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Physiology of the hearing (I)

(Learning objective: 102)

Dr. Attila Nagy

2019

Hearing

• Hearing is a mechanical sense

• A compression wave in air is changed to a vibration of a solid then a wave in a fluid, a displacement of a membrane, a bending of a hair cell and the generation of an action potential

• humans hear sounds 16 to 20000 cycles/second (Hz)

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Phylogenesis of the inner ear

The Hearing System

Basic schematic diagram of the entire auditory system

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Sound System

Source– Any vibrating object

Medium– Any gas, liquid or solid

Receiver– anything designed to

detect the vibrations within the medium originating from the source

A Common Sound System

Illustration of the distribution of molecules surrounding a sound source

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Condensation and Rarefaction

Bands of compression and rarefaction emanating from a sound source

Important Physical Characteristics of Sound

Frequency

• Rate of pressure change as a function of time

• Measured as cycles/sec or Hertz

• The primary determiner of pitch

Intensity

• Magnitude of the pressure change

• Measured as decibel (dB)

• The primary determiner of loudness

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Frequency and Intensity

Sounds a and c share the same frequency and sounds b and c share the same intensity

How to quantify loudness: What is a

decibel?

• 20 lg (p1/p0) dB Sound Pressure Level (SPL)

• (p1: sound of interest, p0: threshold of human hearing at 2 kHz)

• hearing protection: longer exposure of levels above 85 dB

• p0= hearing threshold (2 x 10-5 Pa)

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Loudness and Intensity

Here are some common sounds and their decibel equivalents

Pure (basic) ton: it could be characterized by a single frequency

Sound (musical) tone: a basic pitch tone and and its integral

multiples the harmonics

Noise: it contains several frequencies of the hearable frequency range

(white noise)

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Equal loudness curves, unit (phons)

Psychophysical Measurement

dBPain theshold

Area of speech

hearing theshold

phon

Path of Sound

(air conduction, bone conduction)

AIR CONDUCTION:

• External canal

• Vibrates eardrum

• Vibration moves though ossicles

– Malleus, incus, stapes

• Stapes vibrates oval window of cochlea

• Creates pressure wave in the fluid inside

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The Outer Ear

Major Landmarks:

Pinna

External Auditory Meatus

Tympanic Membrane

Function of the Outer Ear

• Collect and funnel sound to the eardrum

• Protection

• Resonance

• (more esthetic than functional role in

human)

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The Middle Ear

Major Landmarks:

• Eustachian tube

• Oval and round windows

• Ossicles

• Middle ear space

• Middle ear muscles

Muscles of the middle ear

• Stapedius muscle (facial nerve) and Tensor

tympani (trigeminal nerv) may influence of

the movement of the ossicles.

• These muscles contract in response to loud

sounds, thereby reducing the transmission of

sound to the inner ear. This is called Tympanic

reflex.

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incus

stapes

maleus

Thympanic m.

50 mm2

4 mm2

Middle ear amplification (impedance matching)

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BONE CONDUCTION

- Bypasses the outer and the middle ear

- The bones of the scull conduct the vibrations

directly to the inner ear

- Its function in recognition of own woice

•CONDUCTIVE HEARING LOSS

•SENSORY HEARING LOSS

•NEURAL HEARING LOSS

•MIXED HEARING LOSS

Classification of hearing loss

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Located in external ormiddle ear

Temporary

CONDUCTION

DEAFNESS

NERVE

DEAFNESS

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Rinne test –

Weber test

Tuning fork investigations

Audiometry

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Audiometry

Ears closed Failing Eardrum

and ossiclesCochlear or/and

nerval damage

Audiometry

Bone conduction

Air conduction

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Presbyacusis

Presbyacusis

Men Women

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Investigation of pressure relation in the middle ear: Tympanometry

Investigation of pressure relation in the middle ear: Tympanometry

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Investigation of pressure relation in the middle ear: Tympanometry