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Hearing
• Anatomy of the auditory pathway
• Hair cells and transduction of sound
waves
• Regional specialization of the cochlea to
respond to different frequencies
The Ear
Middle ear
Middle Ear
maleus incus
stapes
Eardrum
Oval window
Round window
Amplification
•
The ear has air / water interface
– Poor for transmitting vibration
•
Amplification
1. Large eardrum  small oval window
2. Mechanical levers of the middle ear bones
Cross section of the Cochlea
Scala vestibuli
Scala media
Endolymph – low Na+, high K+
Scala tympani
Perilymph – high Na+, low K+
cochlear nerve
Basilar membrane
Shear force generated
Tectorial membrane
Hair bundle
Outer
Hair Cell
Basilar membrane
Inner Hair Cell
Hinge Points
Vibrates in response
to sound
3 rows of outer
hair cells
1 row of inner
hair cells
• Inner Hair Cell
– The actual sensory receptors
• Outer Hair Cell
– Act as mechanical amplifier to sharpen
response of basilar membrane
Hair Cell
Hair Cell
Kinocilium – the tallest one
Stereocilia
30-100
Tip Link
Rest
Active
Tip Link
K+
Adaptation
K+
K+
Depolarization
Voltage gated Ca channel
Ca++
Ca++
Synaptic vesicles
Sensory neuron
Sensory neuron
Sequence of Events
• Sound waves transmitted to oval window
of cochlea
• Compression of oval window vibrates the
basilar membrane
• Shear forces between basilar membrane
and tectorial membrane deflect stereocilia
of hair cells
• mechanical opening of cation channel via
the ‘tip link’
• K+ flows into cilia depolarizing the hair cell
• Opens voltage-gated Ca++ channel
• Leads to fusion of synaptic vesicles
• Activates neurotransmitter receptors on
the sensory neuron
K+
At rest, some channels open
Deflection away from kinocilium, all
channels close  hyperpolarization
Inhibition
excitation
Vm
0
• How do the hair cells repolarize?
Tight Junctions – separate the extracellular fluids, & create two extracellular
environments
K+
Repolarization
K+
Endolymph
High K+, Low Na+
Voltage gated K channel
K+ K+
Ca++
Ca++
perily
Perilymph
Low K+, High Na+
Tuning of the sensory response
1. Basilar membrane is specialized to
respond to certain frequencies along its
length
Frequency response of the basilar membrane
Unrolled cochlea
Oval window
Basilar Membrane
Base
Membrane Displacement
Round
Window
10,000 Hz
1000 Hz
100 Hz
Distance from oval window
20 Hz
Apex
Basilar membrane
• At the base, narrow & stiff  high
frequency vibration
• At the apex, wide & flexible  low
frequency vibration
• Therefore, sensory neurons originating
from different regions of the cochlea carry
frequency information
• Apical end  low frequency information
• Basal end  high frequency information