1. The Special Senses: Part A15
The Special Senses: Part A

2. The Eye and Vision
70% of all sensory receptors are in the eye
Nearly half of the cerebral cortex is involved in processing visual information!
Most of the eye is protected by a cushion of fat and the bony orbit

3. (a) Surface anatomy of the right eye
Eyebrow
Eyelid
Eyelashes
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Lateral
commissure
Iris
Eyelid
Pupil
Sclera
(covered by
conjunctiva)
Lacrimal
caruncle
Medial
commissure
(a) Surface anatomy of the right eye
Figure 15.1a

4. Overlie the supraorbital margins Function in
Eyebrows
Overlie the supraorbital margins
Function in
Shading the eye
Preventing perspiration from reaching the eye

5. Eyelids
Protect the eye anteriorly
Palpebral fissure—slit separating eyelids
Medial & Lateral Canthi – eye angles
Caruncle—fleshy elevation at medial canthus; contains oil and sweat glands
Tarsal plates—internal supporting C.T. sheet
Levator palpebrae superioris—gives the upper eyelid mobility

6. (a) Surface anatomy of the right eye
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Lateral
commissure
Eyelid
Pupil
Sclera
(covered by
conjunctiva)
Lacrimal
caruncle
Medial
commissure
(a) Surface anatomy of the right eye
Figure 15.1a

7. Lubricating glands associated with the eyelids
Eyelashes
Nerve endings of follicles initiate reflex blinking
Lubricating glands associated with the eyelids
Tarsal glands-sebaceous glands that produce oily secretion to lubricate the eye

8. Mucous membranes of the eye
Conjunctiva
Mucous membranes of the eye
Palpebral conjunctiva: lines the eyelids
Bulbar conjunctiva: covers the white of the eyes (anteriorly)

9. Levator palpebrae
superioris muscle
Orbicularis oculi muscle
Tarsal plate
Palpebral conjunctiva
Tarsal glands
Cornea
Palpebral fissure
Bulbar conjunctiva
Orbicularis oculi muscle
(b) Lateral view; some structures shown in sagittal section

10. Lacrimal Apparatus
Consists of: Lacrimal gland and ducts that connect to nasal cavity
Releases: dilute salt solution (tears)
The solution also contains:mucus, antibodies, and lysozyme (bacteria destroying enzyme)
Blinking spreads the tears towards the medial canthus, then into lacrimal puncta to lacrimal canaliculi then drain into nasolacrimal duct

11. Lacrimal sac Lacrimal gland Lacrimal punctum Lacrimal canaliculus
Nasolacrimal duct
Figure 15.2

12. Extrinsic Eye Muscles
The movement of the eye is controlled by six muscles on the external surface of each eye:
Superior rectus: elevates eye
Inferior rectus: depresses eye
Lateral rectus: moves eye laterally
Medial rectus: moves eye medially
Inferior oblique: moves eye up and out
Superior oblique: moves eye down and out

13. (a) Lateral view of the right eye
Superior oblique
muscle
Superior oblique
tendon
Superior rectus
muscle
Lateral rectus
muscle
Inferior rectus
muscle
Inferior oblique
muscle
(a) Lateral view of the right eye
Figure 15.3a

14. (b) Superior view of the right eye
Trochlea
Superior oblique
muscle
Superior oblique
tendon
Axis at center
of eye
Superior rectus
muscle
Inferior
rectus muscle
Medial
rectus muscle
Lateral
rectus muscle
Common
tendinous ring
(b) Superior view of the right eye
Figure 15.3b

15. Extrinsic Eye Muscles
The innervations to each muscle can be remembered by the following equation: (LR6SO4)O3
Which means:
Lateral rectus: controlled by CN # 6 (Abducens)
Superior oblique: controlled by CN # 4 (Trochelar)
All others: controlled by CN # 3 (oculomotor)

16. Structure of the Eyeball
The eyeball is composed of three layers
Fibrous layer
Vascular layer
Sensory layer

17. Anterior
segment (contains
aqueous humor)
Lens
Posterior segment
(contains vitreous humor)

18. Outermost layer of dense avascular CT Two regions: sclera and cornea
Fibrous Layer
Outermost layer of dense avascular CT
Two regions: sclera and cornea
1. Sclera:
Opaque posterior region
2. Cornea:
Transparent anterior part

19. Three regions: choroid, ciliary body, and iris
Vascular Layer
Middle layer
Three regions: choroid, ciliary body, and iris
Choroid region
Posterior portion
Supplies blood to most of eyeball
Brown pigment absorbs light to prevent its scattering

20. Vascular Layer 2. Ciliary body
Ring of smooth muscle attached to and surrounding the lens
Ciliary zonule (suspensory ligament): holds lens in position

21. Vascular Layer 3. Iris (smooth muscle)
The anterior colored part of the eye
Pupil: central opening of iris; the iris regulates the amount of light entering the eye
Close vision and bright light- pupils constrict
Distant and dim light- pupils dilate

22. Pupillary Sphincter
muscle contraction
decreases pupil size.
Iris (two muscles)
• Sphincter pupillae
• Dilator pupillae
Dilator
muscle contraction
increases pupil size.

23. Ciliary body Ciliary zonule (suspensory ligament) Sclera Choroid Iris
Pupil
Optic nerve
Lens
Optic disc
(blind spot)
Figure 15.4a

24. Sensory Layer (The Retina)
Delicate two-layered membrane
Pigmented layer -Outer layer that absorbs light and prevents its scattering
Neural layer – contains photoreceptor cells (rods and cones), bipolar cells (connect photoreceptors to ganglion cells), and ganglion cells (axons form optic nerve)

25. (a) Posterior aspect of the eyeball
Pathway of light
Neural layer of retina
Pigmented
layer of
retina
Choroid
Sclera
Optic disc
Central artery
and vein of retina
Optic
nerve
(a) Posterior aspect of the eyeball
Figure 15.6a

26. Pathway of signal output Pigmented layer of retina Pathway of light
Photoreceptors
Bipolar
cells
Ganglion
cells
• Rod
• Cone
Amacrine cell
Horizontal cell
Pathway of signal output
Pigmented
layer of retina
Pathway of light
(b) Cells of the neural layer of the retina
Figure 15.6b

27. Photoreceptors
Rods
More numerous at peripheral region of retina, away from the fovea centralis (area of retina containing only cones)
Operate in dim light
Provide indistinct, fuzzy, non color peripheral vision

28. Photoreceptors
Cones
Densest in center of retina and concentrated in an area called the fovea centralis (next to blind spot)
Operate in bright light
Provide high-acuity color vision

29. Photoreceptors
Photoreceptor cells are distributed over entire retina except where the optic nerve leaves the eye called the blind spot

30. Cones
There are three types of cones named for the colors of light absorbed: blue, green, and red
Intermediate hues are perceived by activation of more than one type of cone at the same time
Color blindness is due to a congenital lack of one or more of the cone types

31. Central artery and vein emerging from the optic disc Macula lutea
Retina
Figure 15.7

32. Internal Chambers and Fluids
The lens and ciliary zonule separate the anterior and posterior segments
Posterior segment contains vitreous humor that:
Is a clear gel like substance which helps reinforce the eye internally

33. (contains vitreous humor) Optic disc (blind spot)
Ciliary body
Ciliary zonule
(suspensory
ligament)
Sclera
Choroid
Retina
Macula lutea
Fovea centralis
Pupil
Posterior pole
Optic nerve
Anterior
segment (contains
aqueous humor)
Lens
Posterior segment
(contains vitreous humor)
Optic disc
(blind spot)
Figure 15.4a

34. Internal Chambers and Fluids
Anterior segment is composed of two chambers
Anterior chamber—between cornea and iris
Posterior chamber—between iris and lens

35. Internal Chambers and Fluids
Anterior segment contains aqueous humor
Watery fluid continuously produced and drained
Glaucoma: compression of the retina and optic nerve if drainage of aqueous humor is blocked

36. Iris Posterior segment (contains Lens vitreous humor) Cornea2
Ciliary zonule
(suspensory
ligament)
Aqueous humor
Anterior
chamber
Anterior
segment
(contains
aqueous
humor)
Posterior
chamber
Ciliary body 3 1
Scleral venous
sinus
Ciliary
processes
Corneal-
scleral junction
Ciliary
muscle
Cornea
Lens
Figure 15.8

37. Lens Biconvex, transparent, flexible,and avascular
Allows precise focusing of light on the retina
Cataracts (clouding of lens) occur as a consequence of aging, diabetes mellitus, heavy smoking, and frequent exposure to intense sunlight

38. Figure 15.9

39. (contains vitreous humor) Optic disc (blind spot)
Ora serrata
Ciliary body
Ciliary zonule
(suspensory
ligament)
Sclera
Choroid
Retina
Cornea
Macula lutea
Iris
Fovea centralis
Pupil
Posterior pole
Optic nerve
Anterior pole
Anterior
segment (contains
aqueous humor)
Lens
Scleral venous
sinus
Central artery
and vein of
the retina
Posterior segment
(contains vitreous humor)
Optic disc
(blind spot)
(a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
Figure 15.4a

40. Refraction and Lenses Refraction
Bending of a light ray when light passes from one transparent medium to another

41. Refraction and Lenses
Light passing through a convex lens is bent so that the rays converge at a focal point
The image formed at the focal point is upside-down and reversed right to left

42. (a) Focusing of two points of light.
Point sources
Focal points
(a) Focusing of two points of light.
(b) The image is inverted—upside down and reversed.
Figure 15.12

43. Focusing Light on the Retina
Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, neural layer of retina, photoreceptors
Light is refracted
At the cornea
Entering the lens
Leaving the lens
Change in lens curvature allows for fine focusing of an image

44. Focusing for Distant Vision
Light rays from distant objects are parallel and need little refraction
Ciliary muscles are relaxed
Lens is stretched flat by tension in the ciliary zonule

45. Sympathetic activation Nearly parallel rays from distant object
Lens
Ciliary zonule
Ciliary muscle
Inverted
image
(a) Lens is flattened for distant vision.
Figure 15.13a

46. Focusing for Close Vision
Light from a close object diverges as it approaches the eye; requires that the eye make three active adjustments

47. Focusing for Close Vision
Close vision requires
Accommodation—changing the lens shape by ciliary muscles to increase refractory power
Presbyopia—loss of accommodation over age 50
Constriction—constriction of pupils to prevent the most divergent rays from entering eye
Convergence—medial rotation of the eyeballs toward the object being viewed

48. Parasympathetic activation Divergent rays from close object Inverted
image
(b) Lens bulges for close vision. Parasympathetic
input contracts the ciliary muscle, loosening the
ciliary zonule, allowing the lens to bulge.
Figure 15.13b

49. Problems of Refraction
Myopia (nearsightedness)—focal point in front of retina
Hyperopia (farsightedness)—focal point behind retina
Astigmatism—caused by unequal curvatures in different parts of the cornea or lens

50. Emmetropic eye (normal)
Focal
plane
Focal point is on retina.
Figure (1 of 3)

51. Myopic eye (nearsighted)
Eyeball
too long
Uncorrected
Focal point is in front of retina.
Concave lens moves focal
point further back.
Corrected

52. Hyperopic eye (farsighted)
Eyeball
too short
Uncorrected
Focal point is behind retina.
Convex lens moves focal
point forward.
Corrected
Figure (3 of 3)

53. Functional Anatomy of Photoreceptors
Rods and cones
Outer segment of each contains visual pigments -molecules that change shape as they absorb light
Once light is absorbed by the visual pigments, this leads to a series of chemical reactions which results in an AP along the optic nerve

54. Process of bipolar cell Synaptic terminals Rod cell body Nuclei
segment
Inner
Discs containing
visual pigments
Outer segment
Pigmented layer
Melanin
granules
(border
with choroid)

55. In the dark Photoreceptor cell (rod) Bipolar cell Ganglion cell
Na+
Ca2+
Photoreceptor
cell (rod)
Ca2+
Bipolar
cell
Ganglion
cell
No action potentials occur
along the optic nerve. 7

56. In the Light Photoreceptor cell (rod) Bipolar cell Ganglion cell
Na+
Ca2+
Photoreceptor
cell (rod)
Ca2+
Bipolar
cell
Ganglion
cell
Action potentials occur
along the optic nerve. 7

57. The Visual Pathway
Axons of ganglion cells form the Optic Nerve Optic chiasma optic tracts thalamus primary visual cortex in the occipital lobe of the brain conscious perception of the image

58. (primary visual cortex)
Fixation point
Right eye
Left eye
Optic nerve
Optic chiasma
Optic tract
thalamus
Occipital lobe
(primary visual cortex)

59. Hearing and balance

60. The Ear
Three parts of the ear
External ear
Middle ear
Internal ear

61. The Ear: Hearing and Balance
External and middle ear are involved with: hearing only
Internal ear (labyrinth) functions in both: hearing and equilibrium

62. (a) The three regions of the ear
Middle
ear
Internal ear
(labyrinth)
External
ear
Auricle
(pinna)
Helix
Lobule
External
acoustic
meatus
Tympanic
membrane
Pharyngotympanic
(auditory) tube
(a) The three regions of the ear
Figure 15.25a

63. The auricle (pinna) is composed of:
External Ear
The auricle (pinna) is composed of:
Helix (rim)
Lobule (earlobe)
External acoustic meatus (auditory canal)
Short, curved tube lined with skin bearing hairs, and ceruminous glands

64. Tympanic membrane (eardrum)
Middle Ear
Tympanic membrane (eardrum)
Boundary between external and middle ear
CT membrane that vibrates in response to sound
Transfers sound energy to bones of middle ear

65. Pharyngotympanic tube—connects the middle ear to the nasopharynx
Equalizes pressure in the middle ear cavity with the external air pressure

66. Oval window
(deep to stapes)
Semicircular
canals
Malleus
Vestibule
Incus
Auditory
ossicles
Vestibular
nerve
Stapes
Cochlear
nerve
Tympanic membrane
Cochlea
Round window
Pharyngotympanic
(auditory) tube
(b) Middle and internal ear

67. Three small bones in middle ear cavity: the malleus, incus, and stapes
Ear Ossicles
Three small bones in middle ear cavity: the malleus, incus, and stapes
Transmit vibratory motion of the eardrum to the oval window
Tensor tympani and stapedius muscles contract reflexively in response to loud sounds to prevent damage to the hearing receptors

68. Epitympanic recess Malleus Incus Superior Lateral Anterior View
Pharyngotym-
panic tube
Tensor
tympani
muscle
Tympanic
membrane
(medial view)
Stapes
Stapedius
muscle
Figure 15.26

69. Internal Ear Bony labyrinth Winding channels in the temporal bone
Three parts: vestibule, semicircular canals, and cochlea
Filled with perilymph
Series of membranous sacs within the bony labyrinth
Filled with endolymph

70. Superior vestibular ganglion
Inferior vestibular ganglion
Temporal
bone
Semicircular
ducts in
semicircular
canals
Facial nerve
Vestibular
nerve
Anterior
Posterior
Lateral
Cochlear
nerve
Cristae ampullares
in the membranous
ampullae
Maculae
Spiral organ
(of Corti)
Utricle in
vestibule
Cochlear
duct
in cochlea
Saccule in
vestibule
Stapes in
oval window
Round
window
Figure 15.27

71. Vestibule Central egg-shaped cavity of the bony labyrinth
Contains two membranous sacs that:
House equilibrium receptor regions called maculae
Maculae respond to pull of gravity and changes in head position (linear movment)

72. Otolithic membrane Kinocilium Stereocilia Hyperpolarization Receptor
potential
Depolarization
Nerve impulses
generated in
vestibular fiber
When hairs bend toward
the kinocilium, the hair
cell depolarizes, exciting
the nerve fiber, which
generates more frequent
action potentials.
When hairs bend away
from the kinocilium, the
hair cell hyperpolarizes,
inhibiting the nerve fiber,
and decreasing the action
potential frequency.
Figure 15.35

73. Semicircular Canals
Three canals are oriented along all three planes (x,y,z)
Membranous semicircular ducts line each canal
Ampulla of each canal houses equilibrium receptor region called the crista ampullaris
Receptors respond to rotational movements of the head

74. Crista
ampullaris
Endolymph
Hair cell
Crista
ampullaris
Supporting
cell
(a) Anatomy of a crista ampullaris in a
semicircular canal
Cupula
(b) Scanning electron
micrograph of a
crista ampullaris
(200x)

75. Equilibrium
Impulses sent from the vestibule (maculae) or from ampulla (crista ampullaris) travel along the vestibular nerve which quickly merges with the cochlear nerve to form the vestibulocochlear nerve (CN #8)

76. Temporal
bone
Semicircular
ducts in
semicircular
canals
Vestibular
nerve
Anterior
Posterior
Lateral
Cochlear
nerve
Cristae ampullares
in the membranous
ampullae
Maculae
Spiral organ
(of Corti)
Utricle in
vestibule
Cochlear
duct
in cochlea
Saccule in
vestibule
Stapes in
oval window
Round
window

77. A spiral, conical, bony chamber
The Cochlea
A spiral, conical, bony chamber
Extends from the vestibule
Contains the cochlear duct, which houses the spiral organ (of Corti)

78. Sound and the Cochlea Organ of Corti
In cochlear duct which runs through center of cochlea
Has hair cells(nerve cells) and supporting cells
Tectorial membrane- gel-like mass that cilia of hair cells are embedded in
Basilar membrane-fibrous “floor” of organ of corti
Bending of the cilia: excites hair cells

79. Tectorial membrane Inner hair cell Hairs (stereocilia) Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
(c)
Figure 15.28c

80. Sounds with frequencies below hearing travel through
Auditory ossicles
Malleus
Incus
Stapes
Cochlear nerve
Scala vestibuli
Oval
window
Helicotrema
Scala tympani
Cochlear duct 2 3
Basilar
membrane 1
Sounds with frequencies
below hearing travel through
the helicotrema and do not
excite hair cells.
Tympanic
membrane
Round
window
Sounds in the hearing range
go through the cochlear duct,
vibrating the basilar membrane
and deflecting hairs on inner
hair cells.
(a) Route of sound waves through the ear
Sound waves vibrate
the tympanic membrane. 1
Pressure waves created by
the stapes pushing on the oval
window move through fluid in
the scala vestibuli. 3
Auditory ossicles vibrate.
Pressure is amplified. 2
Figure 15.31a

81. Transmission of Sound to the Internal Ear
Transmission of Sound to the Inner Ear
Sound waves enter the external acoustic canal and cause tympanic membrane to vibrate
Ossicles vibrate and amplify the pressure at the oval window
Pressure waves move through perilymph
Sounds in the hearing range go through the cochlear duct, ultimately causing bending of hair cells
Impulses from the cochlea pass: along the cochlear nerve, which then merges with the vestibular nerve (forming vestibulocochlear N.) that travels to the primary auditory cortex in temporal lobe of brain

82. Deafness Hearing loss can be temporary or permanent
Common causes:
Middle ear infections
Conduction deafness
Can be caused by:
Impacted earwax
Ruptured eardrum
Middle ear inflammations
Otosclerosis

83. Deafness Nerve Deafness Can be caused by:
Gradual loss of hair cells throughout life
Single explosive loud noise
Prolonged exposure to loud noise
Degeneration of cochlear nerve, tumors in auditory cortex, etc.

84. Tinnitus
Ringing or clicking sound in ears in the absence of auditory stimuli
One of the first symptoms of cochlear degeneration
Can be caused by middle ear inflammation