Light on the Retina (4 of 5)
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 the lenses
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

16. Focusing Light on the Retina (5 of 5)
Constriction of the pupils
Accommodation pupillary reflex involves constriction of pupils to prevent most divergent light rays from entering eye
Mediated by parasympathetic nervous system
Convergence of the eyeballs
Medial rotation of eyeballs causes convergence of eyes toward object being viewed
Controlled by somatic motor neuron innervation on medial rectus muscles

17. Figure 15.13c Focusing for Distant and Close Vision

18. Clinical – Homeostatic Imbalance 15.8
Problems associated with refraction related to eyeball shape:
Myopia (nearsightedness)
Eyeball is too long, so focal point is in front of retina
Corrected with a concave lens
Hyperopia (farsightedness)
Eyeball is too short, so focal point is behind retina
Corrected with a convex lens
Astigmatism
Unequal curvatures in different parts of cornea or lens
Corrected with cylindrically ground lenses or laser procedures

19. Figure 15.14-1 Problems of Refraction

20. Figure 15.14-2 Problems of Refraction

21. 15.3 Phototransduction (1 of 3)
Functional Anatomy of Photoreceptors
Photoreceptors (rods and cones) are modified neurons that resemble upside-down epithelial cells
Consists of cell body, synaptic terminal, and two segments:
Outer segment: light-receiving region
Contains visual pigments (photopigments) that change shape as they absorb light
Inner segment of each joins cell body
Inner segment is connected via cilium to outer segment and to cell body via outer fiber

22. 15.3 Phototransduction (2 of 3)
Cell body is connected to synaptic terminal via inner fibers
Plasma membrane of outer segment folds back to form many discs
Photopigments are embedded in discs

23. 15.3 Phototransduction (3 of 3)
Photoreceptors are vulnerable to damage
Degenerate if retina detached
Destroyed by intense light
Vision is maintained because outer segment is renewed every 24 hours
Tips fragment off and are phagocytized

24. Figure 15.15a Photoreceptors of the Retina

25. Comparing Rod and Cone Vision (1 of 2)
Rods are very sensitive to light, making them best suited for night vision and peripheral vision
Contain a single pigment, so vision is perceived in gray tones only
Pathways converge, causing fuzzy, indistinct images
As many as 100 rods may converge into one ganglion

26. Comparing Rod and Cone Vision (2 of 2)
Cones have low sensitivity, so require bright light for activation
React more quickly than rods
Have one of three pigments, which allow for vividly colored sight
Nonconverging pathways result in detailed, high- resolution vision
Some cones have their own ganglion cell, so brain can put together accurate, high-acuity resolution images

27. Table 15.1 Comparison of Rods and Cones
Noncolor vision (one visual pigment)
Color vision (three visual pigments)
High sensitivity; function in dim light
Low sensitivity; function in bright light
Low acuity (many rods converge onto one ganglion cell)
High acuity (one cone per ganglion cell in fovea)
More numerous (20 rods for every cone)
Less numerous
Mostly in peripheral retina
Mostly in central retina

28. Clinical – Homeostatic Imbalance 15.9
Color blindness: lack of one or more cone pigments
Inherited as an X-linked condition, so more common in males
As many as 8–10% of males have some form
The most common type is red-green, in which either red cones or green cones are absent
Depending on which cone is missing, red can appear green, or vice versa
Rely on different shades to get cues of color

29. Visual Pigments (1 of 2)
Retinal: key light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments
Synthesized from vitamin A
Four opsins are rhodopsin (found in rods only), and three found in cones: green, blue, red (depending on wavelength of light they absorb)
Cone wavelengths do overlap, so same wavelength may trigger more than one cone, enabling us to see variety of hues of colors
Example: yellow light stimulates red and green cones, but if more red are triggered, we see orange

30. Visual Pigments (2 of 2) Retinal isomers are different 3-D forms
Retinal is in a bent form in dark, but when pigment absorbs light, it straightens out
Bent form called 11-cis-retinal
Straight form called all-trans-retinal
Conversion of bent to straight initiates reactions that lead to electrical impulses along optic nerve

31. Figure 15.15b Photoreceptors of the Retina

32. Phototransduction (1 of 3)
Phototransduction: process by which pigment captures photon of light energy, which is converted into a graded receptor potential
Capturing light
Deep purple pigment of rods is rhodopsin
Arranged in rod’s outer segment
Three steps of rhodopsin formation and breakdown:
Pigment synthesis, pigment bleaching, and pigment regeneration
Similar process in cones, but different types of opsins and cones require more intense light

33. Phototransduction (2 of 3)
Capturing light
Pigment synthesis
Opsin and 11-cis retinal combine to form rhodopsin in dark
Pigment bleaching
When rhodopsin absorbs light, 11-cis isomer of retinal changes to all-trans isomer
Retinal and opsin separate (rhodopsin breakdown)
Pigment regeneration
All-trans retinal converted back to 11-cis isomer
Rhodopsin is regenerated in outer segments

34. Figure 15.16 The Formation and Breakdown of Rhodopsin (1 of 3)

35. Figure 15.16 The Formation and Breakdown of Rhodopsin (2 of 3)

36. Figure 15.16 The Formation and Breakdown of Rhodopsin (3 of 3)

37. Phototransduction (3 of 3)
Light transduction reactions
Light-activated rhodopsin activates G protein transducin
Transducin activates PDE, which breaks down cyclic GMP (cGMP)
In dark, cGMP holds cation channels of outer segment open
enter and depolarize cell
In light cGMP breaks down, channels close, cell hyperpolarizes
Hyperpolarization is signal for vision!

38. Figure 15.17 Events of Phototransduction (1 of 5)

39. Figure 15.17 Events of Phototransduction (2 of 5)

40. Figure 15.17 Events of Phototransduction (3 of 5)

41. Figure 15.17 Events of Phototransduction (4 of 5)

42. Figure 15.17 Events of Phototransduction (5 of 5)

43. Information Processing in the Retina
Photoreceptors and bipolar cells generate only graded potentials (EPSPs and IPSPs), not APs
When light hyperpolarizes photoreceptor cells, they stop releasing inhibitory neurotransmitter glutamate to biopolar cells
Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells
Ganglion cells generate APs transmitted in optic nerve to brain

44. Figure 15.18-1 Signal Transmission in the Retina (1 of 7)

45. Figure 15.18-1 Signal Transmission in the Retina (2 of 7)

46. Figure 15.18-1 Signal Transmission in the Retina (3 of 7)

47. Figure 15.18-1 Signal Transmission in the Retina (4 of 7)

48. Figure 15.18-1 Signal Transmission in the Retina (5 of 7)

49. Figure 15.18-1 Signal Transmission in the Retina (6 of 7)

50. Figure 15.18-1 Signal Transmission in the Retina (7 of 7)

51. Figure 15.18-2 Signal Transmission in the Retina (1 of 7)

52. Figure 15.18-2 Signal Transmission in the Retina (2 of 7)

53. Figure 15.18-2 Signal Transmission in the Retina (3 of 7)

54. Figure 15.18-2 Signal Transmission in the Retina (4 of 7)

55. Figure 15.18-2 Signal Transmission in the Retina (5 of 7)

56. Figure 15.18-2 Signal Transmission in the Retina (6 of 7)

57. Figure 15.18-2 Signal Transmission in the Retina (7 of 7)

58. Light and Dark Adaptation (1 of 3)
Rhodopsin is so sensitive that bleaching occurs even in starlight
In bright light, bleaching occurs so fast that rods are virtually nonfunctional
Cones respond to bright light
So, activation of rods and cones depends on:
Light adaptation
Dark adaptation

59. Light and Dark Adaptation (2 of 3)
Light adaptation
When moving from darkness into bright light we see glare because:
Both rods and cones are strongly stimulated
Large amounts of pigments are broken down instantaneously, producing glare
Pupils constrict
Visual acuity improves over 5–10 minutes as:
Rod system turns off
Retinal sensitivity decreases
Cones and neurons rapidly adapt

60. Light and Dark Adaptation (3 of 3)
When moving from bright light into darkness, we see blackness because:
Cones stop functioning in low-intensity light
Bright light bleached rod pigments, so they are still turned off
Pupils dilate
Rhodopsin accumulates in dark, so retinal sensitivity starts to increase
Transducin returns to outer segments
Sensitivity increases within 20–30 minutes

61. Clinical – Homeostatic Imbalance 15.10
Nyctalopia (night blindness): condition in which rod function is seriously hampered
Ability to drive safely at night is impaired
Due to rod degeneration, commonly caused by prolonged vitamin A deficiency
If administered early, vitamin A supplements restore function
Can also be caused by retinitis pigmentosa
Degenerative retinal diseases that destroy rods
Tips of rods are not replaced when they slough off

62. 15.4 Processing and Relaying of Visual Information (1 of 3)
Visual Pathway to the Brain
Axons of retinal ganglion cells form optic nerve
Medial fibers from each eye cross over at the optic chiasma then continue on as optic tracts, which means each optic tract:
Contains fibers from lateral (temporal) aspect of eye on same side and medial (nasal) aspect of opposite eye, and
Each carries information from same half of visual field

63. 15.4 Processing and Relaying of Visual Information (2 of 3)
Most fibers of optic tracts continue on to lateral geniculate nuclei of thalamus
From there, thalamic neurons form optic radiation, which projects to primary visual cortex in occipital lobes
Conscious perception of visual images occurs here

64. 15.4 Processing and Relaying of Visual Information (3 of 3)
Other optic tract fibers send branches to midbrain
One set ends in superior colliculi, area controlling extrinsic eye muscles
A small subset of ganglion cells in retina contains melanopsin (circadian pigment), which projects to:
Pretectal nuclei: involved with pupillary reflexes
Suprachiasmatic nucleus of hypothalamus: timer for daily biorhythms

65. Figure 15.19a Visual Pathway to the Brain and Visual Fields, Inferior View

66. Figure 15.19 Visual Pathway to the Brain and Visual Fields, Inferior View

67. Depth Perception
Both eyes view same image from slightly different angles
Visual cortex fuses these slightly different images, resulting in a three-dimensional image, which leads to depth perception
Requires input from both eyes

68. Clinical – Homeostatic Imbalance 15.11
Loss of an eye or destruction of one optic nerve eliminates true depth perception entirely
Peripheral vision on damaged side is also affected
If neural destruction occurs beyond optic chiasma, then part or all of opposite half of the visual field is lost
Example: stroke can affect left visual cortex, which leads to blindness in right half of visual field

69. Visual Processing (1 of 3)
Retinal cells split input into channels that include information about:
Color and brightness, but also complex info such as angle, direction, and speed of movement of edges (sudden changes in brightness or color)
Lateral inhibition decodes “edge” information
Job of amacrine and horizontal cells

70. Visual Processing (2 of 3)
Ganglions pass information to lateral geniculate nuclei of thalamus to be processed for depth perception, with cone input emphasized
Primary visual cortex contains topographical map of retina
Neurons here respond to dark and bright edges and to object orientation
Provide form, color, motion inputs to visual association areas

71. Visual Processing (3 of 3)
Info is also passed on to temporal, parietal, and frontal lobes, where objects are identified and location in space determined