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. Lateral canthus 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. Superior oblique muscle
Superior oblique tendon
Superior rectus muscle
Conjunctiva
Lateral rectus muscle
Optic nerve
Inferior rectus muscle
Inferior oblique muscle
(a)
Figure 8.3a

13. Why are the eye muscles named this way?
Trochlea
Superior oblique muscle
Superior oblique tendon
Axis at
center of
eye
Superior rectus muscle
Inferior
rectus muscle
Why are the eye muscles named this way?
Medial rectus muscle
Lateral rectus 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. Fovea centralis (macula) Iris Pupil Optic nerve Aqueous humor
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
Pathway of light
Ganglion
cells
(a)
Figure 8.5a

22. Pigmented layer of retina Neural layer of retina
Central artery and vein of retina
Optic disc
Sclera
Optic nerve
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. Light absorption by cone populations
560 nm
(red cones)
530 nm
(green cones)
420 nm
(blue cones)
Light absorption by cone populations
380
450
500
550
600
650
700
750
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. Fovea centralis (macula) Iris Pupil Optic nerve Aqueous humor
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. Fovea centralis Macula Blood vessels Optic disc Retina 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. Light from distant source Focal point
Retina
Light from distant source
Focal point
(a)
Light from near source
Focal point
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. Occipital lobe (visual cortex)
Fixation point
Right eye
Left eye
Optic nerve
Optic chiasma
Optic tract
Optic radiation
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
malleus
incus
stapes
Auditory canal
Auditory ossicles
Figure 8.12

51. The External Ear A. Involved in hearing only
B. Structures of the external ear
1. Pinna (auricle)
2. Auditory Canal (External acoustic meatus)
Narrow chamber in the temporal bone
Lined with skin and ceruminous (wax) glands
Ends at the tympanic membrane

52. The Middle Ear (Tympanic Cavity)
A. Air-filled cavity within the temporal bone
B. Only involved in the sense of hearing
When does the middle ear fill with fluid? Is that bad or good??

53. The Middle Ear (Tympanic Cavity)
Two tubes are associated with the middle ear
The opening from the auditory canal is covered by the tympanic membrane
The auditory tube connecting the middle ear with the throat
Allows for equalizing pressure during yawning or swallowing
This tube is otherwise collapsed
Why do pacifier-babies get more ear infections??

54. Bones of the Middle Ear (Tympanic Cavity)
Three bones (ossicles) span the cavity
Malleus (hammer)
Incus (anvil)
Stapes (stirrup)
Function
Vibrations from eardrum move the malleus  incus  stapes  inner ear

55. 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
malleus
incus
stapes
Auditory canal
Auditory ossicles
Figure 8.12

56. Inner Ear or Bony Labyrinth
A. Includes sense organs for hearing and balance
B. Filled with perilymph
C. Contains a maze of bony chambers within the temporal bone
Cochlea
Vestibule
Semicircular canals (balance only!)

57. 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
malleus
incus
stapes
Auditory canal
Auditory ossicles
Figure 8.12

58. Organs of Hearing Organ of Corti A. Located within the cochlea
B. Receptors = hair cells on the basilar membrane
C. Gel-like tectorial membrane is capable of bending hair cells
D. Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe

59. Mechanism of Hearing
A. Vibrations from sound waves move tectorial membrane
B. Hair cells are bent by the membrane
C. An action potential starts in the cochlear nerve
Impulse travels to the temporal lobe
D. Continued stimulation can lead to adaptation
Why do you not notice a repetitive sound?

60. Figure 8.16 EXTERNAL EAR MIDDLE EAR INTERNAL EAR Auditory canal
Ear- drum
Hammer,
anvil, stirrup
Oval
window
Fluids in cochlear canals
Pinna
Upper and middle
lower
Pressure
Spiral organ
of Corti
stimulated
Time
One
vibration
Amplitude
Amplification
in middle ear
Figure 8.16

61. Perilymph in scala vestibuli Spiral organ of Corti Vestibular membrane
Temporal bone
Perilymph in scala vestibuli
Spiral organ of Corti
Vestibular
membrane
Afferent fibers
of the cochlear
nerve
Temporal
bone
Cochlear
duct (contains endolymph)
Perilymph in scala tympani
(a)
Figure 8.15a

62. Hair (receptor) cells of spiral organ of Corti Tectorial membrane
Vestibular membrane
Hair (receptor) cells of spiral organ of Corti
Tectorial membrane
Fibers of the cochlear nerve
Supporting cells
Basilar membrane
(b)
Figure 8.15b

63. Pathway of sound waves from pinna to temporal lobe of brain…..
pinna → external auditory canal → tympanic
membrane vibration → (malleus → incus →
stapes) → oval window → fluids of inner ear
movement (perilymph) → Organ of Corti
(basilar/tectorial membranes → hair cells) →
Electrical impulse and neurotransmitter release
→ auditory nerve (VIII) → temporal lobe
Slide 8.40

64. Mechanism of Hearing- No notes!
High-pitched sounds disturb the short, stiff fibers of the basilar membrane
Receptor cells close to the oval window are stimulated
Low-pitched sounds disturb the long, floppy fibers of the basilar membrane
Specific hair cells further along the cochlea are affected

65. 20,000 (High notes) 2,000 200 20 (Low notes)
Stapes
Fibers of sensory neurons
Scala
vestibuli
Oval window
Perilymph
Round window
Scala tympani
Basilar membrane
Cochlear duct
(a)
Fibers of basilar membrane
Apex (long, floppy fibers)
Base (short, stiff fibers)
20,000 (High notes)
2,000
200 20
(Low notes)
Frequency (Hz)
(b)
Figure 8.17

66. Organs of Equilibrium
A. Equilibrium receptors of the inner ear are called the vestibular apparatus
B. Vestibular apparatus has two functional parts
Static equilibrium
Dynamic equilibrium

67. Semicircular canals Ampulla Vestibular nerve Vestibule (a)
Figure 8.14a

68. Membranes in vestibule
Otoliths
Otolithic
membrane
Hair tuft
Hair cell
Supporting cell
Nerve fibers of
vestibular division
of cranial nerve VIII
(a)
Figure 8.13a

69. Force of gravity Otolithic Otoliths membrane Hair cell Head upright
Head tilted
(b)
Figure 8.13b

70. Semicircular canals Ampulla Vestibular nerve Vestibule (a)
Figure 8.14a

71. Cupula of crista ampullaris
Endolymph
Flow of endolymph
Cupula of crista ampullaris
Cupula
Nerve
fibers
Direction of body movement
(b)
(c)
Figure 8.14b-c

72. Chemical Senses: Taste and Smell
A. Both senses use chemoreceptors
1. Stimulated by chemicals in solution
2. Taste has five types of receptors
3. Smell can differentiate a large range of chemicals
B. Both senses complement each other and respond to many of the same stimuli

73. Olfaction—The Sense of Smell
Receptors: Neurons with long cilia (1000’s)
Chemicals must be dissolved in mucus for detection
Location: Olfactory receptors are in the roof of the nasal cavity
Impulses are transmitted via the olfactory nerve (I)
Interpretation of smells is made in the olfactory cortex (tied to limbic system)

74. Olfactory receptor cell
Olfactory bulb
Cribriform plate
of ethmoid bone
Olfactory tract
Olfactory filaments
of the olfactory nerve
Supporting cell
Olfactory
mucosa
Olfactory receptor cell
Olfactory hairs (cilia)
Mucus layer
(a)
Route of inhaled air containing odor molecules
(b)
Figure 8.18

75. The Sense of Taste
Receptors: Taste buds- Only 4 types (gustatory cells with long microvilli- substances must be dissolved)
Location of taste buds
Most are on the tongue
Soft palate
Cheeks

76. Epiglottis Palatine tonsil Lingual tonsil Fungiform papillae (a)
Figure 8.19a

77. Circumvallate papilla
Taste buds
(b)
Figure 8.19b

78. Structure of Taste Buds
Nerves (different places = different nerves)
Facial nerve
Glossopharyngeal nerve
Vagus nerve
Brain Area- gustatory cortex (insular/frontal lobes)

79. Taste Sensations Sweet receptors (sugars) Saccharine Some amino acids
Sour receptors
Acids
Bitter receptors Umami- Glutamate
Alkaloids
Salty receptors **No “spicy” sense
Metal ions

80. Developmental Aspects of the Special Senses
A. Formed early in embryonic development
B. Eyes are outgrowths of the brain
C. All special senses are functional at birth

81. Developmental Aspects of the Special Senses
Eye problems
A. Strabismus—“crossed eyes” results from unequal pulls by the external eye muscles in babies
B. Ophthalmia neonatorum—conjunctivitis resulting from mother having gonorrhea. Baby’s eyelids are swollen and pus is produced

82. Developmental Aspects of the Special Senses
Eye problems
C. Presbyopia—“old vision” results from decreasing lens elasticity that accompanies aging

83. Developmental Aspects of the Special Senses
Ear problems
A. Presbycusis—type of sensorineural deafness
B. Otosclerosis—ear ossicles fuse