Hearing
• Anatomy of the auditory pathway
• Hair cells and transduction of sound waves
• Regional specialization of the cochlea to respond to different frequencies
Amplification
• The ear has air / water interface– Poor for transmitting vibration
• Amplification1. Large eardrum small oval window
2. Mechanical levers of the middle ear bones
Scala tympaniPerilymph – high Na+, low K+
Scala vestibuli
Scala mediaEndolymph – low Na+, high K+
Basilar membrane
cochlear nerve
Cross section of the Cochlea
Hair bundle
OuterHair Cell
Tectorial membrane
Basilar membrane
Inner Hair CellVibrates in response to sound
Shear force generated
Hinge Points
• Inner Hair Cell – The actual sensory receptors
• Outer Hair Cell– Act as mechanical amplifier to sharpen
response of basilar membrane
K+
K+
Depolarization
Ca++Ca++
Voltage gated Ca channel
Sensory neuron Sensory neuron
Synaptic vesicles
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
At rest, some channels open Deflection away from kinocilium, all
channels close hyperpolarization
K+
perily
K+
K+
Ca++Ca++
K+ K+
PerilymphLow K+, High Na+
Repolarization
EndolymphHigh K+, Low Na+
Voltage gated K channel
Tuning of the sensory response
1. Basilar membrane is specialized to respond to certain frequencies along its length
Oval window
Round Window
Basilar Membrane
Frequency response of the basilar membraneUnrolled cochlea
Distance from oval windowMem
bran
e D
ispl
acem
ent
10,000 Hz 1000 Hz 100 Hz
BaseApex
20 Hz
Basilar membrane
• At the base, narrow & stiff high frequency vibration
• At the apex, wide & flexible low frequency vibration