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Auditory System (Parts of Ear (Process (1) sound wave move tympanic…

- - - - amplifies sound
        
        2 impedance matching mechanisms to overcome energy mismatch b/w air & water
        
        1) lever-arm ratio
        
        ossicles act as levers
        
        malleus arm = shorter than incus arm
        
        small movements in malleus –> large movements in incus
        
        2) area ratio:
        
        amplification due to larger area of tympanic membrane compared to footplate of stapes in oval window
        
        pressure = force/area
  - - - 3 scalae of cochlea
        
        scala vestibuli
        
        scala media
        
        scala tympani
        
        perilymph = fluid in scala vestibuli & tympani
        
        composition = like extracellular fluid = low K+ & high Na+
        
        endolymph = fluid in scala media
        
        composition = like intracellular fluid = high K+ & low Na+
        
        stria vascularis = pumps K+ into scala media
- - - - frequency at which it responds the most
      - frequency w/ lowest threshold intensity
    - - tonotopic maps (place code) – different frequencies displace different parts of basilar membrane
        
        higher frequencies = base & low frequencies = apex
    - - Phase Locking
        
        only works up to about 4 kHz
        
        intermediated frequencies use volley principle– pooled active of many neurons each code different phases on wave to code whole cycle
        
        low frequencies = phase locking @ every cycle
        
        high frequencies can't phase lock – use tonotopy alone
  - - - 1) increase firing rate
      - 2) increase # of active neurons
        
        different nerve fibers have same CF but different threshold intensity
  - - - interaural timing difference for low frequencies
        
        wavelength > width of head
        
        wave diffracts around head w/ no intensity loss
        
        sound onset/phase differences (for continuous sound)
        
        Binaural neurons in medial superior olive = tuned to different interaural time delays
        
        AP created when excitatory inputs from right & left coincide @ neuron
        
        aka EE cells/incidence detectors
      - interaural intensity difference for high frequencies
        
        wavelength < width of head
        
        sound reflect off head –> sound shadow
        
        sound = less intense in ear w/ sound shadow
        
        binaural neurons in lateral superior olive = tuned to different interaural intensity differences
        
        ipsilateral ear provides excitatory (E) input to LSO
        
        contralateral ear provide inhibitory (I) input to LSO cell
        
        aka EI cells
    - - pinna = outer ear structure
        
        important for sound localization
        
        directs sound into external auditory meatus differently for sounds above & below
- - - - range of frequencies for humans = 20Hz - 20 KHz
- - - - basilar membrane moves up & down w/ movement of perilymph
        
        apex = wide & floppy – low frequencies
        
        PLACE CODE (tonotopy) along length of membrane – diff frequencies move diff part of membrane ^
        
        base = narrow & stiff – high frequencies
  - - - top of hair cells = stereocilia; kinocilium = tallest stereocilia
        
        3 rows outer hair cells & 1 row inner hair cells
        
        inner hair cells receive most of afferent innervation (affect brain)
        
        OHCs = cochlear amplier
        
        OHCs receive most efferents from CNS & modulate sensitivity of IHCS (receives effects of brain)
        
        as cell membrane potential fluctuates up & down, motor protein prestin molecules expands & contract
        
        cell expands & contracts
        
        makes basilar membrane move –> amplifying movement of basilar membrane –> IHC movement also amplified –> signal to brain amplified
        
        displacement of basilar membrane –> deflection of stereocilia on hair cells
        
        movement of stereocilia toward kinocilium open mechanically-gated channels & depolarizes hair cell
        
        tip links: elastic filaments that connect & open channels
        
        resting Vm of cell relative to perilymph = -70mV
        
        Very high K+ in endolymph, Ek = 0 mV
        
        K+ moves in cell through mechanically-gated channels
        
        Depolarization opens voltage-gated Ca2+ channels
        
        Ca2+ triggers release of neurotransmitter –> 8th nerve afferent depolarized
        
        movement away from kinocilium hyperpolarizes cell
- - - - EE & EI cells organized in rows