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
maleus incus stapes Eardrum Oval window Round window Middle Ear
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
Scala tympani Perilymph – high Na+, low K+ Scala vestibuli Scala media Endolymph – low Na+, high K+ Basilar membrane cochlear nerve Cross section of the Cochlea
Hair bundle Outer Hair Cell Tectorial membrane Basilar membrane Inner Hair Cell Vibrates in response to sound Shear force generated Hinge Points
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
Kinocilium – the tallest one Stereocilia
Tip Link
K+ Rest Tip Link Active Adaptation
K+ Depolarization Ca++ Voltage gated Ca channel 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+
Vm 0 excitation Inhibition
How do the hair cells repolarize?
Tight Junctions – separate the extracellular fluids, & create two extracellular environments
perily K+ Ca++ K+ Perilymph Low K+, High Na+ Repolarization Endolymph High 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 membrane Unrolled cochlea Distance from oval window Membrane Displacement 10,000 Hz 1000 Hz 100 Hz Base Apex 20 Hz
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