Download Hearing and Equilibrium Human Ear Major questions Anatomy of

Hearing and Equilibrium
• Sound wave: disturbance of air molecules
into areas of compression (high pressure)
and rarefaction (low pressure)
• Hearing: our perception of the energy in
these waves
• Travel in all directions (344 m/sec in air) and
energy dissipates
• Frequency determines pitch
• Amplitude determines intensity (loudness)
Major questions
• How can ear be so sensitive (little
energy in soft sound)
(AMPLIFICATION)
• How can ear distinguish pitch?
Human Ear
• Sensitivity - 20 to 20,000 hertz
(cycles/sec)
• ~2000 pitches distinguished (pure tones)
• ~400,000 sound qualities, learned
overlaid frequencies --> timbre)
• Intensity - logarithmic scale (decibel)
• Detect differences of about 0.1 to 0.5
dB
• Sensitivity varies with frequency
Anatomy of the ear (Fig 16.17)
• External Ear
– Channelizes sound
– Air-filled
• Middle Ear
– Transfers sound energy from eardrum to cochlea
– Air-filled
• Internal Ear
– Transduces sound energy into neural signal
– Fluid filled
External and middle ear (Fig 16.18)
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Pinna or auricle
External auditory canal
Eardrum = tympanic membrane
Auditory ossicles (bones)
– Malleus (hammer), incus (anvil) stapes (stirrup)
• Eustachian Tube (Auditory tube)
• Tensor typani and stapedius muscles
(protective)
• Oval window
Internal ear (Fig 16.20a)
• Semicircular canals and vestibule discuss later
• Cochlea (snail-like shape)
–Scala vestibuli (perilymph)
–Cochlear duct (endolymph)
–Scala typani (perilymph)
–Helicotrema
• Round Window
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How to transmit energy from airborne
vibration to liquid-borne vibration
• Not trivial,different viscosities
• Requires amplification
• Ossicles provide mechanical advantage
–lever action
– Malleus absorbs over ~ 50 mm 2
– Stapes transmits to oval window ~ 3 mm 2
– Increases force/unit area pushing against fluid in
scala vestibuli
• 3 bones can buckle, change tension of
tympanic membrane and position of stapes
on OW (tympanic reflex)
Sound WaveTransmission
Cochlea anatomy (Fig 16.20c)
• Encased in temporal bone
• Cochlear duct (CD) divides cochlea into
3 chambers
• Base of CD = basilar membrane
• Organ of Corti (spiral organ)
• Tectorial Membrane
• Auditory nerve= vestibulocochlear nerve
Sound Sensory Receptors
(Fig 16.20d)
• Hair cells sit on basilar membrane
• Apical surface stereocilia- longest
embedded in overlying tectorial
membrane
• Perilymph vibrating -->basilar
membrane--> stereocilia flex back and
forth in or against tectorial membrane
• Mechanical opening of ion channels
Signal Transduction
Receptor potential/action potentials
• Potassium influx from endolymph depolarizes cell
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Hair cells
• Inner hair cell - afferent fibers in nerve
• Three outer hair cells - efferent fibers in
nerve
• Motor input makes them vibrate
• Change the mechanical coupling of
inner hair cell and tectorial membrane?
Loss of hair cells with exposure
to loud noise
• Basilar membrane is stiff and narrow at
windows end and broad and elastic at apex
end
Sound intensity
• Louder noise -->greater energy
--->greater movement of basilar
membrane --> great receptor potential
amplitude --> increased frequency of
action potentials
• Hair cells easily damaged by exposure
to loud noise
Pitch Discrimination: Different regions of
basilar membrane vibrate maximally at different
frequencies
Pitch discrimination
• Lateral inhibition is necessary to
precisely locate input from basilar
membrane vibration (i.e. discriminate
pitch)
• Interconnecting processes inhibit
neighbors.
• Sensitivity: displacement of hair cells in
range of Brownian motion, width of
hydrogen molecule
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