Hearing

Ronald Allan Cruz, MDDepartment of PhysiologyFEU-NRMF Institute of Medicine

External ear Pinna

External auditory cannal

Cerumen

Middle ear Tympanic membrane

Sensitive Periodic Well damped Distorts sound waves

>45 db

Middle ear Ear ossicles

Malleus, incus, stapes Reinforce sound

waves Impedance

matching High amplitude-low

pressure sound waves converted to low amplitude-high pressure waves

Sensitive: 300-3000 cps

Middle ear Tensor tympani

Stapedius

Eustachian tube

Middle ear Acoustic reflex

Attenuation reflex tension dampens

vibrations by 30-40db sensitivity to own

speech Protects the cochlea

from loud tones Loud low tones Masks low tones

Middle ear Impedance matching

Tympanic membrane, oval window, ossicles

the force and pressure of waves going to the oval window: 22x

the sensitivity of hearing

Internal ear Vestibule

Semi-circular canals

Cochlia

Internal ear Cochlea

Scala vestibuli Oval window

Scala media Scala tympani

Round window

Endolymph Perilymph

Reisner’s membrane Basilar membrane

Internal ear Endocochlear

potential Endolymph – K, Na Perilymph – K, Na Positive scala media

80 mv electrical potential

Bodies of hair cells: perilymph, -70 mv

Apex of hair cells: endolymph, -150 mv

Internal ear Organ of Corti

Basilar membrane, basilar fibers

Rods of Corti, reticular lamina

Hair cells Inner hair cells Outer hair cells

Tectorial membrane

CN VIII

Steriocilia Kinocilium

Vibration of bodies which can evoke an auditory sensation

Molecular motion in the direction of energy transmission

Rarefaction in pressure

length

rarefaction

compression

pressure

amplitude

Propagated with specifiable velocity according to the characteristics of the medium and independently of the intensity

density of the medium, velocity

Air 1000 ft/sec

Water 4700 ft/sec

Wood 13000 ft/sec

Steel 16500 ft/sec

Distance travelled by sound in one period

W=V/F W – wavelength V – velocity of sound F – frequency

500 cycle tone in air has a wavelength of 2 ft

Absorption

Reflection

Refraction

Diffraction

Properties of sound Frequency

Pitch

Intensity

Loudness

Quality of the sensation that permits a sound to be classified in the scale from high to low

Determined by the frequency of the sound waves

Hertz

Amplitude modifies pitch 1000-3000 cps: frequency, pitch

Pitch does not change perceptively with changes in intensity

amplitude of high frequency waves, pitch >3000 cps: intensity, pitch

amplitude of low frequency waves, pitch <1000 cps: intensity, pitch

Telephone theory Pitch is a function of

the auditory center Basilar membrane

moves more as a unit

Place theory Pitch is a function of

the cochlea Pitch is determined by

the vibrating portion of the basilar membrane

Helmholtz’s resonance theory

Bekesy’s travelling wave theory

Helmholtz’s Resonance Theory Basilar membrane has fibers of different lengths

and diameter Basilar fibers

Length from oval window to helicotrema Diameter from oval window to helicotrema

Fibers vibrate selectively to tones of different frequencies

Short fibers near the oval window vibrate to higher frequencies

Long fibers near the apex vibrate to lower frequencies

Bekesy’s Travelling Wave Theory The cochlea is a tuned structure

Width of the basilar membrane increases from base to helicotrema

The basilar membrane exhibits graded stiffness Mass of organ of Corti increases towards the apex

Resonant frequency of cochlear partition is highest at the stapes, and decreases along its length

high frequency: displacement at the stapes low frequency: displacement at the apex Short wavelengths will die out more quickly

Bekesy’s Travelling Wave Theory

Length and diameter of hair cells Decreasing elasticity coefficient of basilar fibers

from the oval window to the apex Place principle

Frequencies are detected based on the area of the basilar membrane that is most stimulated

20-20000 cps 500-5000 cps at

60db 50-8000 cps at old

age Presbycusis

Loudness Psychologic reaction

to the intensity of the sound wave

Intensity Force or strength of

sound Depends on

amplitude

amplitude, intensity Inner and outer hair

cells are stimulated More nerve impulses

are produced Spatial summation

Stimulation of hair cells on the fringes of the resonating portion of the basilar membrane

Amplitude modifies pitch amplitude of high frequency waves, pitch

>3000 cps: intensity, pitch amplitude of low frequency waves, pitch

<1000 cps: intensity, pitch

1000-3000 cps: pitch does not change perceptively with changes in intensity

Loud sounds cause the width of the resonating membranes to increase in oscillations

Outer hair cells are stimulated with louder sounds

I = log E1/E2

I – intensity E1 – intensity of

observed sound E2 – intensity of

reference sound If intensity of sound A

is 10x that of sound B Ratio is 10:1 Log of ratio is 1 The intensities differ

by 1 bel

0.1 bel = 1 decibel 1 decibel = 1.26x

1 bel = 10 decibel 10 decibel = 12.6x

Least intensity that can be heard by the average person

Varies for each frequency within the pitch range

500 cps: threshold is highest2048 cps: threshold is lowest

<1000 cps: pitch , threshold

Low frequency: vibratory pressure

>3000 cps: pitch , threshold

High frequency: pain

1000-3000 cps: minimal thresholdThreshold increase for tones higher and lower than this

frequency

120 decibels Highest intensity of

sound that can be heard without pain

1000000 times the lowest auditory threshold

db hrs

60 Normal conversation

90 8 Shouting at 2 ft

100 2 LRT train passing

120 <0.25

Jet take-off, sandblasting

Louder sounds: Evoke vibrations of

greater amplitude in the basilar membrane and hair cells, thus nerve endings are stimulated at faster rates

Some hair cells are stimulated only when a certain intensity is reached

Spatial summation

Timbre Quality of sound Relative amplitudes of the various harmonics

yield a unique wave form for each sound source

Property of complex sounds Enables us to distinguish musical

instruments, voices, etc

Musical Regularly repeated

wave patterns

Noise Non-periodic

vibrations

Spatial pattern of neuronal stimulation Medial superior olivary nucleus

Difference in time arrival of the sound waves to each ear

Difference in the phase of the sound waves to each ear: For low pitch sound

Lateral superior olivary nucleus: Difference in intensities of the sound waves to each ear:

Localization is difficult for continuous sounds, pure tones, and noises

Ossicular route

Air route conduction

Osseous route

Cochlea CN VIII

Medial geniculate

body

Temporal lobeBA 41, 42:

6 tonotopic maps, isofrequency

columnsBA 22: Wernickie’s

area

Auditory radiation

Cochlear nuclei of restiform bodyDorsal nucleiVentral nuclei

Superior olivary nucleus

Superior olivary

complex

Lateral lemniscus

Inferior colliculus

Crossing-over in the brain stem Trapezoid body Lateral lemniscus

Commissure of Probst Inferior colliculi

Reticular activating system of the brain stem

Vermis

Retrograde pathways Cortex to cochlea

Superior olivary nucleus to organ of Corti

Final pathway Inhibitory: 15-20 db

Tuning of the receptor system

Brain stem to hair cells Shortening of outer hair

cells Change in stiffness of

outer hair cells

Whisper test Maximum distance

at which sound can be heard is determined and compared with that which can be heard by the normal ear

Expressed as a fraction of the normal

Whisper: 30ft

Tuning fork test Test fork is held close

to the ear Time from the moment

the fork is struck until the sound becomes inaudible is determined

It is compared with the normal time

Degree of hearing loss is determined by the difference

Audiometry Precise testing for auditory function Tones of varied intensity and pitch are generated Tests air and bone conduction

Conduction deafness Otosclerosis Otitis media Perforated tympanic

membrane Trauma

Central deafness Psychologic CVA

Sensory-neural deafness Ototoxic drugs Presbycusis Tumors Trauma

Webber test Tuning fork at

midline Check for laterality

Rinne test Tuning fork at ear

with lateralized sound Tuning fork at

mastoid process then at pinna

Air conduction > bone conduction

I’m gonna be woundedI’m gonna be your wound I’m gonna bruise youYou’re gonna be my bruise

- Steven SaterFrank Wedekind

Spring Awakening