AUDITORY SYSTEMMiguel B. Sebastian Jr. M.D., D.P.P.S.

Department of PhysiologyOLFU

The Human Ear

Functional Anatomy

External Ear

Middle Ear

Middle Ear

Function of the muscles and ossicles Functions

◦ Protects the cochlea from damaging vibrations by excessively loud sounds

◦ Mask low frequency sounds in loud environments◦ Decrease persons sensitivity to his or her own

speech◦ Latency – 40 to 80 milliseconds◦ Contraction of tensor tympani and stapedius

dampens the movement of ossicles and decreases the sensitivity of acoustic apparatus

◦ Reduces intensity of sound transmission by 30 – 40 dB

Tympanic / Attenuation Reflex

Impedance Matching Device (tympanic membrane and ossicles)

Acoustic impedance higher in fluid-filled cochlea than in air

Depends on the following◦ Ratio of the surface area of large tympanic

membrane to that of the smaller oval window◦ Mechanical advantage of the lever system

formed by the ossicle chain

Impedance Matching

Eustachian tube – equalizes pressure differences between external ear and middle ear

Infection: fluid collects in middle ear E.T. blocked pressure differences pain, displacement of the Tympanic Membrane rupture

Otitis Media

Inner Ear

Inner Ear

Inner Ear

Organ of Corti

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Sound◦ Produced by compression and decompression

waves that are transmitted in air or in other elastic media such as water

Physiology of hearing

Sound Frequency◦ Measured by cycles per second – hertz (Hz)◦ Travels more slowly than light

Light – 300,000 km/sec (186,000 miles/sec) Sound – 0.331 – 0.334 km/sec (0.2 miles/sec) – at 20

oC at sea level (↑ with temperature and altitude)- 335 m/sec in air (Berne & Levy)

Speed of sound: Solid > Liquid > Air

Physiology of hearing

Sound wave◦ Expressed as Sound Pressure Level (SPL)◦ Decibel (dB)◦ SPL = 20 log P/Pr

P= sound pressurePr = reference pressure (0.0002

dyne/cm2)- the absolute threshold for

human hearing at 1000 Hz

Physiology of hearing

Normal Human Ear◦ Sensitive to pure tone with frequency of 20 to

20,000 Hz◦ >100 dB – damage the auditory apparatus◦ >120 dB – pain and permanent damage

* As people age, their threshold at high frequency rises thereby reduces the ability to hear such tone (presbycusis)

Physiology of hearing

Sound waves cause T.M. to oscillate Oscillations transmitted to the scala vestibuli creates a pressure difference between the S.V. and S.T. displaces the basilar membrane stereocilia of the hair cells bend.

Upward displacement bends the stereocilia toward the tallest cilium (away from the modiolus) depolarizes the haircells.

Downward deflection – bends the stereocilia away - hyperpolarized

Sound Transmission

Basilar Membrane

• apex is wider than the base

• tension is higher at the base than at the apex

• base vibrate at higher frequency than the apex (frequency analyzer)

Basilar Fibers

• length of the fibers is greater at the apex than at the base

• fiber diameter is greater at the base than at the apex

• base -- shorter and wider• apex – taller and slender• high –frequency

resonance (base), low frequency resonance (apex)

Ossicular Conduction◦ Main pathway for normal hearing

Air Conduction◦ Unimportant for normal hearing◦ Mediated by vibration of round window

Bone Conduction◦ Involves skull bone◦ Plays a role in transmission of extremely loud

sounds

Conduction of sound waves

Presence of one sound decreases an individual’s ability to hear other sounds

Due to the relative and absolute refractoriness of previously stimulated auditory receptors and nerve fibers to other stimuli

Masking

Brodmann’s area 22◦ Concerned with the processing of auditory signals

related to speech◦ Right side – melody, pitch and sound intensity

Planum Temporale◦ Portion of posterior superior temporal gyrus◦ Involved in language-related auditory processing

Other Cortical Areas concerned in Audition

Central Auditory Pathway

Conductive Deafness◦ Due to impaired sound transmission in external

and middle ear◦ Impacts all sound frequencies◦ Causes:

Plugging of the EAC with cerumen or foreign body Otitis externa and otitis media Perforation of eardrum osteosclerosis

Types of Deafness

Sensorineural Deafness◦ Due to loss of cochlear hair cells◦ Problems with CN VIII◦ Lesions within the Central Auditory Pathway◦ Impairs the ability to hear certain pitches

(permanent)◦ Causes:

Aminoglycosides Prolonged exposure to noise Tumors and vascular damage

Types of Deafness

Audiometer Tuning Fork

Auditory Acuity

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