1. Hearing Anatomy of the auditory pathway Hair cells and transduction of sound waves Regional specialization of the cochlea to respond to different frequencies

2. The Ear Middle ear

4. maleus incus stapes Eardrum Oval window Round window Middle Ear

5. 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

7. 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

8. Hair bundle Outer Hair Cell Tectorial membrane Basilar membrane Inner Hair Cell Vibrates in response to sound Shear force generated Hinge Points

9. 3 rows of outer hair cells 1 row of inner hair cells

10. Inner Hair Cell –The actual sensory receptors Outer Hair Cell –Act as mechanical amplifier to sharpen response of basilar membrane

11. Hair Cell

12. Kinocilium – the tallest one Stereocilia 30-100

13. Tip Link

14. K+ Rest Tip Link Active Adaptation

15. K+ Depolarization Ca++ Voltage gated Ca channel Sensory neuron Synaptic vesicles

16. 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

17. 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

18. At rest, some channels open Deflection away from kinocilium, all channels close  hyperpolarization K+

19. Vm 0 excitation Inhibition

20. How do the hair cells repolarize?

21. Tight Junctions – separate the extracellular fluids, & create two extracellular environments

22. perily K+ Ca++ K+ Perilymph Low K+, High Na+ Repolarization Endolymph High K+, Low Na+ Voltage gated K channel

23. Tuning of the sensory response 1.Basilar membrane is specialized to respond to certain frequencies along its length

25. 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

26. Basilar membrane At the base, narrow & stiff  high frequency vibration At the apex, wide & flexible  low frequency vibration

27. Therefore, sensory neurons originating from different regions of the cochlea carry frequency information Apical end  low frequency information Basal end  high frequency information