Download Hearing Physiology

Hearing Physiology
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Auditory Physiology
• Sense organ that responds to sound vibrations over
a frequency range of 16-20,000 Hz
• Middle ear- Mechanical
• Inner Ear- Hydraulic
• How do these pieces send coded messages to the
brain?
– Encoding frequencies & intensities
– Brain assembles elements of sound (pitch, loudness and
quality)
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Hydraulic Process
• Both frequency & intensity
characteristics arrive at the oval window
of inner ear as mechanical vibrations
• Within the cochlea, the hydraulic waves
that result correspond to these vibrations
– Frequency is reflected in the # of waves of
compression generated per second,
intensity is reflected in their amplitudes.
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Hydraulic Process
• How does the cochlea respond to these
waves?
– Basilar membrane shape
• Short and stiff at the basal end near oval
window
• Wide and lax at the other end near
helicotrema
• “Tuned” membrane responds selectively to
different frequencies
– High frequencies at narrow end
– Low frequencies at the wide end
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Basilar Membrane “Tuning”
Vestibule
More Stiff
Narrow
Basal
End
Helicotrema
Wide
Apex
Less Stiff
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Traveling Waves
• Vibration transmitted along the basilar membrane -ex. Shake a bed sheet
• Fluid in cochlea moves with movement of stapes
& round window
• Tuning of wave also dictated by stapes
• Wave crest= Frequency of that place on membrane
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Frequency Analysis
• Sound generating traveling wave- Tuning fork
(vibrates single frequency)
– Air-conducted energy delivered to stapes
– Rocking in and out of perilymph in vestibule
• greater sound, greater movement
– Rocking creates compression wave; moves toward exit
(round window)
– Round window displaced outward
– Rarefraction (bounce back) pushes footplate backwards
and doing this sucks in the round window
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Generation of Hydraulic Wave
Compression Wave
Vestibular Canal
Rarefaction Wave
Basilar membrane
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Frequency
• Low frequency (50 Hz)
– Wave will travel to far end of basilar membrane before
peaking (near apex)
• Mid Frequency (1,000 Hz)
– Wave will grow to maximum amplitude about half-way
along basilar membrane (higher frequency=shorter
distance traveled)
• High Frequency (up to 20,000 Hz)
– Crests near basal end of membrane
• Higher frequency, the more resistance the
perilymph offers to being moved by stapes
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Traveling Wave Peaks at Different Frequencies
Low
Basilar Membrane
Mid
High
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Neural Processes
• How does the mechanical motion of the
basilar membrane encode into neural
auditory signals?
– Organ of Corti mounted on the basilar
membrane
– Bending the cilia of hair cells
– Key to bending action is the manner of
attachment to basilar and tectorial membranes
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Shearing Force Bending of Hair Cell Cilia
Tectorial Membrane
Pivot Point
Shearing force
Fluid Pressure
Basilar Membrane
Pivot Point
Shearing force
Fluid Pressure
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Cilia Bending
• When tectorial membrane is displaced
downward, basilar membrane will move
downward; these two membranes will also
move upward together
– Lateral movement of cilia = up & down
movement of basilar membrane
– Radial movement= shearing force of cilia
• Result in complex bending of cilia
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Cilia
Hair Cell
Traveling
Wave
Directions of
Cilia bending
Basilar
Membrane14
Traveling Wave
• Another aspect of the complex motion:
– Wave Envelope
• Summarizes amplitudes of vibration
• Peak at about the same frequency
1,000 Hz
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Generating the Auditory Signal
• Base of hair cell in contact with auditory nerve end
• Outer hair cell primarily responsive to lateral shear
• Inner hair cells, do not drag against tectorial
membrane, have different function, activated by
basilar membrane movement rather than shearing
• Base of hair cell makes a synaptic contact with
auditory nerve ends when cilia move
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Auditory Pathways to Brain
• 30,000 nerve fibers from organ of Corti
join to form auditory nerve
• Organized like two parallel railway
systems between the same city, each
having its own passenger terminals:
– Neural traffic travels from one line to another at
several terminals
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Auditory Pathways to Brain
• Auditory nerve feeds into cochlear nucleus
(first terminal in auditory pathway)
• From cochlear nucleus transfer to ascending
pathways then to auditory cortex, one in each
temporal lobe.
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Auditory Pathways to Brain
• Between cochlear nucleus and auditory cortex:
– 3 sets of terminals
• Superior olive- lowest & smallest (auditory information can
be matched with infor from other ear)
• Lateral lemniscus- next highest level (Info from both ears
provides a basis for a quick reflexive response)
• Auditory projection fibers- last terminal in brainstem
(transfer of auditory neural impulses from one side of brain to
the other at three levels:
– Cochlear nucleus
– Superior olive
– Inferior colliculus
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Auditory Pathways to Brain
• Input from both ears are well represented on
both sides of the brain
– permits:
• Comparison of information about frequency,
intensity and time of arrival of the acoustic signal to
both ears
• “Main line” contralateral auditory pathway
does make it slightly easier to understand
speech better with right ear (main line to
temporal lobe)
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Auditory Pathway
Auditory
Cortex
Medial
Geniculate
Inferior
Colliculus
Lateral
Lemniscus
Superior Olive
Cochlear
Nerve
Cochlear
Nucleus
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Ascending-Descending
Auditory Pathways
Afferent Pathways
Middle Ear
Muscles
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Descending Pathways
• Sensory nerve- Auditory nerve
• 98% of fibers carry afferent information from the
cochlea to brain
• 500 nerve fibers carry efferent neural impulses
from brain to ear
• This information controls the operation of ear
– Some goes to middle ear muscles (protection)
– Most goes to or near the hair cells of the cochlea
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Reading/Assignments
• Seikel: Pgs.565-588
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