1 hearing physiology. 2 auditory physiology sense organ that responds to sound vibrations over a...
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1
Hearing Physiology
2
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”
High FrequenciesMore Stiff
Less Stiff
WideApex
Low Fre
quencie
s
NarrowBasalEnd
Vestibule
Helicotrema
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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
Tympanic Canal
Basilar membrane
Rarefaction Wave
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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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Low
Mid
High
Traveling Wave Peaks at Different Frequencies
Basilar Membrane
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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
Shearing forceTectorial Membrane
Pivot Point
Basilar MembranePivot Point
Fluid Pressure
Shearing forceFluid 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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Directions ofCilia bending
Cilia
Hair Cell
Traveling Wave
BasilarMembrane
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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 Cortex
Medial Geniculate
InferiorColliculus
LateralLemniscus
CochlearNucleusCochlear
Nerve
Superior Olive
Auditory Pathway
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Ascending-DescendingAuditory Pathways
Afferen
t Path
ways
Efferen
t Path
ways
Middle EarMuscles
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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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