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