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Chapter 16
The Special Senses
• Smell, taste, vision, hearing and
equilibrium
• Housed in complex sensory organs
• Ophthalmology is science of the eye
• Otolaryngology is science of the ear
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Chemical Senses
• Interaction of molecules with receptor cells
• Olfaction (smell) and gustation (taste)
• Both project to cerebral cortex & limbic
system
– evokes strong emotional reactions
2
Olfactory Epithelium
• 1 square inch of
•
•
membrane holding 10100 million receptors
Covers superior nasal
cavity and cribriform
plate
3 types of receptor
cells
3
Olfaction: Sense of Smell
• Odorants bind to
•
•
•
receptors
Na+ channels open
Depolarization
occurs
Nerve impulse is
triggered
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Adaptation & Odor Thresholds
• Adaptation = decreasing sensitivity
• Olfactory adaptation is rapid
– 50% in 1 second
– complete in 1 minute
• Low threshold
– only a few molecules need to be present
– methyl mercaptan added to natural gas as
warning
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Gustatory Sensation: Taste
• Taste requires dissolving
•
•
of substances
Four classes of stimuli-sour, bitter, sweet, and
salty
10,000 taste buds found
on tongue, soft palate &
larynx
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Anatomy of Taste Buds
• An oval body
•
•
consisting of 50
receptor cells
surrounded by
supporting cells
A single gustatory hair
projects upward
through the taste pore
Basal cells develop
into new receptor cells
every 10 days.
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Physiology of Taste
• Complete adaptation in 1 to 5 minutes
• Thresholds for tastes vary among the 4
primary tastes
– most sensitive to bitter (poisons)
– least sensitive to salty and sweet
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Accessory Structures of Eye
• Eyelids or palpebrae
– protect & lubricate
• Tarsal glands
– oily secretions keep lids
from sticking together
• Conjunctiva
– stops at corneal edge
– dilated BV--bloodshot
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Eyelashes & Eyebrows
Eyeball = 1
inch diameter
5/6 of Eyeball
inside orbit &
protected
• Eyelashes & eyebrows help protect from foreign
•
•
objects, perspiration & sunlight
Sebaceous glands are found at base of
eyelashes (sty)
Palpebral fissure is gap between the eyelids
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Lacrimal Apparatus
• About 1 ml of tears produced per day. Spread over
eye by blinking. Contains bactericidal enzyme called11
Extraocular Muscles
• Six muscles that
•
•
•
insert on the
exterior surface of
the eyeball
.
4 rectus muscles -superior, inferior,
lateral and medial
2 oblique muscles -inferior and superior
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Tunics (Layers) of Eyeball
• Fibrous Tunic
•
•
(outer layer)
Vascular Tunic
(middle layer)
Nervous Tunic
(inner layer)
13
Fibrous Tunic -- Description of
Cornea
• Transparent
• Helps focus light(refraction)
– astigmatism
• Transplants
– common & successful
– no blood vessels so no antibodies to cause rejection
14
Fibrous Tunic -- Description of
Sclera
• “White” of the eye
• Dense irregular
•
connective tissue layer -collagen & fibroblasts
Provides shape &
support
15
Vascular Tunic -- Choroid & Ciliary Body
• Choroid
– pigmented epithilial cells
(melanocytes) & blood
vessels
– provides nutrients to retina
– black pigment in
melanocytes absorb
scattered light
• Ciliary body
– ciliary processes
• folds on ciliary body
• secrete aqueous humor
– ciliary muscle
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• smooth muscle that alters
Vascular Tunic -- Iris & Pupil
• Colored portion of eye
• Shape of flat donut
•
•
suspended between
cornea & lens
Hole in center is pupil
Function is to regulate
amount of light entering
eye
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Vascular Tunic -- Description of lens
• Avascular
• Crystallin proteins
•
arranged like layers
in onion
Clear capsule &
perfectly transparent
18
Nervous Tunic -- Retina
• Posterior 3/4 of
•
eyeball
Optic disc
– optic nerve exiting
back of eyeball
• Central retina BV
– fan out to supply
nourishment to retina
– visible for inspection
• hypertension &
diabetes
View with Ophthalmoscope
• Detached retina
– trauma (boxing)
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• fluid between layers
Rods & Cones--Photoreceptors
• Rods----rod shaped
– shades of gray in dim light
– 120 million rod cells
– discriminates shapes &
movements
– distributed along
periphery
• Cones----cone shaped
– sharp, color vision
– 6 million
– fovea of macula lutea
• densely packed region
• at exact visual axis of eye
• 2nd cells do not cover 20
cones
Pathway of Nerve Signal in Retina
• Light penetrates retina
• Rods & cones
•
•
•
transduce light into
action potentials
Rods & cones excite
bipolar cells
Bipolars excite ganglion
cells
Axons of ganglion cells
form optic nerve
leaving the eyeball
(blind spot)
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Aqueous Humor
• Continuously produced
•
•
by ciliary body
Flows from posterior chamber
into anterior through the pupil
Glaucoma
– increased intraocular pressure that could produce
blindness
– problem with drainage of aqueous humor
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Major Processes of Image
Formation
• Refraction of light
– by cornea & lens
– light rays must fall upon the retina
• Accommodation of the lens
– changing shape of lens so that light is focused
• Constriction of the pupil
– less light enters the eye
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Definition of Refraction
• Bending of light as it passes from one substance
•
(air) into a 2nd substance with a different
density(cornea)
In the eye, light is refracted by the anterior &
posterior surfaces of the cornea and the lens
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Refraction by the Cornea &
Lens
• Image focused on retina is inverted
•
•
& reversed from left to right
Brain learns to work with that
information
75% of Refraction is done by
cornea -- rest is done by the lens
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Near Point of Vision and Presbyopia
• Near point is the closest distance from the
eye an object can be & still be in clear
focus
– 4 inches in a young adult
– 8 inches in a 40 year old
• lens has become less elastic
– 31 inches in a 60 to 80 year old
• Reading glasses may be needed by age
40
– presbyopia
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– glasses replace refraction previously provided
Correction for Refraction Problems
• Emmetropic eye (normal)
– can refract light from 20 ft
away
• Myopia (nearsighted)
– eyeball is too long from front
to back
– glasses concave
• Hypermetropic (farsighted)
– eyeball is too short
– glasses convex (coke-bottle)
• Astigmatism
– corneal surface wavy
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– parts of image out of focus
onstriction of the Pupil
• Constrictor pupillae muscle contracts
• Prevents light rays from entering the eye
through the edge of the lens
• Sharpens vision by preventing blurry
edges
• Protects retina very excessively bright light
28
nvergence of the Eyes
• Binocular vision in humans has both eyes
looking at the same object
• As you look at an object close to your
face, both eyeballs must turn inward.
– convergence
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Photoreceptors
• Photopigment is integral membrane
protein of outer segment
membrane
– photopigment membrane folded into
“discs” & replaced at a very rapid rate
• Photopigments = opsin (protein) +
retinal (derivative of vitamin A)
– rods contain rhodopsin
– cone photopigments contain 3
different opsin proteins permitting the
absorption of 3 different wavelengths
(colors) of light
30
Color Blindness & Night Blindness
• Color blindness
– inability to distinguish between certain colors
– absence of certain cone photopigments
– red-green color blind person can not tell red from
green
• Night blindness (nyctalopia)
– difficulty seeing in low light
– inability to make normal amount of rhodopsin
– possibly due to deficiency of vitamin A
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Light and Dark Adaptation
• Light adaptation
– adjustments when emerge from the dark into the
light
• Dark adaptation
– adjustments when enter the dark from a bright
situation
– light sensitivity increases as photopigments
regenerate
• during first 8 minutes of dark adaptation, only cone
pigments are regenerated, so threshold burst of light is
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seen as color
Brain Pathways of Vision
synapse in thalamus
& visual cortex
33
Processing of Image Data in the
Brain
• Visual information in optic nerve travels to
– occipital lobe for vision
– midbrain for controlling pupil size &
coordination of head and eye movements
– hypothalamus to establish sleep patterns
based upon circadian rhythms of light and
darkness
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Visual fields
• Left occipital lobe
•
•
receives visual images
from right side of an
object through
impulses from nasal
1/2 of the right eye
and temporal 1/2 of
the left eye
Left occipital lobe sees
right 1/2 of the world
Fibers from nasal 1/2
of each retina cross in35
Anatomy of the Ear Region
36
External Ear
• Function = collect sounds
• Structures
– auricle or pinna
• elastic cartilage covered with skin
– external auditory canal
• curved 1” tube of cartilage & bone leading into temporal
bone
• ceruminous glands produce cerumen = ear wax
– tympanic membrane or eardrum
• epidermis, collagen & elastic fibers, simple cuboidal epith.
• Perforated eardrum (hole is present)
– at time of injury (pain, ringing, hearing loss,
dizziness)
37
Middle Ear Cavity
38
Middle Ear Cavity
• Air filled cavity in the temporal bone
• Separated from external ear by
eardrum and from internal ear by
oval & round window
• 3 ear ossicles connected by synovial joints
– malleus attached to eardrum, incus & stapes
attached by foot plate to membrane of oval
window
– stapedius and tensor tympani muscles attach to
ossicles
• Auditory tube leads to nasopharynx
– helps to equalize pressure on both sides of
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Inner Ear---Bony Labyrinth
Vestibule
canals
ampulla
• Bony labyrinth = set of tubelike cavities in temporal
bone
– semicircular canals, vestibule & cochlea lined with
periosteum & filled with perilymph
– surrounds & protects Membranous Labyrinth
40
Inner Ear---Membranous
Labyrinth
• Membranous labyrinth = set of membranous tubes
containing sensory receptors for hearing & balance
and filled with endolymph
– utricle, saccule, ampulla, 3 semicircular ducts & cochlea
41
Anatomy of the Organ of Corti
• 16,000 hair cells have 30-100 stereocilia(microvilli )
• Microvilli make contact with tectorial membrane
•
(gelatinous membrane that overlaps the spiral organ of
Corti)
Basal sides of inner hair cells synapse with 1st order
42
Sound Waves
• Vibrating object causes compression of air
around it = sound waves
– audible range is 20 to 20,000 Hz
– hear best within 500 to 5000 cycles/sec or Hz
– speech is 100 to 3000 Hz
• Frequency of a sound vibration is pitch
– higher frequency is higher pitch
• Greater intensity (size) of vibration, the
louder the sound measured in decibels (dB)
– Conversation is 60 dB; pain above 140dB
– OSA requires ear protection above 90dB
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Deafness
• Nerve deafness
– damage to hair cells from antibiotics, high
pitched sounds, anticancer drugs
• the louder the sound the quicker the hearing loss
– fail to notice until difficulty with speech
• Conduction deafness
– perforated eardrum
– otosclerosis
44
Physiology of Hearing
• Auricle collects sound waves
• Eardrum vibrates
– slow vibration in response to low-pitched sounds
– rapid vibration in response to high-pitched sounds
• Ossicles vibrate since malleus attached to
eardrum
• Stapes pushes on oval window producing fluid
pressure waves in scala vestibuli & tympani
– oval window vibration 20X more vigorous than
eardrum
• Pressure fluctuations inside cochlear duct
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Overview of Physiology of Hearing
46
Cochlear Implants
• If deafness is due to destruction of hair
cells
• Microphone, microprocessor &
electrodes translate sounds into electric
stimulation of the vestibulocochlear
nerve
– artificially induced nerve signals follow
normal pathways to brain
• Provides only a crude representation of
47
Physiology of Equilibrium (Balance)
• Static equilibrium
– maintain the position of the body (head)
relative to the force of gravity
• Dynamic equilibrium
– maintain body position (head) during sudden
movement of any type--rotation, deceleration or
acceleration
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Detection of Position of Head
• Movement of stereocilia or kinocilium results in the
release of neurotransmitter onto the vestibular
branches of the vestibulocochler nerve
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Detection of Rotational Movement
• When head moves, the attached semicircular ducts
•
and hair cells move with it
Nerve signals to the brain are generated indicating
which direction the head has been rotated
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