PHYSIOLOGY OF

HEARING

J.P. SOUAID, M.D., C.M., FRCS(C)QUEENSWAY-CARLETON HOSPITAL

THE OTTAWA HOSPITALDEPARTMENT OF OTOLARYNGOLOGY

April 10, 2014J. G. Marsan MD FRCSC

Physics of SoundDEFINITIONMechanical radiant energyTransmitted by longitudinal pressure wavesIn a material medium

CHARACTERISTICSFrequency – pitchAmplitude - loudness

Mismatch of Impedance

Air Water

Sound pressure energy

99.9% Energy reflected

0.1% Energy absorbed

Loss of sound pressure energy: 30dB

OBJECTIVES Explain how sound travels from a source to the

temporal lobe Explain how mechanical energy is transformed

into electrical energy in hearing Define and differentiate between conductive

hearing loss and sensorineural hearing loss Describe the neural pathways involved in the

stapedial reflex Describe pure-tone audiometry and

tympanometry. Describe the physiology of otoacoustic emissions

EXTERNAL EAR Concha and External Auditory Canal act

as acoustic resonators 10-15 db gain in 3-5kHz region. Concha:

Resonance of 5kHz, Irregular surface introduces other

resonances and anti-resonances. Ear canal acts roughly like a simple tube

resonator, open at one end Resonant peak at a frequency of 2.5 kHz

in human ears Effects of the ear canal resonance

contribute substantially toward an increase in sound pressure level at the tympanum

EXTERNAL EAR CANAL2.5kHz

MIDDLE EAR

Purpose: Transfer sound energy from the air space of the external auditory meatus to the fluid in the cochlea.

Accomplished through the vibration of the three middle-ear ossicles.

Piston-like vibration of the stapes in the oval window results in a pressure differential between the oval window (and hence scala vestibuli) and the round window (and scala tympani). This pressure difference between the two scalae is critical to the mechanical excitation of the cochlear structures.

MIDDLE EAR Serves as a transformer, acting to

increase the sound energy transmitted to the cochlear fluids.

Middle-ear transformer effects: A, The ratio of the areas of the

tympanic membrane and the stapes footplate (A1/A2) results in a pressure increase at the oval window. (17:1) (25 dB)

B, The lever effect caused by the unequal displacements of the malleus and incus (L1/L2) about the incudostapedial joint also results in a pressure increase. (1.3x) (2.5 dB)C, Curved membrane effect. Certain areas of TM vibrate more than others.D, Phase difference between oval and round windows (small effect)

TOTAL GAIN: 27.5 dB

17:1

1.3x

22:1=(17x1.3):1

Lever effect

Hydraulic effect

Combined effect

Next Step

Mechanical energyMechanical energy Electrical nerve impulses

External and middle ear Inner ear (cochlea)

COCHLEAR PHYSIOLOGY

35 mm coiled bony tube; 2.5 turns

Divided into :

1) Scala vestibuli and tympani: Contains perilymph ECF like : Na- 140. K- 4-10 Production of perilymph:

Unknown Ultrafiltrate of blood? From CSF?

COCHLEAR PHYSIOLOGY

2) Scala media: Contains endolymph ICF like: K- 144 meq, Na- 15-25meq Bounded by :

Reissner’s membrane Basilar membrane Osseous spiral lamina Lateral wall

COCHLEAR PHYSIOLOGY

Stria vascularis: Lateral wall Highly vascularized Cells contain NA-K-ATPase to produce

endocochlear potential about +80mV in the scala media

Decreases slightly from base to apex

FUNCTION: Endolymph production

ORGAN OF CORTI

HAIR CELL INNERVATION 50,000 neurons innervate cochlea:

90-95% synapse directly on inner hair cells (type 1 neurons)

Predominantly afferent 15- 20 of these neurons innervate each hair

cell 5-10% synapse directly on outer hair cells

(type 2 neurons) Predominantly efferent Each type2 neuron branches to innervate 10

outer hair cells

Characteristic Inner H.C. Outer H.C.

Shape Flask Cylindrical

Number 3500 12000

Stereocilia

No.of hair cells Few Many

Arrangement 3-4 rows, slightly curved

6-7 rows, rows arranged in V or W shape

Attachement to tectorial membrane

None or loosely Long stereocilia firmly embedded

Intracellular electric potential

-40mV -70mV

COCHLEAR MECHANICS

1. Motion is a traveling wave moving longitudinally from the base to the apex

of the cochlea.2. Each point along the cochlear partition

vibrates at a frequency equal to that of the stimulus. Tonotopic organization:This means that specific areas of the basilar membrane respond to specific frequencies

3. High frequency- base of cochlea4. Low frequency- apex of cochlea

COCHLEAR TONOTOPIC ORGANIZATION

PHYSIOLOGY ORGAN OF CORTI

Stereocilia-Hair Cell complex: Deflection of Stereocilia of outer hair cells By travelling wave Opens and closes non-specific ion channels Influx of K, depolarizing of cell Ca mediated K channels -intracellular cascade Release of chemical transmitters (Glutamate) Activate afferent neurons

HAIR CELL DEPOLARIZATION

DEFLECTION TOWARDS KINOCILIUM=EXCITATION DEFLECTION AWAY FROM KINOCILIUM=INHIBITION

KINOCILIUM (longest cilium)

Auditory CNS Cochlea Cochlear Nerve Cochlear Nucleus (CN) Superior Olivary

Complex (SOC) Lateral Lemniscus (B) Inferior Colliculus (IC) Medial Geniculate Body

(MGB) Trapezoid body Auditory Cortex

(Temporal lobe, Brodman area 41)

Extensive crossoverTonotopicity of Cortex also.

MNEMONIC: N.N.S.L.I.M.

CONDUCTIVE HEARING LOSSSENSORINEURAL HEARING

LOSSConductive HL:Related to sound conduction travelling through air via Pinna, External Ear Canal, TM, Ossicles and Middle Ear up to the oval window.

Sensorineural HL:Related to the electrical signal that is travelling via the cochlea, nerve and onwards.

AUDIOMETRY

AIR CONDUCTION TESTING

BONE CONDUCTION TESTING

NORMAL AUDIOGRAM

CONDUCTIVE HEARING LOSS

SENSORINEURAL HEARING LOSS

TYMPANOMETRY

Not a hearing test. Objective test of

middle-ear function. It is a measure of energy transmission through the middle ear.

TYMPANOMETRY A tone of 226 Hz is

generated by the tympanometer into the ear canal, where the sound strikes the tympanic membrane, causing vibration. This is done for different air pressures.

Some of this sound is reflected back and picked up by the instrument.

Admittance is how energy is transmitted through the middle ear. The instrument measures the reflected sound and expresses it as an admittance or compliance, plotting the results on a chart known as a tympanogram.

TYMPANOGRAMS

Under normal conditions, the air pressure in the middle ear is approximately the same as ambient pressure since the eustachian tube opens periodically to ventilate the middle ear and to equalize pressure. In a healthy individual, the maximum sound is transmitted through the middle ear when the ambient air pressure in the ear canal is equal to the pressure in the middle ear.

TYMPANOGRAMS

Type A: normal ear Type As: stiff

TM/ossicles Type Ad: flaccid TM or

discontinuity of ossicles Type B: fluid in ear,

mass in ear Type C: negative

pressure means ET dysfunction.

MIDDLE EAR MUSCLES

5= STAPEDIUS MUSCLE9= TENSOR TYMPANI MUSCLE

STAPEDIUS INNERVATION: CN7TENSOR TYMPANI INNERVATION: V3

STAPEDIAL REFLEX ARC

Otoacoustic Emissions, OAEs

Sounds given off by the inner ear, 0-15dB Triggered by sound stimuli to the cochlea Usual emitted with sound pressure level at 25-30 dB Generated by the outer hair cell Can be measured by sound probe in ear canal Completely objective measure Important part of newborn hearing screening Diagnosis of auditory dysfunction in adults

Mechanism of Otoacoustic Emissions

Types of OAEs

Spontaneous otoacoustic emissions

Transient otoacoustic emissions

Distortion product otoacoustic emissions

Sustained frequency otoacoustic emissions (Evoked OAEs)

Otoacoustic

CURVED MEMBRANE EFFECT

Certain areas of TM vibrate more than others.

Ultrastructure Inner HC Outer HC

Position of nucleus Center Base

Cytoplasmic organelles

Scattered Adjacent to cell membrane

Presynaptic specializations

Large Small or absent

Glycogen content Low High

Relation to supporting cells

Completely surrounded

Only at base and surface

Afferent innervation

Inner HC Outer HC

Ganglion cells TYPE 1 TYPE 2

Number of ganglion cells

27000 2100

Hair cell to gangion cell ratio

1.8:1 5.7:1

Efferent innerv.

Source Lateral superior olivary complex

Medial superior olivary complex

Postsynaptic target

Afferent dendrites Base of hair cell