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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”

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