Download Lecture15 - hearing anatomy and physics of sound

HEARING
ANATOMY
The auditory system
• Peripheral auditory system
– Outer ear →
– Middle ear →
– Inner ear
• Central auditory system
– Brainstem nuclei →
– Auditory centers in the cortex
Peripheral auditory system
Inner ear
Outer ear
Middle ear
Peripheral auditory system (PAS)
Central Auditory System (CAS)
Inner ear (PAS)
PAS: Outer Ear
• Auricle/Pinna
– Localisation
PAS: Outer Ear
• External auditory
meatus
• Ear canal
• Auditory canal
– Funnels and resonates
sound
• Cerumen
• Cilia
Middle ear (PAS)
Ossicular chain
PAS: Middle Ear
• Tympanic Membrane
– Ear drum
– Pressure conversion
• Ossicular Chain
– Malleus, Incus, stapes
– Amplifies
• Eustachian Tube
– Equalizes
Protection mechanism of
middle ear
• Acoustic reflex
– Stapedius muscle
• Attached to stapes
– Tensor tympani muscle
• Attached to malleus
– Stiffens the ossicular chain to protect from
loud sounds.
• http://www.youtube.com/watch?v=qW1z-DjOB2Q
The Eustachian Tube
• From middle ear to
nasopharynx
• Equalizes pressure
• Sits closed at
nasal entrance
PAS: Inner Ear
• Cochlea
– Hearing
• Vestibular Apparatus
(SemiCircular Canals)
– Balance
Amplification via 3 mechanisms
• Ear canal
– resonates speech frequencies (2,500Hz – 4,000Hz)
and these amplify the sound 2-4 times
• Ossicular chain
– Leverage power of ossicular chain multiplies tympanic
vibration slightly
• Ear drum → stapes
– Difference in size between tympanic membrane and
footplate of stapes
– Amplifies sound up to 14 times
Helicotrema
Reissner’s
membrane
Round
window
Scala media/
cochlear duct
Scala vestibuli
Scala tympani
Basilar membrane
The cochlea (inner ear – PAS)
Tonotopical organization of the
organ of corti
Central Auditory System (CAS)
• Auditory Nerve
•30,000 fibres from
Cochlea
• Brain Stem Nuclei
•“cross-over point”
• Auditory Cortex
•Heschl’s gyrus
Inner ear (PAS)
Process of Hearing
History
• Bartolomeo Eustachi (1510-15-74)
–
–
–
–
‘tuba auditiva’
Tensor tympani
Stapedius
Inspiration for Hamlet
• Alfonso Corti (1822-1876)
– Studied over 200 cochleas
– Developed techniques to visualize
different structures in the cochlea
– First to describe the organ of corti
Physics of Sound
• Displacement of air particles
• Intensity = loudness (amplitude)
– Unit of measure is the decibel (dB)
• Frequency = pitch (number of cycles per
second)
– Unit of measure is Hertz (Hz)
Pure tone vs complex sound
Types of Hearing Loss
http://www.muschealth.com/video/Default.aspx?videoId=10266&cId=11&type=rel
Portion of Auditory System Affected:
Conductive Loss
Sensorineural Loss
Mixed Loss
Ear Affected:
Unilateral
Bilateral
Conductive Hearing Loss
Results from interference
of sound waves that
would normally pass
through the outer and
middle ear.
Can be completely
corrected (“reversed”) in
most cases.
Sensorineural Hearing Loss
Results from damage
to the cochlea or the
auditory nerve and
upwards (CAS). The
electrical transmission
of the sound is
affected.
Can generally not be
fully corrected
(irreversible).
Mixed Hearing Loss
Results from
interference of
sound waves
through outer
and/or middle ear
AND the electrical
transmission of the
sound.
Degree of hearing loss
• Severity
•
•
•
•
•
•
0-25dB –
26-40dB
41-55dB
56-70dB
71-90dB
90+dB
NONE
–
MILD
–
MODERATE
–
MODERATE-TO-SEVERE
–
SEVERE
–
PROFOUND
Hearing Loss: Demographics
• 10.5% of NZ population experience hearing loss
(approx 450,000 people)
• NZ European 10%
• Maori 12%
•
•
•
•
•
2 per 1000 newborns (well-baby)
4 per 100 newborns (NICU)
4% of population below 45-years of age
30% of population over 65-years of age
36% of population over 70-years of age
Deafness
Defined: The permanent and profound loss of
hearing in both ears with an auditory threshold of
more than 90 decibels.
Hard of Hearing: Mild to Severe loss of hearing.
Types of Deafness
Congenital & Adventitious
Language & Deafness
Prelingual & Postlingual
Risk Factors for Hearing Loss
• Early Age Factors:
– Genetically inherited (50% of all
hearing impairments “run” in families)
– Prenatal Disease (rubella, meningitis)
– Prematurity (less than 37 weeks
gestation, LBW<1500 grams)
– Chronic Otitis Media
– Medications (Gentamicin)
Risk Factors for Hearing Loss
• Later Age Factors:
– Noise (cochlear damage)
– Ageing (Presbycusis)
– Drugs (asprin, Lasix)