Anatomy of the Ear
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Objectives
o Discuss the anatomy of the outer ear
o Discuss the anatomy of the middle ear
o Discuss the basic physiology of the outer ear
o Discuss the basic physiology of the middle ear

Auditory Mechanism
o Processes the acoustic signals of speech
o Has an amazing range of sound pressures
o Has a frequency range of around 19 octaves spanning 20 to 30 Hz
 An octave is a doubling in frequency
o Audition
 The process associated with hearing
 Vital part of verbal communication
 The ear changes acoustic energy into electrochemical energy

Structures & Functions of Hearing
o Structures of the ear
 Outer, middle, inner ear, auditory pathways
o Outer ear
 Collects sound; “shapes” frequency components
o Middle ear
 Matches airborne acoustic signal with fluid medium of cochlea
o Inner ear
 Performs temporal and spectral analysis on ongoing acoustic signal
o Auditory pathway
 Conveys and further processes the signal
o Cerebral Cortex
 Interprets the signal

Structures of the Outer Ear
o The pinna (auricle)
 Prominence referred to as “the ear”
 Funnels acoustical information to the external auditory meatus
 Auricle and external auditory canal reach adult size @ age 9
 Sensitive
 Skin is thin
 Structure provided by a cartilaginous framework
 Yellow elastic cartilage
 Also found in larynx
 Continuous with the cartilage of the lateral 1/3 of the external auditory canal
 Collector of sound that is processed in the middle ear and cochlea
 Helps localize sound sources
 Directs sound into the ear
 Each individual's pinna creates a distinctive imprint on the acoustic wave traveling into
the auditory canal

Landmarks of pinna
o Helix (1)
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Crus of helix (2)
Auricular tubercle (3)
 Darwin’s
Antihelix (4)
 Antihelix has 2 crura- ant and post. between the 2 is a fossa- triangular fossa
Crura of antihelix (5)
Triangular fossa (6)
 Between the 2 cruras
Scaphoid fossa (7)
 Scaphoid fossa between helix and antihelix.
Tragus (10)
Antitragus (11)
 Antitragus is across from tragus
Cavum concha (14)
 Cavum concha: cave. leads to EAM.
Cymba concha (15)
Lobule (13)

Innervation of the Outer Ear
o Sensory
o Great auricular nerve
 Cervical plexus, Ventral rami of C2 & C3
 From cranial surface, part of lat. surface; helix, antihelix, tragus
o Auricular branch of CN X (Arnold’s nerve)
 Arises from superior ganglion of CN X
 Through mastoid canaliculus
 Contribution from auricular branch of CN IX
 From concha & ponticulus
o Facial Nerve
 From concha & ponticulus
o Auriculotemporal Nerve
 Branch of mandibular division of Trigeminal Nerve - CN V3
 From ant. part of auricle; tragus & ant. limb of helix
o Lesser Occipital Nerve
 Ventral ramus of C2 or C2 & C3
 From upper part of cranial surface

Innervation of the outer ear- Sensory
o Lateral surface
 Auriculotemporal n. (CN V3)
o Medial surface
 Great auricular n. (C2, C3)
 Lesser occipital n. (C2 (C3))
o Concha
 Facial n. (CN VII)
 Vagus n. (CN X)

Structures of the Outer Ear (Cont’d)
o External auditory meatus (EAM)
 External ear canal
 Funnels sound to eardrum
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Protects eardrum
Ends at tympanic membrane. 2/3 of it is bony (temporal bone); 1/3 cartilaginous
Terminates at the tympanic membrane
 2/3 of ear canal housed in bone
 1/3 of ear canal composed of cartilaginous parts
Cerumen
 Repel water
 Trap dust, sand particles, micro-organisms, and other debris
 Moisturize epithelium in ear canal
 Odor discourages insects
 Antibiotic, antiviral, antifungal properties
 Cleanse ear canal
 Keeps the ear canal moist,Has a distinct smell that keeps insects away.
 Continually being produced and moved through

Diseases of the EAM
o Otitis externa
 Swimmer’s ear
 Inflaming lining of outer ear
 If you pull on it and it hurts, its an outer ear infection. If it doesn’t hurt, its probably
middle ear
 Inflammation of the EAM
o Otitis externa with ear wick in place
 Referred sensation from the external acoustic meatus is medicated by the vagus n.
 Irritations may cause nausea or cough
o Otitis media
 Bottom pic looking at pus that is trapped
 Perforated eardrum: Pressure and fluid behind eardrum, it actually bursts.

Tympanic Membrane
o Separates the middle ear from the outer ear
 Lining is continuous with these- both middle ear and outer ear
o Responsible for initiating mechanical impedance-matching process of middle ear
o Three layered
 First layer: outer (cuticular) layer
 Stratified squamous of skin (external ear)- continuous with outer covering of TM
 Second layer: intermittent (fibrous) layer
 Third layer: inner (mucus) layer
 Deepest layer is stratified columnar epithelium
 *know what these are made of?!!
o CT between mucosa of middle ear and skin of outer ear. This layer provides strength of the TM
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Second layer: intermittent (fibrous) layer – 2 layers
 Radial fibers
 Circular fibers
 They overlap in the CT layer, keeps the TM taut to withstand exceptional forces
TM is conical. Has a small depression. The apex of the cone is where it attaches to your middle ear
bones. Creates a distinctive feature of the middle ear called the umbo. That is where handle of malleus
attaches to TM and helps pull it taut.
Cone of light tells you that TM is very healthy. It’s a reflection of the light from the otoscope.
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Majority of eardrum held taut-pars tensa
Small area of eardrum superior to malleus lacks CT layer (only has skin and mucosa). So it’s not nearly
as strong as the rest- pars flaccida. The weak part of the TM
Small folds coming away from this process that create tents in eardrum- anterior and posterior
malleolar folds

Middle Ear Structure: contains….
o Connects outer ear that receives sound, with inner ear that processes it
o Tympanic membrane
 Most proximal portion of outer ear
 Most distal portion of middle ear
o The ossicles
 The bones that actually articulate
o The entry to the cochlea
 The oval window
 The round window- opens to the cochlea, but not considered the entrance
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Middle Ear Mechanism
o Increases the pressure arriving (from air filled outer ear) at the cochlea.. to be able to move the fluid
filled inner ear
 Cochlea is a fluid-filled cavity
o Acts to overcome impedance
 Impedance is resistance to the flow of energy
 Fluid of any type resists the flow of energy
o Sound waves travel through air well, but not through fluid well. So having fluid-filled inner ear creates a
problem. So we needed a means of amplifying sound waves in air to be strong enough to work in a
fluid-filled cavity
o So ME amplifies sound waves traveling thru air to be able to still make changes in the fluid-filled inner
ear. Has to overcome impedance

Middle Ear Function
o Primary function: Match the impedance of two different conductive systems
 The outer ear (air) and the cochlea (fluid)
 ~30dB loss to air/fluid interface
 In water, causes reduction of 30 dB of acoustic energy. So we have to overcome this 30
dB loss- that’s what middle ear does.
 Acts as a piston concentrating all the force that hits the large eardrum onto the tiny
stapes
 Eardum 20x larger in volume than the footplate of the stapes. That concentration of force
from large area to small accomplishes gain of 25 dB.
o
Three impedance matching mechanisms
 Area ratio provides a 25 dB gain
 Lever advantage provides a 2 dB gain
 Buckling effect provides a 4-6 dB gain
 These three impedence matching devices combined cause a signal gain of around 31 dB
 Other advantages: length of long process of malleus is longer than incus, so when it moves the
incus moves the same amount but requires less energy- transferred to stapes.
 Buckling: Force is concentrated on the umbo and we receive 5 dB gain
 We concentrate the acoustic waves in air down to a tiny area that moves fluid, and overcome that
air fluid interface
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Middle Ear
o Contains the three smallest bones of the body
o Bones of middle ear called ossicles
o Ossicular chain: collective name for ossicles
 Malleus
 Hammer
 Incus
 Anvil
 Stapes
 Stirrup
o Ossicles act as hydraulic press
 Concentrate force on (on TM) large area of eardrum into small area of footplate of stapes
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Malleus
 Largest of the ossicles
 Provides point of attachment with tympanic membrane
 Bulk of bone, majority of its weight is the head or caput
 Anterior process: ligaments suspend these ossicles in the ear. One of them comes off of this
anterior process.
Incus
 Shaped like an anvil
 Provides intermediate link of ossicular chain
 Incus and malleus articulate by means of a synovial saddle joint
 Has a short process and a long process (lenticular). This process reorients the joints on either
side of the incus.
 Stapes is almost 90 degrees from the malleus
 Incus joints
 Saddle joint with malleus
 Ball and socket w/ stapes
Stapes (stirrup)
 Third bone of ossicular chain, SMALLEST
 Footplate sits in one of the openings to the cochlea- the oval window. And communicates with the
cochlea
 Helps to transmit sound vibrations from eardrum to oval window
 Articulation of the incus and stapes of ball and socket type
 Head articulates with incus.
 Short neck, ant and post crus (legs) that connect to footplate (which sits in oval window)
Ossicular/Tympanic Ligaments that help suspend it
o Malleus
 Superior
 Anterior- very tiny
 Lateral- attaches it to the middle ear cavity. Not in the pic
o Incus
 Posterior- large
 Superior- tiny
o Stapes
 *Annular (ring) ligament- completely surrounds footplate and holds it in place in the oval
window. The stapes always has access to the cochlea and isn’t gonna move!
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Tympanic Muscles
o Muscles of middle ear attached to ossicles
o Smallest muscles of human body
 Stapedius muscle
 Imbedded in posterior wall of middle ear
 Smallest of the body!
 Attaches to neck of stapes
 Contracts and limits rocking ability of stapes.
 Innervated by facial nerve
 Tensor tympani
 Inserts into upper mandibular malli
 Attaches to long process (manubrium) of malleus
 Limits movement of malleus. Pulls TM even more taut, so not able to deform and rock
malleus as much.
 Innervated by V3 Mandibular of trigeminal
 These 2 muscles: protect us against loud or strong sounds. When they contract, they prevent
rocking (how much malleus moves)
 Skeletal muscles. If you constantly use them, they fatigue. Tinnitus- you wore out your safety
systems, so you have an overstimulated ear. Nerves have been firing for so long

Landmarks of the Middle Ear
o Landmarks of medial wall of middle ear cavity
 Oval window (fenestra ovalis)
 Round window (fenestra rotunda)
 Promontory of the cochlea
 Prominence of facial nerve
o Eustachian tube (auditory tube)
 Responsible for aeration of middle ear
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MIDDLE EAR is a box with roof, floor, 4 walls
Boundaries of Tympanic Cavity
o Roof
o Floor
o Four walls
 Anterior
 Posterior
 Medial
 Lateral- Tympanic membrane makes up this wall
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Roof (of middle ear, also called tegman tympani)
o Tegmental wall
o Thin layer of bone, separates middle ear from middle cranial fossa- called the Tegman tympani
o Bad infection can eat through the roof and get to middle cranial fossa
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Floor
o Jugular wall
 Separates middle ear from internal jugular v.
 Bump on the wall is caused by the internal jugular vein.
 ABOVE ME is cranial fossa of brain
 BELOW M.E. is internal jugular vein
o Tympanic br. of CN IX- this nerve enters through the floor of the middle ear
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Lateral Wall
o Membranous wall- because the TM forms this lateral wall!!!!
o Tympanic membrane
o Wall of epitympanic recess
o ME cavity extends UP above the tympanic membrane into the epitympanic recess
o This recess is where the majority of the mass of the ossicles sit
o Chorda Tympani crosses the lateral wall. It’s the cord that runs between malleus and incus on
lateral wall of middle ear. Just passes through.
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Posterior wall
o *Mastoid Wall
o Incomplete
 Aditus- hole, opening to mastoid air cells. Communicates w/ mastoid air cells.
o Pyramidal eminence- tiny pyramid of bone. Sticks out into middle ear. Inside this pyramid, the
stapedius muscle is housed
 Stapedius m.
 Tiny part sticks out of pyramidal eminence
o Facial N. (CN VII)
 Chorda tympani enters middle ear through the posterior wall
 Exits via anterior wall; travels along lateral wall.
 FACIAL N directly behind the posterior wall!
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Anterior Wall
o Incomplete wall
o POSTERIOR TO INTERNAL CAROTID ARTERY
o Inferior portion
 Internal carotid a. comes into skull anterior to the anterior wall
o Superior portion
 Has an opening for the Pharyngotympanic tube
 This is where it is incomplete. 2 things associated w/ opening
 Eustacian tube communicates w/ ME via anterior wall
 Tensor tympani m comes through anterior wall to connect to malleus
o Tensor tympani m.
o Chorda tympani n. exits middle ear via anterior wall
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Pharyngotympanic Tube
o Connects Middle ear to nasopharynx
o Equalizes pressure* FUNCTION! Example of when you’re on an airplane
o Looking from medial at the lateral wall
o Less air pressure the higher you go. Low pressure outside, high pressure in ME. Nature wants to
equalize- high pressure area wants to go out. TM is being pushed out by high pressure inside. When
starting to rise in an airplane, everything sounds dull bc can’t move ossicular chain as much.
o Swallowing opens up ET. Take in that air & exchange it with high pressure air in ME. Then it pops
o Opposite: scuba diving… High pressure outside, low pressure inside
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Auditory Tube
o Proximal 1/3 within temporal bone
o Distal 2/3 cartilaginous- torus tubarius
o Continuous with middle ear and mastoid air cells
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Eustachian Tube
o The eustachian tube also operates like a valve
 Opens during swallowing and yawning
 Equalizes pressure on either side of the eardrum
 Necessary for optimal hearing.
 Without this function, a difference in the static pressure in the middle ear and the outside
pressure may develop, causing the eardrum to displace inward or outward
 Reduces efficiency of the middle ear and less acoustic energy transmitted to the inner
ear.
o Undergoes significant changes with age
 In infants it is horizontal. Angles down as we get older. So the drainage isn’t as efficient in
infants- children more prone to ME infections. Tube doesn’t drain well down towards
nasopharynx like it does in adults
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Muscles of Eustachian Tube
o Tensor veli palatini
 Pulls down on tube, pulls it open
 Main opener of the ET
o Levator veli palatini
 Near ET, but doesn’t open or close it
 Just originates from a portion of it
o Salpingopharyngeus
 Pulls on cartilaginous portion, but much more weakly
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Medial Wall
o Labyrinthine wall. Bony labyrinth is on other side
o Lateral wall of inner ear
 Promontory- bulge of lateral wall into inner ear cavity
 Tympanic plexus
 Tympanic n. (CN IX)
 Internal carotid plexus
 Lesser petrosal n.
o Prominence of the facial canal
 Facial n is moving from medial to posterior
o Prominence of the lateral semicircular canal
 Intruding into the ME just like cochlea is, down at promonotory
o Oval window- footplate of stapes sits here. Stapes which communicates w/ cochlea (inner ear), sits
here. Cochlea is medial to medial wall??
o Round window
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Diseases of the Middle Ear
o Conductive deafness
 Problem with sound waves from TM to footplate of stapes
 A lot of times stapes fuses inside oval window; if fused it can’t rock, so can’t create motion. So
they go in and remove crura of stapes. Cut hole in footplate, remake the stapes. Stapedotomy
o Tympanoplastes
 People who get frequent ME infections
 Alternate drainage route through TM
 Put plug through TM and fluid can drain out medially through ET and laterally through TM;
allow TM to be taut
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Inner Ear
o Series of tubes and rooms all w/in temporal bone
o Osseous labyrinth
o Osseous bony labyrinth- not a feature at all. It’s a cave- caves that you’ve dug out of temporal bone.
Cave w/in temporal bone. Inside of that cave is your membranous labyrinth.
o Membranous labyrinth- floating inside of the fluid that filled up your cave
o
System of membranous ducts
 Contain receptors for hearing and balance
 Membranous labyrinth
 Enclosed within matching system of osseous canals
 Bony labyrinth
 Embedded in petrous portion of temporal bone
July 18, 2013
Anatomy of the Ear

Objectives
o Discuss the anatomy of the vestibular system
o Discuss the anatomy of the cochlear system
o Discuss the basic physiology of the vestibular system
o Discuss the basic physiology of the cochlear system

The Inner Ear
o Contains sensors for balance
 The vestibular system
o Contains sensors for hearing
 The cochlea
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The Inner Ear (Cont’d)
o Osseous or bony labyrinth
 Cavities within temporal bone
 Osseous vestibule
 Osseous semicircular canals
 Osseous cochlear labyrinth (we just call it the cochlea)
 Filled with perilymph
 These cavities are filled with fluid.
 High sodium concentration, LOW potassium
 Fluid helps to suspend and cushion a series of membranous structures that float
w/in fluid in the cave.
 Membranous structures mimic the shape of the caves and cause the caves to be
formed.
 Organs for balance and hearing reside here
 ↑Na+ ↓K+
 Houses inner ear structures
 Membranous labyrinth
 Filled with endolymph
 High in potassium, LOW in sodium
 ↑K+ ↓Na+
 Vestibule provides entrance to structures
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Osseous Vestibule
o Continuous with vestibular system and cochlear system
o Lateral wall
 Oval window
 In pic the lateral wall was removed. In here sits the oval window.
o Medial wall
 Vestibular aqueduct- an opening
 Looking at the medial wall in the pic
o Recesses
 Spherical
 Saccule- contains one of your organs of balance, position.
 Saccule and utricle are swellings
 Elliptical
 Utricle
 Superior & Lateral semicircular canals
 Portions of semicircular canals open into elliptical recess
 Cochlear
 Basal cochlea
 Most proximal portion of coch????????
 Lots of holes in these recesses. Nerves go through them
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Otolithic Organs
o Sense linear acceleration
 Movement in absence of rotation
 Driving
 Riding in elevator
 Not rotating, but accelerating linearly.
 *****Linear acceleration is sensed by the saccule and the utricle (otolithic organs)
o Saccule
 Endolymphatic duct- impt because endolymph inside whole membrane being produced and
absorbed. It is moved out to the sack. w/in the sack it allows ions to escape, and some of the
endolymphatic fluid to escape.
 If you close off the sack, you will hear for a little while but end up developing
menier’s(???) disease- excess of endolymphatic fluid. HL, vertigo, fullness of the ear,
pain. Not regulating endolymphatic fluid properly.
 Ductus reuniens
 Communication with cochlear duct
o Utricle
 Utriculosaccular duct
 Communication with endolymphatic duct via the utriculosacular(?) duct.
o
Maculae- collection of hair cells; arranged in characteristic way
 Sensory organ of Utricle and Saccule
 Hair cells- 2 types
 Stereocilia (50)- numerous per hair cell
 Kinocilium (1)- one per hair cell
 How stereos bend relative to the kinocilium. Moved away or towards kinocilium?
 All based on the hair cells; how they move relative to the kinocilium determines the firing of that
hair cell
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Osseous Semicircular Canals
 Now out of the vestibule, into semicircular canals. Talking about the BONE surrounding the
membranous semicircular ducts.
 Canals are features of bone
 Ducts are talking ab endolymphatic tubes that contain sensory organs
 Ducts sit inside canals
 House sense organs for rotational movement in space (3 canals, separate arches connected to
the vestibule)
 1. Anterior (superior) arch
 2. Posterior (inferior) arch
 3. Horizontal (lateral) arch
 They all open into vestibule
 Common crus- ant and post semicircular canals share one leg
 As the 2 legs enter… on each of them, on one leg you have a swelling called an
ampulla
 Ampulla- 3 distinct swellings
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o
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Gelatinous matrix
 Hair cells imbedded in matrix. That has calcium based salt crystals w/in it called
otoconia or otoliths, INCREASE MASS of whole gelatinous structure. By having that
mass it helps to cause hair cells to shift, move, sense linear acceleration
 Putting grapes into the jello- increases the weight
 Contain otoconia
 Increase weight
 Causes haircells to shift
Otolithic membrane
 Fibrous
Anterior semicircular canal of 1 ear is parallel to posterior semicircular canal of contralateral ear
 The 2 inner ears are almost perpendicular to one another
 Being parallel helps us perceive rotational info in the same direction at the same time.
Helps us determine movement to the L vs. the R
Horizontal are in same plane
 Ampullae (swelling) are mirror images
 Swelling is medial on both sides, facing inward
Helps differentiate movement to left vs. right
Sense rotational acceleration
 Roll
 Anterior
 Moving your head to your shoulder
 Pitch
 Posterior
 Moving the head up and down (Yes movement)
 Yaw
 Horizontal
 Moving head left to right (No movement)
Semicircular Ducts
o Housed within osseous (BONY) semicircular canals
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o
o
o
Contain endolymph- fills semicircular duct
Surrounded by perilymph- inside bone; cushions semicircular duct which is filled w/ endolymph
Sensory organs located at ampullae
o
Ampulla
 Swelling at base of duct/canal
 Hair cells
 Stereocilia
 Kinocilium
 Hair cells only in ampulla. Only thing that goes through the whole duct is endolymph
Cupula
 Gelatinous matrix
 Spans the width of ampulla
 Forms barrier through which endolymph cannot flow
 Therefore distorted by endolymph movements
 Prevents endolymphatic fluid from flowing continuously. When you start rotating your bod, the
endolymphatic fluid lags behind a little bit. That lag/movement deforms this cupula in the
direction that you’re moving. Cupula is pushed in direction you’re moving, so stereocilia are
moved either towards or away from the kinocilium- that’s how you know which direction you’re
spinning, or rotating.
o
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o
Ahhhh!
 Semicircular canals give us rotational input
 Hair cells ONLY in ampulla. The whole structure is the crista ampularis (located in ampulla).
They take the form of hair cells that span the width of SC duct, but only at the ampulla
 Hair cells embedded in gelatin. Gelatin tries to go to top of amp, but doesn’t make it bc
endolymphatic fluid can’t pass thru
 Shoot a hose at jello- it’ll deform in direction hose is pointing. Hair cells embedded in jello so
they get moved too. That is what is sensed by your cranial nerve- the movement towards or away
from the kinocilium. How we know if we’re moving R or L
 Jello is there just to cause the hair cells to fire! NOT protective gel
o
Vestibular Mechanism
 Provide major input to proprioceptive system
 Responsible for the sense of one’s body in space
 Information is integrated with
 Joint sense
 Muscle spindle afferents
 Visual input
Osseous Cochlea
o Looks like a coiled snail shell
o Wraps around itself 2⅝ times
 Apex
o Cored by modiolus
 Bony core that cochlea spirals around
 Perforated by CN VIII fibers
 Continuous with IAM
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o
o

Take a can and remove top and bottom. Cover top and bottom with saran wrap… top is oval window.
Stapes sits in oval window. The bottom is the round window (RW is covered by a membrane that
prevents perilymph from flowing around). Cochlear duct houses the organ of hearing.
 Top part closer to stapes is scala vestibuli.
 The one in the middle that divides it is scala media. Separates sc vestibuli from sc tympani
 Bottom part near round window is scala tympani.
 3 chambers all part of 1 feature. Our organ of hearing extends the length of the chamber, except
gonna cut a little out of the top. By doing this you make the top and bottom chamber continuous
 Pull the can and make it longer. All the contents come with me. Scala tymp, vestibuli, media
become longer and thinner.
CN 8 goes through spiral lamina- innervates cochlear duct
o
Osseous spinal lamina
 Around the modiolus; crucial bc one of the attachment sites for cochlear duct
 Scala media: where the actual organ of hearing is located w/in the cochlear duct
 Divides cochlea into top and bottom:
 Scala vestibuli
 Scala tympani
 Decreases in size as it approaches apex (top)
 Hamulus
 Helicotrema- defect at the top. Where chambers are continuous
 Attachment point for scala media
 Houses sensory organ for hearing
o
Three openings
 Round window
 Covered by a membrane
 Communicates between scala tympani and middle ear
 Opens up into scala tympani
 Oval window
 Stapes sits here
 Communicates between scala vestibuli and middle ear space
 Opens up into scala vestibuli
 *DON’T WORRY AB Cochlear canaliculus
 Connects upper duct and subarchnoid space
Cochlear Duct (SAME THING AS SCALA MEDIA)
o Resides between scala vestibuli and scala tympani
o Scala media
o Houses sensory apparatus for hearing (organ of hearing- organ of corti)
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Whole triangle in pic is scala media
Osseous spiral lamina
Reissner’s membrane- the roof of the cochlear duct
 Separates scala vestibuli from scala media
Stria vascularis
 Unique epithelium- the only epithelium that is vascularized
 Possible reason fo dat: secretes endolymph
 Feature of scala media, of cochlear duct
 ENDOLYMPH in scala media
 PERILYMPH in the canals
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 Ducts are self-contained in the canals, filled with ENDOLYMPH
 Scala vestibuli and tympani have PERILYMPH
Spiral ligament
 Transformed periosteum of temporal bone
Basilar membrane
 Separates scala media from scala tympani
Supports organ of hearing
 Organ of Corti
o
Organ of Corti
 Sensory organ of hearing within the inner ear
 Has four rows of hair cells
 Supported by bed of Deiters’ cells
 Has three rows of outer cells (12,000 cells)
 Has a single row of inner hair cells (3,500 cells)
 Outer hair cells (FURTHER AWAY from osseous spiral lamina) supported by Deiters’ cells
 Run the entire length of the cochlear duct
 Remember, 50 stereocilia and 1 kinocilium
 Outer hair cells have more of a V shape
 Inners are more of a U shape
o
Spiral limbus
 Tectorial membrane- gelatinous mass; stems from the spiral limbus. Has ONLY the outer hair
cells embedded in it
 Overlays hair cells
 **OHCs embedded in tectorial membrane
 Inner hair cells are not!
Auditory Mechanism
o Performs frequency and temporal acoustic analysis of incoming acoustical signal
o Establishes first level of auditory processing of incoming acoustic signals
 Determines frequency components of signal
 Pitch
 Determines amplitude of signal
 Loudness, intensity, power
 Identifies temporal aspects of signal
 L ear will hear something before the right, if the sound is coming from the L
o
Sound is disturbance of air
 Causes TM to move
 Movement transmitted to oval window
 When TM moves inward, footplate of stapes moves inward
 Stapes recreates the wave that hit the TM (the original airborne wave)
 When TM moves out, footplate of stapes moves out
 Direct analog to airborne source sound translated to cochlear fluid
 Perilymph
 Oval window communicates w/ scala vestibuli (which is filled w/perilymph). Wave of
peri will deform the roof of Reissners membrane. Deforms it into the scala media. If you
pushed down on the roof (which is filled w/ fluid), youll push down on the floor as well.
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so now have distended basilar membrane (organ of corti sits on it). So now the hair cells
embedded in the tectorial membrane have been moved- this causes them to fire.
o
Stapes compresses perilymph of scala vestibuli
 Distends Reissner’s membrane toward scala media
 Distends basilar membrane toward scala tympani
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Frequency of sound is number of oscillations/second
100Hz signal moves TM/footplate in and out 100 times/second
Fluid wave in perilymph of same freq. that hit eardrum. That fluid wave now traveling through
perilymph is called the traveling wave
Initiates wave-like action of basilar membrane
 Traveling wave
 Frequency it came in at determines frequency data going to brain (bc of perilymph being
induced to form a wave w/ freq of 100 hz)
 Arises from stimulation of perilymph of vestibule
 Moves along basilar membrane until it reaches point of maximum growth (which is 100
hz in this example)
 After it reaches this, it dies out instantly.
 Wave damps after reaching maximum growth
 Only one strong point of disturbance
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High frequency sounds vibrate the basilar membrane closer to vestibule
 Base
 Stimulate the cochlea close to the base of the cochlea
Low frequency sounds generate longer traveling waves
 Reach the apex of cochlea
 Low freq travel longer, have more power
Brain enabled to separate out different freq components; depending on where you are on the basilar
membrane, determines the freq that you respond to
Traveling wave separates out frequency components of complex sounds
 High frequency processed at base
 Low frequency processed at apex
Deformation of basilar membrane determines ability to analyze frequency
 How?
 Basal end is stiffer than apical
 Flaccid at helicotrema
 Gains mass from base of apex
 As mass increases resonant frequency decreases
 Widens from base to apex
 Traveling wave always travels from base to apex due to impedance gradient of basilar membrane
 Because of these properties, traveling wave propagates from BASE to APEX; can only travel in
one direction!
 **FROM BASE TO APEX: Decrease stiffness, increase mass and width ***
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Basilar membrane is diff shape along the length of it. By changing shape, we increase the amt of
frequencies it can interpret.
Basilar membrane spans distance between osseous spiral lamina and outer edge of cochlea. If
BM is narrow at base, the osseous lamina extends further.
The wider region close to apex is where LOW frequencies are interpreted
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o
APEX: low frequencies, WIDER
 Apex is not as stiff- more FLACCID
 Gains mass closer to apex; alters resonant freq, changes how the wave stimulates organ
of corti
BASE: high frequencies, NARROW
 Basilar end is STIFFER
 Smaller mass
Preventing the wave from rebounding- vibration at round window dissipate the energy you just
had at the stapes. Pressure release valve
 If you just had solid bone, (esp low frequency sounds) wave would hit that then rebound
and you’d have to process continually incoming sound AND background noise that is
now working against waves coming in
 You need release valve to hear properly
Excitation of hair cells
 Hair cells displaced as traveling wave moves along basilar membrane
 Excitation of outer hair cells
 Result of shearing effect on cilia
 Code intensity
 Outer hair cells interpret intensity (LOUDNESS)
 Excitation of inner hair cells
 Results from fluid flow and endolymph turbulence
 Code frequency
 Inner hair cells [not embedded in Tectorial Membrane] process frequency
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