1. Special Senses: Vision
Chapter 8

2. Agenda Anatomy of the eye – internal and external
Light and light refraction
Diseases and disorders of the eye

3. The Eye and Vision
Special sense receptors – large, complex sensory organ
70% of sensory receptors
Optic nerve – 1+ million nerve fibers
Eyes and ears, localized cluster of receptors – taste buds and olfactory epithelium

4. External and Accessory Structures of Eye
Extrinsic eye muscles
Figure 8.3a

5. External and Accessory Structures of Eye
Eyelids – eyelashes
Tarsal glands - lubrication
Conjunctiva
Lacrimal Apparatus
6 extrinsic eye muscles
Eyelids – protect eye
Eyelashes – protect border of eyelids, catch dust etc from entering eye
Lactrimal apparatus- glands continuously release dilute salt solution, lacrimal canaliculi, into sac, nasolacrimal duct
Lacrimal secretions also contain Ab and lysozyme (enzyme that destroys bacteria) – cleans and protects as moistens and lubricates
Crying - over production
Over production can occur with foreign objects or chemicals – washing away
Conjuctiva – lines eyelids and covers part of outer surface of eye. Ends at edge of cornea by fusing with corneal epithelium, secretes mucous to keep moist

6. Conjunctivitis – Pink Eye
Conjunctivitis – inflammation of conjunctiva
Pink Eye – Infection of conjunctiva by bacteria or virus, highly contagious
Leads to red, irritated eyes

7. Internal Anatomy of Eye
Figure 8.4a
Humors
Vitreous - posterior
Aqueous – anterior
Lens
Wall – 3 layers
Fibrous layer
Vascular layer
Sensory layer
Hollow sphere filled with fluids called humors that maintain shape and integrity.
Wall composed of three layers
Lens – main focusing
Figure 8.4b

8. Wall of Eye Fibrous Layer – 2 parts Sclera – protective, white of eye
Cornea – crystal clear, many nerve endings, vulnerable, extraordinary healing
Only tissue in body that can be transplanted w/o fear of rejection – no blood vessels immune system can’t reach
Mostly pain fibers – touched, blinking, increased tearing

9. Wall of Eye Vascular Layer – 3 regions
Choroid – blood rich, dark pigment, light scatter
Ciliary body – muscle structure, lens attached via ciliary zonule (suspensory ligament)
Iris – muscle structure, pigmented, rounded opening (pupil), smooth muscle
Dark pigment prevents light scattering within eye
Circular and radial muscle regulate size of pupil and therefore light entering. See as clearly as possible,
Close vision and bright light – circular muscles contract, pupil constricts
Distant vision and dim light radial fibers constrict to dilate pupil

10. Wall of Eye Sensory Layer – 2 layer retina
Extends to ciliary body
Outer layer – Pigmented Layer
Pigmented cells – absorb and prevent light scattering
What other structure also does this?
Cells act as phagocytes
Phagocytes – remove dead or damaged receptor cells
Figure 8.5b

11. Wall of Eye Sensory Layer – 2 layer retina Inner Layer – Neural Layer
Extends to ciliary body
Inner Layer – Neural Layer
Transparent
Contains millions of photorectptors- cones and rods
Figure 8.5

12. Nerve Impulses from Retina
Electrical signals from photoreceptors leave retina as nerve impulses via optic nerve
Transmitted to optic cortex = vision
Photoreceptors on entire retina except where optic nerve leaves eyeball = optic disk or blind spot
Optic nerve composed of ganglion cell axons
Light focused on optic disc – object disappears

13. Photoreceptors
Cones – most dense at center of retina, decrease on periphery
Color under bright conditions
Macula lutea, fovea centralis
3 types – sensitive to different wavelengths
Blue
Green
Green and Red (called red)
‘mixing’ in brain
Macula lutea, fovea centralis – contains only cones, greatest visual acuity
If blue and red stimulated will see purple
All stimulated see white
Shine green in one eye red in other see yellow

14. Photoreceptors
Rods – more dense at periphery, decrease in number towards center
See grey tones in dim light
Provides peripheral vision

15. Night or Color Blindness
Color Blindness – More common in men, most common is lack of red or green receptors
See red or green as same color, depends on cones present
Can see differences in intensity of color
Night Blindness – most commonly caused by Vitamin A deficiency, leads to deterioration of neural retina
Vitamin A supplements can restore if degenerative changes have not occurred

16. Lens Divides eye into 2 segments Both maintain intraocular pressure
Anterior (Aqueous) Segment
Similar to blood plasma
Continuously secreted by part of choroid
Provides nutrients for lens and cornea, lack blood supply
Reabsorbed into venous blood through canal of Schlemm
Posterior (Vitreous) Segment
Gel-like substance
Both maintain intraocular pressure

17. Glaucoma Drainage of aqueous fluid via canal of Schlemm blocked
pressure increases compressing retina and optic nerve, causes pain and possibly blindness
Common in elderly
Slow process

18. Lens Focuses light entering retina Biconvex
Held upright by suspensory ligaments – ciliary zonule attached to ciliary body

19. Light Refraction Refraction = bending of light due to change in speed
Refractive power of cornea, and humors constant
Refractive power of lens changes
Goal to focus light at one point on back of eye
How does lens change to do this?
Distant vision – light enters as parallel rays, lens does not need to change shape
Close vision – divergent rays, lens must bulge more = accommodation

20. Nearsighted or Farsighted

21. Cataracts With age, lens becomes hard and opaque Diabetes mellitus
Heavy smoking
Vision becomes hazy and distorted, will lead to blindness
Surgical removal or glasses

22. Binocular vision “two-eyed” vision Provides depth perception 3D vision

23. Eye Reflexes
Convergence = movement of eyes medially when viewing close objects
Photopupillary reflex = pupils constrict in bright light to protect photoreceptors
Accommodation pupillary reflex = pupils constrict when viewing close objects
Think of reading

24. The End