1. 8
Special Senses

2. The Senses
Special senses
Smell
Taste
Sight
Hearing
Equilibrium

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

4. Accessory Structures of the Eye
Eyelids and eyelashes
Conjunctiva
Lacrimal apparatus
Extrinsic eye muscles

5. Eyebrow Site where conjunctiva merges with cornea Eyelid Eyelashes
Pupil
Palpebral
fissure
Lacrimal
caruncle
Lateral
commissure
(canthus)
Medial
commissure
(canthus)
Sclera
(covered by
conjunctiva)
Iris
Eyelid
Figure 8.1

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

7. Accessory Structures of the Eye
Conjunctiva
Membrane that lines the eyelids
Connects to the outer surface of the eye
Secretes mucus to lubricate the eye and keep it moist

8. Homeostatic Imbalance
Conjunctivitis  inflammation of the conjunctiva that results in reddened, irritated eyes
Pinkeye  the infectious form of conjunctivitis cause by bacteria or viruses is highly contagious

10. Accessory Structures of the Eye
Lacrimal apparatus = lacrimal gland + ducts
Lacrimal gland  produces lacrimal fluid (tears); situated on lateral aspect of each eye
Lacrimal canaliculi  drain lacrimal fluid from eyes medially
Lacrimal sac  provides passage of lacrimal fluid towards nasal cavity
Nasolacrimal duct  empties lacrimal fluid into the nasal cavity

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

12. Lacrimal sac Lacrimal gland Excretory ducts of lacrimal gland
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
(b)
Figure 8.2b

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

14. Accessory Structures of the Eye
Extrinsic eye muscles
Six muscles attach to the outer surface of the eye
Produce eye movements

15. 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

16. Superior oblique muscle
Trochlea
Superior oblique muscle
Superior oblique tendon
Axis at
center of
eye
Superior rectus muscle
Inferior
rectus muscle
Medial rectus muscle
Lateral rectus muscle
(b)
Figure 8.3b

17. Structure of the Eye Layers forming the wall of the eyeball
Fibrous layer
Outside layer
Vascular layer
Middle layer
Sensory layer
Inside layer

18. humor (in anterior segment)
Sclera
Ciliary body
Choroid
Ciliary zonule
Retina
Cornea
Fovea centralis
Iris
Pupil
Optic nerve
Aqueous
humor (in anterior segment)
Lens
Scleral venous sinus
(canal of Schlemm)
Central artery and
vein of the retina
Vitreous humor
(in posterior segment)
Optic disc
(blind spot)
(a)
Figure 8.4a

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

20. Structure of the Eye: The Fibrous Layer
Sclera
White connective tissue layer
Seen anteriorly as the “white of the eye”
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

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

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

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

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

25. Pigmented layer of retina Rod Cone Bipolar cells Pathway of light
Ganglion
cells
(a)
Figure 8.5a

26. Structure of the Eye: Sensory Layer
Cone sensitivity
Three types of cones
Different cones are sensitive to different wavelengths
Color blindness  is the result of the lack of one cone type
Night blindness  inhibited rod function that hinders the ability to see at night

28. Lens Biconvex crystal-like structure
Held in place by a suspensory ligament attached to the ciliary body

29. Lens 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
Anterior (aqueous) segment
Anterior to the lens
Contains aqueous humor
Posterior (vitreous) segment
Posterior to the lens
Contains vitreous humor

32. humor (in anterior segment)
Sclera
Ciliary body
Choroid
Ciliary zonule
Retina
Cornea
Fovea centralis
Iris
Pupil
Optic nerve
Aqueous
humor (in anterior segment)
Lens
Scleral venous sinus
(canal of Schlemm)
Central artery and
vein of the retina
Vitreous humor
(in posterior segment)
Optic disc
(blind spot)
(a)
Figure 8.4a

33. Anterior Segment Aqueous humor
Watery fluid found between lens and cornea
Similar to blood plasma
Helps maintain intraocular pressure
Provides nutrients for the lens and cornea
Reabsorbed into venous blood through the scleral venous sinus, or canal of Schlemm

34. Homeostatic Imbalance
Glaucoma
If drainage of aqueous humor is blocked, fluid backs up like a clogged sink
Pressure within the eye may increase to dangerous levels and compress the delicate retina and optic nerve
Eventually causes pain and possibly blindness if not detected early

35. Posterior Segment Vitreous humor
Gel-like substance posterior to the lens
Prevents the eye from collapsing
Helps maintain intraocular pressure

36. Ophthalmoscope
Instrument used to illuminate the interior of the eyeball
Can detect diabetes, arteriosclerosis, degeneration of the optic nerve and retina

37. Fovea centralis Macula Blood vessels Optic disc Retina Lateral Medial
Figure 8.8

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

39. 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

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

41. Figure 8.10

42. Visual Fields and Visual Pathways
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
Optic tracts
Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye

43. 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

44. A Closer Look Emmetropia Eye focuses images correctly on the retina
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

45. A Closer Look 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”

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

47. The Ear Houses two senses Hearing Equilibrium (balance)
Receptors are mechanoreceptors
Different organs house receptors for each sense

48. Anatomy of the Ear The ear is divided into three areas
External (outer) ear
Middle ear (tympanic cavity)
Inner ear (bony labyrinth)

49. External (outer) ear Middle ear Internal (inner) ear Vestibulocochlear
nerve
Auricle (pinna)
Semicircular
canals
Oval window
Cochlea
Vestibule
Round window
Pharyngotympanic
(auditory) tube
Tympanic
membrane
(eardrum)
Hammer (malleus)
Anvil (incus)
Stirrup (stapes)
External acoustic
meatus
(auditory canal)
Auditory ossicles
Figure 8.12

50. The External Ear Involved in hearing only
Structures of the external ear
Auricle (pinna)
External acoustic meatus (auditory canal)
Narrow chamber in the temporal bone
Lined with skin and ceruminous (wax) glands
Ends at the tympanic membrane

51. The Middle Ear (Tympanic Cavity)
Air-filled cavity within the temporal bone
Only involved in the sense of hearing

52. The Middle Ear (Tympanic Cavity)
Two tubes are associated with the inner 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

53. 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 hammer  anvil  stirrup  inner ear

54. External (outer) ear Middle ear Internal (inner) ear Vestibulocochlear
nerve
Auricle (pinna)
Semicircular
canals
Oval window
Cochlea
Vestibule
Round window
Pharyngotympanic
(auditory) tube
Tympanic
membrane
(eardrum)
Hammer (malleus)
Anvil (incus)
Stirrup (stapes)
External acoustic
meatus
(auditory canal)
Auditory ossicles
Figure 8.12

55. Inner Ear or Bony Labyrinth
Includes sense organs for hearing and balance
Filled with perilymph (plasma like fluid)
Contains a maze of bony chambers within the temporal bone
Cochlea
Vestibule
Semicircular canals

56. External (outer) ear Middle ear Internal (inner) ear Vestibulocochlear
nerve
Auricle (pinna)
Semicircular
canals
Oval window
Cochlea
Vestibule
Round window
Pharyngotympanic
(auditory) tube
Tympanic
membrane
(eardrum)
Hammer (malleus)
Anvil (incus)
Stirrup (stapes)
External acoustic
meatus
(auditory canal)
Auditory ossicles
Figure 8.12

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

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

59. Static Equilibrium
Changes in position of the head in space with respect to the pull of gravity when the body is not moving
Maculae  receptors in the vestibule
Report on the position of the head
Send information via the vestibular nerve

60. 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

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

62. Dynamic Equilibrium
These receptors respond to angular or rotary movements
Crista ampullaris (in the ampulla of each semicircular canal)—dynamic equilibrium receptors are located in the semicircular canals
Tuft of hair cells covered with cupula (gelatinous cap)
If the head moves, the cupula drags against the endolymph

63. 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

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

65. Mechanism of Hearing
Vibrations from sound waves move tectorial membrane
Hair cells are bent by the membrane
An action potential starts in the cochlear nerve
Impulse travels to the temporal lobe
Continued stimulation can lead to adaptation

66. 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

67. Mechanism of Hearing
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

68. 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

69. Homeostatic Imbalance
Deafness
Hearing loss of any degree (slight loss to total inability to hear sound)
Conduction Deafness
Results when something interferes with the conduction of sound vibrations (earwax)
Sensorineural Deafness
Degeneration or damage to the receptor cells in the spiral organ of Corti, cochlear nerve, or neurons of auditory cortex (extended listening to excessively loud sounds)

70. Olfaction  The Sense of Smell
Olfactory receptors are in the roof of the nasal cavity
Neurons with long cilia
Chemicals must be dissolved in mucus for detection
Impulses are transmitted via the olfactory nerve
Interpretation of smells is made in the cortex

71. 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

72. The Sense of Taste Taste buds house the receptor organs
Location of taste buds  specific receptors for the sense of taste
Most are on the tongue
Soft palate
Cheeks

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

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

75. The Tongue and Taste
The tongue is covered with projections called papillae
Filiform papillae  sharp with no taste buds
Fungiform papillae  rounded with taste buds
Circumvallate papillae  large papillae with taste buds
Taste buds are found on the sides of papillae

76. Structure of Taste Buds
Gustatory cells are the receptors
Have gustatory hairs (long microvilli)
Hairs are stimulated by chemicals dissolved in saliva

77. Structure of Taste Buds
Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas
Facial nerve
Glossopharyngeal nerve
Vagus nerve

78. Taste Sensations Sweet receptors (sugars) Saccharine Some amino acids
Sour receptors
Acids
Bitter receptors
Alkaloids
Salty receptors
Metal ions

79. Chemical Senses: Taste and Smell
Both senses use chemoreceptors
Stimulated by chemicals in solution
Taste has four types of receptors
Smell can differentiate a large range of chemicals
Both senses complement each other and respond to many of the same stimuli