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Dr. Gabriella Kékesi

Hearing: the function of the outer, the

middle and inner ear. Hearing tests. The

auditory pathways

74. Hearing: the function of the outer, the middle and inner ear. Hearing tests. The auditory pathways

Define the following categories: pure (basic) ton, sound (musical tone), noise, frequency, loudness and intensity

of the sound, propagation of the sound, sound pressure level (dB).

Describe the function of the outer, middle, and inner ear structures in the mechano-electrical transduction

process of sound energy into nerve impulses. Describe the acustic impedance matching.

Describe the differences between bone and air conduction

Describe the nerves and muscles in the middle ear and explain their role in withdrawal reflexes Define the

difference between conductive, sensory and neural loss of hearing.

Explain the frequency analysis performed by the cochlea on the basis of its physical properties (Békésy theory,

tonotopy).

Identify the neuronal elements of the organ of Corti. Explain the function of inner and outer hair cells.

Explain how deformations of the basilar membrane are converted into action potentials in auditory nerve fibers.

Describe the auditory pathway. Describe the role of frequency code and population code in hearing and explain

the binaural hearing.

Normal values: frequency range of human hearing: 20-20000 Hz, sound pressure level of human hearing:

0-120 dB, reference sound pressure level: 20 µPa, threshold of human hearing: 0 dB, frequency range of

human speech: 250-4000 Hz, reference frequency of the phon scale: 1000 Hz

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Sound: longitudinal waves that propagates through compressible media

(cannot travel through vacuum)

Sound is the oscillation of pressure (vibration), a series of compressions (where molecules are dense) and rarefactions (where molecules are sparse)– pressure difference

Sound characteristics• Sound characteristics:

– Frequency: measure of how many vibrations occur in one second, and directly corresponds to the pitch of a sound (Hz) -

The higher the frequency the higher the pitch

– Amplitude: the higher the amplitude the higher the volume

– Wave length

– Sound pressure

– Intensity

– Velocity

• Acoustic impedance indicates how much sound pressure is generated by the vibration of

molecules of a particular acoustic medium at a given frequency = resistance of the medium

against the sound propagation

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• A pure tone is a tone with a sinusoidal waveform

• A complex tone is any musical tone that is periodic and can be

described as a sum of simple tones with harmonically related

frequencies of a single frequency.

• A harmonic series is the sequence of all multiples of a base

frequency.

Human hearing

• Sensitivity range: 20-20 000 Hz (0-120 dB)

• Speach range: 1 000 – 4 000 Hz

• Hearing threshold: 2 000 Hz (0 dB)

• Infrasounds: below 20 Hz (elephant, owl)

• Ultrasounds: upper than 20 000 Hz (bat, delfin)

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Sound Pressure Level (SPL)

• Intensitiy of the sound: reference sound

pressure of 20 micropascals (μPa), which

is considered the threshold of human

hearing at 2000 Hz – Alexander Graham

BELL – 0 dB

– Expression unit: dB (decibel)

• SPL [dB] =20 log P/20µPa

• The phon is a unit of loudness level for

pure tones. Its purpose is to compensate

for the effect of frequency on the

perceived loudness of tones

• At 1000 Hz: dB=phon

Ear: is the organ that detects sound. It not only receives sound,

but also aids in balance and body position. The ear is part of the

auditory system.

Portions:

• Outer ear: auricle (pinna) and earcanal, surface of ear drum (tympanicmembrane)

• Middle ear: Couple the sound fromthe opening of the ear canal to thecochlea. Increases the soud pressureand transmits sound waves.

• Inner ear: organ of hearing (cochlea) and vestibular apparatus (labyrinth)

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Outer ear

Auricula (pinna)

• Flesh covered cartilage appendage

Ear canal

• Partially cartilage, partially lies on the bone of the skull

• Ear wax (cerumen) is produced by glands. Hairs

• Functions as a resonator

Tympanic membrane (ear drum)

responsibilities: plays an important role in the conduction of sound.

• Help to get sound (imposes filtering)

• Help localize the direction of the sound source

• Serving as a resonator: it increases the pressure of the incoming acoustic signal by

some dB

• Serving as a transducer, the membrane converts acoustic pressure waves into

mechanical motion.

Figure 5 Otoscopical images

Schilder, A. G. M. et al. (2016) Otitis mediaNat. Rev. Dis. Primers doi:10.1038/nrdp.2016.63

Parts a, c and d reproduced with permission from Onerci, M. in Diagnosis in

Otorhinolaryngology: an Illustrated Guide Ch. 1 (ed. Onerci, M.)

(Springer, 2010), Springer. Part b courtesy of D. McCormick, University of Texas Medical

Branch, Galveston, Texas, USA

Otoscopynormal acute otitis media

otitis media with effusionventillation tube

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Middle ear – air filled cavity

cavum thympani

• Ear drum

• Eustachian-tube (tuba auditiva)

• Connects from the chamber of the middle ear to the back of the nasopharynx

• ventilation

• Oval and round window

• Oval window connects to the stapes; round window is closed by the „secondary eardrum”

Ear bones - ossicles

• malleus – hummer

• incus – anvil

• Stapes - stirrup

Muscles in the middle ear:

• Tensor tympanic muscle: attached to the malleus and keeps the tympanic membrane tensed –

allowes sound vibrations on the tympanic membrane to be transmitted to the ossicles. Pulls the

malleus inward.

• Stapedius muscle: pulls the stapes outward (from ovale window), protects against overly loud

vibrations

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Tympanic (acoustic) reflex - attenuation reflex

• physical response to overly loud noises

• protect the cochlea from damage

• The movement of the ossicles may be stiffened by two muscles, the stapedius and

tensor tympani, which are under the control of the facial nerve and trigeminal nerve,

respectively. These muscles contract in response to loud sounds, thereby reducing the

transmission of sound to the inner ear.

•

– bilateral reflex

– protect against low frequency sounds

– do not protects against single sounds with high intensity (fulmination, rifle shot)

Tympanometry

Tympanometry is an examination used to test the condition of the middle ear and mobility of the tympanic membrane.

A tone of 226 Hz is generated by the tympanometer into the ear

canal, where the sound strikes the tympanic membrane, causing

vibration of the middle ear, which in turn results in the conscious

perception of hearing. Some of this sound is reflected back and

picked up by the instrument. Most middle ear problems result in

stiffening of the middle ear, which causes more of the sound to be

reflected back.

There is a normal pressure in

the middle ear with normal

mobility of the eardrum and

ossicles.

fluid in the middle ear, (b) perforation of the tympanic

consistent with negative pressure in the middle ear space

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Ear bones – ossicles

malleus, incus, stapes: they are suspended by ligaments in such a

way that the combined malleus and incus act as a single lever.

responsibilities: ossicles works to efficiently couple the sound from

the opening of the ear canal to the cochlea. There are several

simple mechanisms that combine to increase the sound

pressure - Impedance matching

1. „hydraulic principle”:

Sound energy strikes the tympanic membrane and is concentrated to the smaller

footplate.

2. „lever principle”:

increase in the force applied to the stapes footplate compared with that applied to

the malleus

3. „round window protection”

channels the sound pressure to one end of the cochlea, and protects the other

end from being struck by sound waves

abnomralities: conductive hearing loss

Conduction of sound from the tympanic

membrane to the cochlea

• Ossicles conduct sound from the

tympanic membrane through the

middle ear to the cochlea

• Handle of malleus is attatched to the

eardrum

• Incus moves with malleus (bounded

with ligaments)

• Incus also articulates with the stem

of the stapes

• Footplate (base) of the stapes lies

against membraneous labyrinth of

the cochlea in the openinig of the

oval window

• Every time the tympanic

membrane moves inward the

stapes push forward the oval

window (cochlear fluid); and to

pull backward on the fluid every

time the malleus moves outward

• Impedance matching by the

ossicular system

– Increase the force of movement

about 1.3 times

– Surface area differences of the

tympanic membrane and stapes (17

times)

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Inner ear

Consists bony labyrinth in the temporal bone of the skull with a

system of passages comprising two main functional parts:

– organ of hearing (cochlea)

– vestibular apparatus (labyrinth): vestibule of the ear and

semicircular canals

• Innervation: VIII. cranial nerve

• Membraneous labyrinth runs inside of the bony labyrinth

(between the perilymph fluid)

• Frequence analyzing

• Mechano-electrical transduction

• Air conduction: outer ear→ middle ear ossicles→ inner ear

• Bone conduction: vibration of skull bones → inner ear

(without middle ear)

Cochlea

• System of coiled tubes

• Stapes – foramen ovale

• portions:

– Fluid filled hollows

– Scala vestibuli, perilympha

– Reissner’s membrane

– Scala media: Corti-organ, endolympha, n. cochlearis

– Basilar membrane

– Scala tympani, perilympha

• role: frequence encoding

Faceplate

of stapes

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Organ of

Corti

Contains electromechanically sensitive hair cells. They are the

receptors and organ that generate nerve impulses in response to soud

vibration.

Structure: two specialized types of epthelial cells

– Three raws of outer/external hair cells:

• Attach to tectorial membrane

• Stereocilia are contracted as the result of depolarization, thus pull the

membrana tectoria closer to the inner hair cells (amplifier)

• Sensitive to sounds with high intensity

• PRESTIN

• Certain medicines may destroy them (pl. streptomycin, ASA)

– Single row of inner/internal hair cells:

• Do not attache to tectorial membrane

– Tectorial membrane

• The bases of the hair cells synapse with a

network of cochlea nerve endings

• Generates nerve impulses in response to

vibration of the basilar membrane.

The auditory sense organ.

Martin Schwander et al. J Cell Biol 2010;190:9-20

© 2010 Schwander et al.

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Tonotopy: is the spatial arrangement of where sounds of

different frequency are processed in the

brain. Tones close to each other in terms of

frequency are represented in topologically

neighbouring regions in the brain.

Different regions of the basilar membrane in the

organ of Corti, vibrate at different sinusoidal

frequencies due to variations in thickness

and width along the length of the

membrane. Nerves that transmit

information from different regions of the

basilar membrane therefore encode

frequency tonotopically:

– base: sounds of high pitch

– helicotrema: sounds of low pitch

mechanisms:

• Traveling wave along the basilar membrane

• Cochlear amplifier

• Best frequency – phase locking

Georg von Békésy (Békésy György)hungarian biophycist

In 1961, he was awarded the Nobel Prize in Physiology or Medicine for

his research on the function of the cochlea in the mammalian hearing

organ.

Research

Békésy developed a method for dissecting the inner ear of human cadavers while

leaving the cochlea partly intact. By using strobe photography and silver flakes as a

marker, he was able to observe that the basilar membrane moves like a surface

wave when stimulated by sound. Because of the structure of the cochlea and the

basilar membrane, different frequencies of sound cause the maximum

amplitudes of the waves to occur at different places on the basilar membrane

along the coil of the cochlea.

He concluded that his observations showed how different sound wave frequencies

are locally dispersed before exciting different nerve fibers that lead from the

cochlea to the brain. He theorized that the placement of each sensory cell (hair

cell) along the coil of the cochlea corresponds to a specific frequency of sound

(the so-called tonotopy). Békésy later developed a mechanical model of the

cochlea, which confirmed the concept of frequency dispersion by the basilar

membrane in the mammalian cochlea. But this model could not provide any

information as to a possible function of this frequency dispersion in the process of

hearing.

1899-1972

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A. J. Hudspeth, Integrating the active process of hair cells with cochlear function.

Nature Reviews Neuroscience 15, 600–614 (2014) doi:10.1038/nrn3786

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Inner ear hair cells. Coloured scanning electron micrograph

(SEM) of sensory hair cells from the cochlea of the inner ear.

The hairs are surrounded by a fluid (endolymph). As sound enters the ear it causes waves to

form in the endolymph, which in turn cause these hairs to move. The movement is

converted into an electrical signal, which is passed to the brain. Each crescent-shaped

arrangement of hairs lies on the top of a single cell.

Nervus cochlearis

• Action potential

– 1 n. cochlearis afferent fiber – 1 inner hair cell

– 1 inner hair cell receives more inputs

– Trm release – ggl. spirale

• Analysis of intensity in the cochlea

– Frequence code:

• the intensity of the sound is

proportional to the frequency of the

action potential

– Population code:

• the sound intensity is proportional to

the deflection of the m. basilaris –

adjacent afferent fibers are also

stimulated with higher sound

intensity

– Phase locking:

• Characteristic frequence

• The maximum of the frequency of the

action potential correlates with the

maximum of the soud pressure

Efferentation

• lateral olivocochlear bound – decreases

the transmitter release from the inner

hair cells - adjusts the sensitivity of the

cochlear afferent fibers

• medial olivocochlear bound – inhibits

the amlifier role of the outer hair cells

on the contralateral cochlea

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Phase locking: an observed phenomenon (in support of the volley principle) where neurons fire in synchrony with the phase of a stimulus. No individual neuron could fire at each peak, but a bunch of phase-locked neurons working together can produce a burst of activity at each peak, and so the firing frequency of a collection of neurons can indeed mimic the frequency of the stimulus.

Central auditory processing

Br 41, 42

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Examination of hearing loss

• Air vs bone conduction

• Rinne test - presence of conductive hearing loss.

• The Rinne test is performed by placing a vibrating

tuning fork (512 Hz) against the patient's mastoid

bone and asking the patient to tell you when the

sound is no longer heard. Quickly position the still

vibrating tuning fork 1-2 cm from the auditory canal,

and again ask the patient to tell you if they are able

to hear the tuning fork.

• A normal or positive Rinne test is when the sound

heard outside of the ear (air conduction) is louder

than bone conduction.

• In conductive hearing loss, bone conduction is better

than air , a negative Rinne.

• Conductive or sensorineural

• Weber test. It can detect unilateral (one-sided)

conductive hearing loss (middle ear hearing

loss) and unilateral sensorineural hearing loss

(inner ear hearing loss).

• a vibrating tuning fork (256Hz) is placed on top

of the head equi-distant from the patient's ears.

The patient is asked to report in which ear the

sound is heard louder.

• A normal Weber test has a patient reporting the

sound heard equally in both sides. In an affected

patient, if the defective ear hears the Weber

tuning fork louder, the finding indicates a

conductive hearing loss in the defective ear. In

an affected patient, if the normal ear hears the

tuning fork sound better, there is sensorineural

hearing loss on the other ear (defective ear).

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Audiometer – Audiogram: to determine the nature of

hearing disabilities

Figure 52-13 Audiogram of air conduction deafness resulting from

middle ear sclerosis.

Figure 52-12 Audiogram of the old-age type of nerve deafness.