Middle ear

Functions of the middle ear

impedance matching

filtering

protection

pressure equalization

impedance matching

amplification of sounds to overcome difference in impedance between the air of EAM and the fluid in the inner ear

Filtering

resonant frequency is approximately 1k Hz function as a bandpass filter

Protection

acoustic reflex- contraction of stapedius muscle in response to loud sounds

pressure equalization

ET opening and closing

How much dB do we lose due to impedance mismatch?

about 30 dB

ME transformer 3 mechanisms

buckling effects of TM curved membrane structure

area differential between TM and stapes footplate/ oval window

lever action of ossicular chain

Bekesy TM vibration

hypothesized that for frequencies up to 2k Hz the eardrum moves as a stiff plate hinged on the axis of the ossicles

greatest displacement of TM occurs inferiorly

55 mm² vibrates to sound energy

Helmholtz TM vibration

in reality eardrum does not move as stiff plate

up to 1500 Hz there are 2 areas of peak displacement with a buckling of the TM between them

above 3k Hz pattern becomes increasingly complex

concave shape of TM end up with multiple areas of movement with complex sound

Methods to overcome impedance mismatch

the curved TM coupling principle increases the vibratory pressure by 2 the pressure is doubled at the manubrium as a result of curved TM

buckling increases pressure on manubrium 2x

areal reduction

stiletto heal effect, walking on grass flat vs heel

decrease area increase the pressure

stapes footplate is small, thus increasing pressure

for the ossicular level mechanism what ossicle has the most movement

malleus

how do the 3 mechanisms of the ossicles work together?

curved membrane coupling = 2

areal reduction = 17.3

lever system= 1/3

45 to 33 dB increase in sound

ideal transformer model

calculations for ossicles oversimplified

factors of ideal transformer model

motion of stapes

acoustic reflex

middle ear resonance

middle ear pressure equalization

factors result in a band pass filtering of the response of the middle ear

frequency

pitch

intensity

loudness

white noise

broadband noise

what does a bandpass filter use

high and low pass filters at same time

resistance

largest impedance due to density of inner ear fluids

resistance consists of processes that absorb and reflect energy

what causes resistance in the middle ear

inner fluids and air in EC impedance mismatch

impedance of TM

friction between components

explain the path of resistance in the ME

when acoustic reflex stimulate the ME muscles increase the stiffness of system

results in more sound pressure being reflected back into EC and less sound pressure through to the inner ear

when acoustic reflex occurs we’re increasing resistance of system because the system has become stiffer

reactance

components that store energy

mass of ME structures and spring action of muscles/ ligaments contribute to the systems reactance

anything that has mass has intertia

mass of ME doesn’t change

stiffness changes

ME transfer function

all factors result in band pass filtering of response of ME

actual transformation of sound in ME dependent on frequency of sound

ME transformer function at low frequencies

more resistance and sounds are reflected back into EC

ME transformer function high frequencies

more positive reactance and sound energy is stored as acceleration

ME transformer function at middle frequencies

combination of resistance and reactance

ME transformer function Yost

measuring pressure at TM and then oval window

filters out frequencies at extreme ends of spectrum

movement of stapes

does not vibrate in simple pattern for all intensities and all frequencies

middle frequencies like swinging door (footplate)

movement of stapes at high intensities/ low frequencies

below 150 Hz

rocks on axis through crura like seesaw

protects inner ear from over stimulation

what are the first 2 functions of the ME

impedance matching and band pass filter

acoustic reflex

involuntary muscle contraction that occurs in ME in response to high-intensity sound stimuli or with person starts to vocalize

only effector organ of reflex stapedius muscle

how does the stapedius muscle act during acoustic reflex

innervated by stapedius nerve

pulls stapes down and out of oval window

how does the tensor tympani act during acoustic reflex

innervated by mandibular branch of trigeminal nerve

when stimulated pull TM inward

increased tension of TM

stimuli that elicit the acoustic reflex

intense sound

motor activity

speaking

functions of acoustic reflex

inner ear protection

ongoing adjustments to input to ME transfer function

smoothing out the ME transfer function

improving hearing in presence of low frequency competing sounds

how does the acoustic reflex improve hearing in presence of low frequency sounds

affects the transmission of low frequencies sounds more than high frequencies

what does the ET connect

ME to nasopharynx

function of ET

allows fresh air to ME

equalizes any pressure differences ME and nasopharynx

allows small amounts of ME fluid to drain into nasopharynx

ET default

closed

tympanometry

acoustic measures of ME conditions

use an immittance bridge

tympanogram

plot of ME compliance as a function of ear canal pressure

should see peak when pressure on both sides of TM are equal

pressure swept from +200 to -200

compliance

opposition of stiffness

compliance

ME system not massive, largely stiffness controlled system

changes in stiffness/ compliance have large effects on function of system

most sound let through when pressure equal

C peak

negative pressure

B peak

effusion, ET malfunction, perforation

As

stiff, otosclerosis

Ad

floppy or dysarticluation of ossicles

effects of ME problems

as much as 60 dB loss in air conduction

normal hearing in bone conduction

parts of ME

TM

ossicles

middle ear cavity

ME muscles and ligaments

TM layers

outer layer- extension of EC

middle- fibrous material, radial and circular fibers

inner- continuation of mucosal lining of ME cavity

when does the ME start to develop

early, around 25 hours

at birth lying nearly on floor of EC, will end being more perpendicular/ erect

3rd week

tubotympanic recess, 1st cleft ectoderm

6 weeks

ossicular mesenchyme separates cleft and pouch

20th week

tympanic cavity grows to enclose ossicles

22 weeks

exntension to form mastoid antrum

33 weeks

early mastoid pneumatization

pars tensa

main part in charge of vibration, energy transmitter, everything around main area of TM

angular and radial fibers

lines around TM

allows for rigidity

2 sections of TM

pars flaccida- schrapnels membrane

pars tensa

malleolar folds

ligamentous bands for the location of the pars flaccida location

annular ligament (annulus)

perimeter of TM

thickened fibrocartilaginous ring

ossicular chain

bridge between TM and inner ear

malleus

manubrium connects at distal end of TM at umbo

lateral process connects to portion of pars flaccida

tensor tympani inserts at neck

head connects to incus

9 mm long

23-37 mg

incus

lenticular prices connects to stapes

7 mm long

23-32 mg

stapes

most medial

footplate connects to oval window

stapedius inserts at neck

ME cavity/ tympanic cavity was

lateral

superior

inferior

anterior

posterior

medial

lateral ME cavity wall

TM and squamous portion of temporal bone

superior

tegmen tympani

inferior

tympanic plate of temporal bone

anterior

carotid wall

thin plate of bone

posterior

mastoid wall

proteins of temporal bone

medial

portions of temporal bone

3 compartments of middle ear cavity

epitympanum (attic)

mesotympanum

hypotympanum

epiityampnic reccess/ roof

function to separate ME space from jugular vein

ET

connects the ME to nasopharynx

runs from anterior wall of ME cavity to posterior wall of nasopharynx

opens 1x per minute

about 36 mm one in adult

bony first 12 mm

adult like around 8 years old

Middle ear muscles

tensor tympani

stapedius

tensor tympani

innervated by trigeminal nervem5

arises from anterior wall and inserts on a tendon that connects to long process of malleus

stapedius

innervated by facial nerve 7

arises from posterior wall and inserts on a tendon that connects to head/neck of stapes

what are ligament names based on

what there connect to and where they’re connected

other structures in ME

facial nerve 8

chorda tympani nerve

facial nerve

runs alongside auditory nerve 8

chorda tympani nerve

passes through tympanic space

branch of facial nerve

carries taste info