Special Senses

1. 8 Special Senses

2. The Senses • Special senses • Smell • Taste • Sight • Hearing • Equilibrium

3. The Eye and Vision • A. 70 percent of all sensory receptors are in the eyes • B. Each eye has over a million nerve fibers • C. Protection for the eye • Most of the eye is enclosed in a bony orbit • A cushion of fat surrounds most of the eye • Which bones make up the eye orbit??

4. Eyebrow Site where conjunctiva merges with cornea Eyelid Eyelashes Pupil Lateral canthus Medial canthus Sclera (covered by conjunctiva) Iris Eyelid Figure 8.1

5. Accessory Structures of the Eye • A. Eyelids • Meet at the medial and lateral commissure (canthus) • B. Eyelashes • Tarsal glands produce an oily secretion that lubricates the eye • Ciliary glands are located between the eyelashes- modified sweat glands

6. Accessory Structures of the Eye • C. Conjunctiva • Membrane that lines the eyelids • Connects to the outer surface of the eye • Secretes mucus to lubricate the eye and keep it moist • What is pink eye?

7. Accessory Structures of the Eye • D. Lacrimal apparatus = lacrimal gland + ducts • 1. Lacrimal gland—produces lacrimal fluid; situated on lateral aspect of each eye • 2. Lacrimal canaliculi—drain lacrimal fluid from eyes medially • 3. Lacrimal sac—provides passage of lacrimal fluid towards nasal cavity • 4. Nasolacrimal duct—empties lacrimal fluid into the nasal cavity

8. Excretory duct of lacrimal gland Lacrimal gland Conjunctiva Anterior aspect Eyelid Eyelashes Tarsal glands (a) Eyelid Figure 8.2a

9. Lacrimal sac Lacrimal gland Excretory ducts of lacrimal gland Lacrimal canaliculus Nasolacrimal duct Nostril (b) Figure 8.2b

10. Accessory Structures of the Eye • 5. Function of the lacrimal apparatus • Protects, moistens, and lubricates the eye • Empties into the nasal cavity • 6. Lacrimal secretions (tears) contain: • Dilute salt solution • Mucus • Antibodies • Lysozyme (enzyme that destroys bacteria)

11. Accessory Structures of the Eye • G. Extrinsic eye muscles • 1. Six muscles attach to the outer surface of the eye • 2. Produce eye movements • How is the human eye musculature different than a cow eye’s?

12. Superioroblique muscle Superioroblique tendon Superiorrectus muscle Conjunctiva Lateral rectusmuscle Inferiorrectusmuscle Inferiorobliquemuscle Opticnerve (a) Figure 8.3a

13. Trochlea Superioroblique muscle Superioroblique tendon Axis at center of eye Superiorrectus muscle Inferior rectus muscle Why are the eye muscles named this way? Medialrectus muscle Lateralrectus muscle (b) Figure 8.3b

14. Figure 8.3c

15. Structure of the Eye • Layers forming the wall of the eyeball • 1. Fibrous layer • Outside layer • 2. Vascular layer • Middle layer • 3. Sensory layer • Inside layer Why were the layers of the eye called “tunics” in the past?

16. Sclera Ciliary body Choroid Ciliary zonule Retina Cornea Fovea centralis (macula) Iris Pupil Optic nerve Aqueous humor Lens canal of Schlemm Central artery and vein of the retina Vitreous humor Optic disc (blind spot) (a) Figure 8.4a

17. Ciliary body Vitreous humor Iris Retina Margin of pupil Choroid Sclera Aqueous humor ( Fovea centralis Optic disc Optic nerve Lens Cornea Ciliary zonule (b) Figure 8.4b

18. Structure of the Eye: The Fibrous Layer • 1. Sclera • White connective tissue layer • Seen anteriorly as the “white of the eye” • 2. Cornea • Transparent, central anterior portion • Allows for light to pass through • Repairs itself easily • The only human tissue that can be transplanted without fear of rejection Why?

19. Structure of the Eye: Vascular Layer • 1. Choroid is a blood-rich nutritive layer in the posterior of the eye • Pigment prevents light from scattering • 2. Modified anteriorly into two structures • a. Ciliary body—smooth muscle attached to lens • b. Iris—regulates light entering eye • Pigmented layer that gives eye color • Pupil—rounded opening in the iris

20. Structure of the Eye: Sensory Layer • 1. Retina contains two layers: • Outer pigmented layer • Inner neural layer 2. Contains receptor cells (photoreceptors) • Rods • Cones

21. Do you recognize • anything? • 2. What do the axons • of the ganglion cells • form? See next page • if needed. Pigmented layer of retina Rod Cone Bipolar cells Pathwayof light Ganglion cells (a) Figure 8.5a

22. Pigmentedlayer of retina Neural layerof retina Centralarteryand veinof retina Opticdisc Sclera Opticnerve Choroid (b) Figure 8.5b

23. Structure of the Eye: Sensory Layer • Neurons of the retina and vision • A. Rods • 1. Most are found towards the edges of the retina • 2. Allow dim light vision and peripheral vision • 3. All perception is in gray tones

24. Structure of the Eye: Sensory Layer • Neurons of the retina and vision • B. Cones • 1. Allow for detailed color vision • 2. Densest in the center of the retina • 3. Fovea centralis–lateral to blind spot • Area of the retina with only cones • Visual acuity (sharpest vision) is here • C. No photoreceptor cells are at the optic disc, or blind spot

25. Structure of the Eye: Sensory Layer • Cone sensitivity • A. Three types of cones • B. Different cones are sensitive to different wavelengths • C. Color blindness is the result of the lack of one cone type

26. 560 nm (red cones) 530 nm (green cones) 420 nm (blue cones) Light absorption by cone populations 450 500 550 600 650 700 750 380 Wavelengths (nanometers) Figure 8.6

27. Lens • A. Biconvex crystal-like structure • B. Held in place by a suspensory ligament attached to the ciliary body • Put your hands in a biconvex shape

28. Sclera Ciliary body Choroid Ciliary zonule Retina Cornea Fovea centralis (macula) Iris Pupil Optic nerve Aqueous humor Lens canal of Schlemm Central artery and vein of the retina Vitreous humor Optic disc (blind spot) (a) Figure 8.4a

29. Lens- no notes! • Cataracts result when the lens becomes hard and opaque with age • Vision becomes hazy and distorted • Eventually causes blindness in affected eye • Risk factors include: • Diabetes mellitus • Frequent exposure to intense sunlight • Heavy smoking

30. Figure 8.7

31. Two Segments, or Chambers, of the Eye - No Notes • Anterior (aqueous) segment • Anterior to the lens • Contains aqueous humor • Posterior (vitreous) segment • Posterior to the lens • Contains vitreous humor

32. Anterior Segment • A. Aqueous humor • 1. Watery fluid found between lens and cornea • 2. Similar to blood plasma • 3. Helps maintain intraocular pressure • 4. Provides nutrients for the lens and cornea • 5. Reabsorbed into venous blood through the scleral venous sinus, or canal of Schlemm

33. Posterior Segment • B. Vitreous humor • 1. Gel-like substance posterior to the lens • 2. Prevents the eye from collapsing • 3. Helps maintain intraocular pressure

34. Macula Bloodvessels Retina Optic disc Foveacentralis Lateral Medial Figure 8.8

35. Pathway of Light Through the Eye • A. Light must be focused to a point on the retina for optimal vision • B. The eye is set for distance vision (over 20 feet away) • C. Accommodation—the lens must change shape to focus on closer objects (less than 20 feet away)

36. Retina Focal point Light from distant source (a) Focal point Light from near source Retina (b) Figure 8.9

37. Pathway of Light Through the Eye • D. Image formed on the retina is a real image • E. Real images are: • Reversed from left to right • Upside down • Smaller than the object

38. Figure 8.10

39. Visual Fields and Visual Pathways • A. Optic chiasma • Location where the optic nerves cross • Fibers from the medial side of each eye cross over to the opposite side of the brain • B. Optic tracts • Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye

40. Fixation point Left eye Right eye Opticnerve Opticchiasma Optictract Opticradiation Thalamus Occipital lobe(visual cortex) Figure 8.11

41. Pathway of vision from light entering pupil to occipital lobe of brain…. Light →Cornea→ Aqueous humor → Pupil → Lens → Vitreous Humor → Absorbed by Pigment/Choroid → Retina [Rods/Cones Electrical signal → Bipolar cells → Ganglion cells] →Optic nerve II→ Optic tract **→ Thalamus → Occipital lobe (visual cortex) **Axons from both eyes due to crossing over Slide 8.28

42. A Closer Look • Emmetropia—eye focuses images correctly on the retina • A. Myopia (nearsighted) • Distant objects appear blurry • Light from those objects fails to reach the retina and are focused in front of it • Results from an eyeball that is too long

43. A Closer Look • B. Hyperopia (farsighted) • Near objects are blurry while distant objects are clear • Distant objects are focused behind the retina • Results from an eyeball that is too short or from a “lazy lens”

44. Focal plane Correction None required Concave lens (a) Emmetropic eye (b) Myopic eye (nearsighted) Convex lens (c) Hyperopic eye (farsighted)

45. A Closer Look • C. Astigmatism • Images are blurry • Results from light focusing as lines, not points, on the retina due to unequal curvatures of the cornea or lens

46. Homeostatic Imbalances of the Eyes • D. Night blindness—inhibited rod function that hinders the ability to see at night • E.Color blindness—genetic conditions that result in the inability to see certain colors • Due to the lack of one type of cone (partial color blindness) • F.Cataracts—when lens becomes hard and opaque, our vision becomes hazy and distorted

47. Homeostatic Imbalances of the Eyes • G. Glaucoma—can cause blindness due to increasing pressure within the eye • H. Hemianopia—loss of the same side of the visual field of both eyes; results from damage to the visual cortex on one side only

48. The Ear • A. Houses two senses • Hearing • Equilibrium (balance) • B. Receptors are mechanoreceptors • C. Different organs house receptors for each sense

49. Anatomy of the Ear • The ear is divided into three areas • A. External (outer) ear • B. Middle ear (tympanic cavity) • C. Inner ear (bony labyrinth)

50. External (outer) ear Middle ear Internal (inner) ear Vestibulocochlear Nerve (auditory) Pinna (auricle) Semicircular canals Oval window Cochlea Vestibule Round window Auditory tube Tympanic membrane incus stapes malleus Auditory canal Auditory ossicles Figure 8.12