auditory system
TRANSCRIPT
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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