Transcript No Slide Title

Chapter 16
The Special Senses
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•
•
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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
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Olfactory Epithelium
• 1 square inch of
membrane holding 10100 million receptors
• Covers superior nasal
cavity and cribriform
plate
• 3 types of receptor cells
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Cells of the Olfactory Membrane
• Olfactory receptors
– bipolar neurons with cilia or
olfactory hairs
• Supporting cells
– columnar epithelium
• Basal cells = stem cells
– replace receptors monthly
• Olfactory glands
– produce mucus
• Both epithelium & glands
innervated cranial nerve VII.
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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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Olfactory Pathway
• Axons from olfactory receptors form the olfactory
nerves (Cranial nerve I) that synapse in the
olfactory bulb
– pass through 40 foramina in cribriform plate
• Second-order neurons within the olfactory bulb
form the olfactory tract that synapses on primary
olfactory area of temporal lobe
– conscious awareness of smell begins
• Other pathways lead to the frontal lobe
(Brodmann area 11) where identification of the
odor occurs
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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
• Found on sides of
circumvallate & fungiform
papillae
• 3 cell types: supporting,
receptor & basal cells
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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
• Mechanism
– dissolved substance contacts gustatory hairs
– receptor potential results in neurotransmitter release
– nerve impulse formed in 1st-order neuron
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Gustatory Pathway
• First-order gustatory fibers found in cranial nerves
– VII (facial) serves anterior 2/3 of tongue
– IX (glossopharyngeal) serves posterior 1/3 of tongue
– X (vagus) serves palate & epiglottis
• Signals travel to thalamus or limbic system &
hypothalamus
• Taste fibers extend from the thalamus to the
primary gustatory area on parietal lobe of the
cerebral cortex
– providing conscious perception of taste
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Accessory Structures of Eye
• Eyelids or palpebrae
– protect & lubricate
– epidermis, dermis, CT,
orbicularis oculi m., tarsal
plate, tarsal glands &
conjunctiva
• Tarsal glands
– oily secretions keep lids
from sticking together
• Conjunctiva
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– palpebral & bulbar
– 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 called lysozyme.
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Extraocular Muscles
• Six muscles that insert
on the exterior surface
of the eyeball
• Innervated by CN III,
IV or VI.
• 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)
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Fibrous Tunic -- Description of Cornea
• Transparent
• Helps focus light(refraction)
– astigmatism
• 3 layers
– nonkeratinized stratified squamous
– collagen fibers & fibroblasts
– simple squamous epithelium
• Transplants
– common & successful
– no blood vessels so no antibodies to cause rejection
• Nourished by tears & aqueous humor
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Fibrous Tunic -- Description of Sclera
• “White” of the eye
• Dense irregular connective
tissue layer -- collagen &
fibroblasts
• Provides shape & support
• At the junction of the sclera
and cornea is an opening
(scleral venous sinus)
• Posteriorly pierced by
Optic Nerve (CNII)
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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 shape
of lens
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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
• Autonomic reflexes
– circular muscle fibers contract
in bright light to shrink pupil
– radial muscle fibers contract in
dim light to enlarge pupil
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Vascular Tunic -- Muscles of the Iris
• Constrictor pupillae (circular) are innervated by
parasympathetic fibers while Dilator pupillae
(radial) are innervated by sympathetic fibers.
• Response varies with different levels of light
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Vascular Tunic -- Description of lens
• Avascular
• Crystallin proteins
arranged like layers in
onion
• Clear capsule &
perfectly transparent
• Lens held in place by
suspensory ligaments
• Focuses light on fovea
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Vascular Tunic -- Suspensory ligament
• Suspensory ligaments attach lens to ciliary process
• Ciliary muscle controls tension on ligaments &16-23
lens
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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
• Detached retina
View with Ophthalmoscope
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– trauma (boxing)
• fluid between layers
• distortion or blindness
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Layers of Retina
• Pigmented epithelium
– nonvisual portion
– absorbs stray light &
helps keep image clear
• 3 layers of neurons
(outgrowth of brain)
– photoreceptor layer
– bipolar neuron layer
– ganglion neuron layer
• 2 other cell types
(modify the signal)
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– horizontal cells
– amacrine cells 16-25
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
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•
•
•
•
densely packed region
at exact visual axis of eye
2nd cells do not cover cones
sharpest resolution or acuity
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Pathway of Nerve Signal in Retina
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• 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)
• To thalamus & then the
primary visual cortex
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Cavities of the Interior of Eyeball
• Anterior cavity (anterior to lens)
– filled with aqueous humor
• produced by ciliary body
• continually drained
• replaced every 90 minutes
– 2 chambers
• anterior chamber between cornea and iris
• posterior chamber between iris and lens
• Posterior cavity (posterior to lens)
– filled with vitreous body (jellylike)
– formed once during embryonic life
– floaters are debris in vitreous of older individuals
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Aqueous Humor
• Continuously produced
by ciliary body
• Flows from posterior chamber
into anterior through the pupil
• Scleral venous sinus
– canal of Schlemm
– opening in white of eye
at junction of cornea & sclera
– drainage of aqueous humor from eye to bloodstream
• 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
• Light rays from > 20’ are nearly
parallel and only need to be bent
enough to focus on retina
• Light rays from < 6’ are more
divergent & need more refraction
– extra process needed to get additional
bending of light is called accommodation
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Accommodation & the Lens
• Convex lens refract light rays towards each other
• Lens of eye is convex on both surfaces
• View a distant object
– lens is nearly flat by pulling of suspensory ligaments
• View a close object
– ciliary muscle is contracted & decreases the pull of
the suspensory ligaments on the lens
– elastic lens thickens as the tension is removed from it
– increase in curvature of lens is called accommodation
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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
– glasses replace refraction previously provided by
increased curvature of the relaxed, youthful lens
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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
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– corneal surface wavy
– parts of image out of focus
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Constriction of the Pupil
• Constrictor pupillae muscle contracts
• Narrows beam of light that enters the eye
• 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
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Convergence 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
– required so that light rays from the object will
strike both retinas at the same relative point
– extrinsic eye muscles must coordinate this action
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Photoreceptors
• Named for shape of outer segment
• Transduction of light energy into a
receptor potential in outer segment
• 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
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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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Photopigments
• Isomerization
– light cause cis-retinal to
straighten & become transretinal shape
• Bleaching
– enzymes separate the transretinal from the opsin
– colorless final products
• Regeneration
– in darkness, an enzyme
converts trans-retinal back to
cis-retinal (resynthesis of a
photopigment)
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Regeneration of Photopigments
• Pigment epithelium near the photoreceptors
contains large amounts of vitamin A and helps
the regeneration process
• After complete bleaching, it takes 5 minutes to
regenerate 1/2 of the rhodopsin but only 90
seconds to regenerate the cone photopigments
• Full regeneration of bleached rhodopsin takes
30 to 40 minutes
• Rods contribute little to daylight vision, since
they
are
bleached
as
fast
as
they
regenerate.
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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 seen as color
• after sufficient time, sensitivity will increase so that a flash of
a single photon of light will be seen as gray-white
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Formation of Receptor Potentials
• In darkness
– Na+ channels are held open and photoreceptor is
always partially depolarized (-30mV)
– continuous release of inhibitory neurotransmitter onto
bipolar cells
• In light
– enzymes cause the closing of Na+ channels producing a
hyperpolarized receptor potential (-70mV)
– release of inhibitory neurotransmitter is stopped
– bipolar cells become excited and a nerve impulse will
travel towards the brain
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Release of Neurotransmitters
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Retinal Processing of Visual Information
• Convergence
– one cone cell synapses onto one
bipolar cell produces best visual acuity
– 600 rod cells synapse on single bipolar
cell increasing light sensitivity
although slightly blurry image results
– 126 million photoreceptors converge
on 1 million ganglion cells
• Horizontal and amacrine cells
– horizontal cells enhance contrasts in
visual scene because laterally inhibit
bipolar cells in the area
– amacrine cells excited bipolar cells if
levels of illumination change
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Brain Pathways of Vision
synapse in thalamus
& visual cortex
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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
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• 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 in optic
chiasm
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Anatomy of the Ear Region
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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)
– caused by explosion, scuba diving, or ear infection
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Middle Ear Cavity
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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 eardrum
• Connection
to
mastoid
bone
=mastoiditis
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Muscles of the Ear
• Stapedius m. inserts onto stapes
– prevents very large vibrations of stapes from loud noises
• Tensor tympani attaches to malleus
– limits movements of malleus & stiffens eardrum to prevent
damage
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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
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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
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Cranial nerves of the Ear Region
• Vestibulocochlear nerve = CN VIII
– ampullary, utricular & saccular brs. form vestibular branch
– cochlear branch has spiral ganglion in bony modiolus
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Cochlear Anatomy
• 3 fluid filled channels found within the cochlea
– scala vestibuli, scala tympani and cochlear duct
• Vibration of the stapes upon the oval window sends
vibrations into the fluid of the scala vestibuli 16-57
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Tubular Structures of the Cochlea
•
•
•
•
Stapes pushes on fluid of scala vestibuli at oval window
At helicotrema, vibration moves into scala tympani
Fluid vibration dissipated at round window which bulges
The central structure is vibrated (cochlear duct)
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Section thru one turn of Cochlea
• Partitions that separate the channels are Y shaped
– bony shelf of central modiolus
– vestibular membrane above & basilar membrane below form the
central fluid filled chamber (cochlear duct)
• Fluid
vibrations affect hair cells in cochlear duct
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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 sensory
neurons
whose
cell
body is in spiral ganglion
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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
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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 move the
hair cells against the tectorial membrane
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• Microvilli
are bent producing receptor potentials
Overview of Physiology of Hearing
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Distinguishing Different Sounds?
• Sounds at different frequencies vibrate
different portions of the basilar membrane
– high pitched sounds vibrate the stiffer more basal
portion of the cochlea
– low pitched sounds vibrate the upper cochlea which
is wider and more flexible
• Loud sounds vibrate cause a greater vibration
of the basilar membrane & stimulate more hair
cells which our brain interprets as “louder”
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Hair Cell Physiology
• Hair cells convert mechanical deformation into
electrical signals
• As microvilli are bent, mechanically-gated
channels in the membrane let in K+ ions
• This depolarization spreads & causes voltagegated Ca+2 channels at the base of the cell to
open
• Triggering the release of neurotransmitter onto
the first order neuron
– more neurotransmitter means more nerve impulses
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Otoacoustic Emissions
• Cochlea can produce inaudible sounds
– caused by shortening & lengthening of outer hair
cells in response to signals from motor neurons
– vibration travels backwards toward the eardrum
– can be recorded by sensitive microphone next to the
eardrum
• Purpose
– as outer hair cells shorten, they stiffen the tectorial
membrane
– amplifies the responses of the inner hair cells
– increasing our auditory sensitivity
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Auditory Pathway
• Cochlear branch of CN VIII sends signals to
cochlear and superior olivary nuclei (of
both sides) within medulla oblongata
– differences in the arrival of impulses from both
ears, allows us to locate the source of a sound
• Fibers ascend to the
– inferior colliculus
– thalamus
– primary auditory cortex in the temporal lobe
(areas 41 & 42)
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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
sounds
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Physiology of Equilibrium (Balance)
• Static equilibrium
– maintain the position of the body (head) relative to
the force of gravity
– macula receptors within saccule & utricle
• Dynamic equilibrium
– maintain body position (head) during sudden
movement of any type--rotation, deceleration or
acceleration
– crista receptors within ampulla of semicircular ducts
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Vestibular Apparatus
• Notice: semicircular ducts with ampulla, utricle & saccule
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Otolithic Organs: Saccule & Utricle
• Thickened regions called macula within the saccule &
utricle of the vestibular apparatus
• Cell types in the macula region
– hair cells with stereocilia (microvilli) & one cilia (kinocilium)
– supporting cells that secrete gelatinous layer
• Gelatinous otolithic membrane contains calcium carbonate
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crystals
called9/e otoliths
that move when you tip your16-72
head
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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Crista: Ampulla of Semicircular Ducts
• Small elevation within each of three semicircular ducts
– anterior, posterior & horizontal ducts detect different movements
• Hair cells covered with cupula of gelatinous material
• When you move, fluid in canal bends cupula stimulating
hair
cells that release neurotransmitter
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Detection of Rotational Movement
• When head moves, the attached semicircular ducts and hair cells move
with it
– endolymph fluid does not and bends the cupula and enclosed hair cells
• Nerve signals to the brain are generated indicating which direction the
head
has&been
rotated
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Equilibrium Pathways in the CNS
• Fibers from vestibulocochlear nerve (VIII) end in
vestibular nuclei and the cerebellum
• Fibers from these areas connect to:
– cranial nerves that control eye and head and neck
movements (III,IV,VI & XI)
– vestibulospinal tract that adjusts postural skeletal muscle
contractions in response to head movements
• Cerebellum receives constant updated sensory
information which it sends to the motor areas of the
cerebral cortex
– motor cortex can adjust its signals to maintain balance
Tortora & Grabowski 9/e 2000 JWS
16-76