Audition

Auditory structure

Outer ear

Middle ear

Inner ear

Transmission

Signal transduction

Sound basics

Components of sound

Human limits


f: 20Hz to 20,000Hz decreasing with age and exposure to noise exposure --> death of hair cells
Higher frequency loss first - sat, that


Amplitude: 0-80 dB w/o damage, 120 dB - discomfort, 130 dB - damage and pain

Functions:


  • protect tympanic membrane(lies in ext. auditory canal, highly sensitive movement)
  • pinna catches sounds waves and deflects into ext. auditory meatus

Pinna


  • boosts certain sounds by concha and ear canal
  • localisation role

Muscles: stapedius and tensor tympani


  • increase stiffness of bones upon contraction --> dampens transmission
  • contraction activates by intense low f sounds e.g. own speech, enabling focus on ext. sounds

Air filled cavity


  • connects to nasopharynx via Eustachian tube
  • lower tube closed normally: opening leads to equilibration of pressure across tympanic membrane
  • 3 bones in cavity: malleus, incus, stapes

Stapes couples movement of eardrum to cochlea membrane


  • cochlea = fluid filled; force to move tympanic membrane increases
  • tympanic 30x > stapes footplate; concentrates force
  • lever system: decreasing lever arm(displacement) bone to bone; force amplified

Sound arrival for analysis and transduction


  • pass through outer ear across tympanic membrane: amplified through stapes, malleus and incus into the oval window
  • round window vibrates in opposite manner to oval window; allowing cochlea fluid movement

Encased with bone and cochlea coiled


  • outer chambers(V and T) filled with perilymph
  • inner(M) filled with endolymph
  • V and M separated by basilar membrane - organ of Corti located

Organ of corti contains hair cells, supporting cells and nerves


  • hair cells: receptors attached to rigid reticular membrane
  • stiff apical stereocilia vary in height and tip links
  • three rows of OHCs within sterocilia embedded on tectorial membrane
  • single layer of IHCs in stereocilia just short of tectorial membrane

Endolymph establishes endocochlear potental


  • perilymph similar conc to CSF: 7K+ 140Na+
  • endolymph similar to intra: 150K+, 1Na+
  • stria vascularis filters endolymph; transporting K+ into scala media
  • high conc gradient due to tight junctions
  • +80mV potential (80 in endolymph, 0 in perilymph) across basilar membrane
  • +125mV(intra = -45) across cell membrane

Stria vascularis = multi layered


  • main site of energy usage; highly vascularised
  • basal cells linked tightly by Claudin 14 and 9 preventing K+ backflow
  • EEL: Claudin 14 mutation ==> deafness
  • electrical potential: reduce energy usage and reduces vasculature nearby -> pulsatile movement doesn't dirsupt

Pitch is our perception of frequency


  • frequency = cycle rate; pitch = high or low

Speed 343m/s at room temperature and atmospheric pressure


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Amplitude perceived as loudness


Maximum extent of vibration/oscillation

  1. Stapes footplate touches oval window of scala vestibuli - lymph incompressible --> round window on scala tympani must bulge out
  1. Pressure gradient across scala media and endolymph --> basilar membrane vibrates
  1. Vibration/displacement of membrane:
  • bends OHC sterocilia against tectorial membrane
  • IHCs move through endolymph displacing sterocilia
  1. Tip links connecting cilia upon stretch open transduction channels
  • composed of CDH23 and PCDH15 dimer --> Ca2+ dependently linked
  • anchor proteins can move up/down cilium -> sensitivity adjustment

CC: Anchor protein mutation


  • CHD23 mutation causes deafness
  • KO mice = deaf
  1. K+ influx enters along electrochemical gradient
  • endolymph + 80mV
  • intracellular -45mV
  1. Cell depolarisation
  • opening of VGCC leading to the release of glutatamate at the base of the hair cell
  • postsynaptic afferent nerve --> excited

EE: Russell et al, 1986 - Measurement of HC sensitivity


  • 1-3 day postnatal mice cochlea dissection; organ of Cortis cultured for 2-7 days
  • transmembrane p.d measured - glass electrodes - stereocilia displacement monitor
  • 2nm displacement by 0.8mV stimulus(most sensitive)

OHC and IHC differ in spiral ganglion innervation


  • IHCs receive 95% afferent auditory nerve fibres - myelinated
  • 5% of (unmyelinated) fibres innervate 20 OHCs each --> convergence/grouping into receptive fields
    --> low threshold stimulation

EE: Bekesy, 1930s - Human cadaver measurements


  • sound causes travelling wave of displacement along basilar membrane from base --> apex
  • high f: peak = near cochlea base
  • lower f: peak shifts towards apex
  • increase in width and decrease in thickness

OHC electromotility amplifies their sensitivity and sharpens frequency tuning


  • basilar membrane response to weak sounds is amplified --> lowering the threshold for activation
  • Mossbauer technique(gamma ray measurements to qunatify vibrations) --> peak v sharp

Encoding of different sound wave properties


  • broadening of tuning curves and saturation of responses with increasing intensity --> resolution becomes poorer above threshold

Delivery to the auditory cortex

Each IHC innervated by 10 nerve fibres


  • reduced sensitivity as stereocilia dont touch tectorial membrane
  • more discrete than OHC - acuity to small f differences
  • respond more to velocity of basilar membrane rather than displacement --> adaptation

Efferent innervation arrives from superior olive via cholinergic and divergent fibres


  • presynaptic termination on IHCs
  • more extensive and direct onto OHCs -> direct function contrl

Transmembrane p.d drives oscillations in cell length


  • VG conformational changes in the motor protein
  • 70,000 per second - actin-myosin couldn't achieve this

EE: Liberman et al, 2012 - Prestin KO mice


  • OHCs isolated from 5/7 wk old prestin KO and control
  • voltage clamp records changes:

decrease in length of control cells when depolarised
no change in motility of KO cells --> loss of amplification and otoacoustic emissions

Otoacoustic emissions = sounds generated within inner ear


  • dervied from OHC motility
  • spontaneous or evoked by external stimulation
  • used for screening hearing loss in newborns

CC: Tinnitus - derived from OHC motility?


  • imaginary sounds heard as continous high pitched whistling noises
  • feedback too large; unwanted oscillation
  • signifies importance of olivocochlear bundle in modulating

Tonotopicity = point of innervation along basilar membrane; topographic variation within nerve


  • perceived pitch dependent on which fibres active
    -timbre encoded by relative activity of different fibres

Increases in intensity increase firing rate up to 30-50dB above fibre threshold


  • ganglion cells sensitivity to glutamate varies --> fire at different intensities

Two tone suppression: 2nd frequency supresses excitatory tone


  • due to cochlear amplification and basilar membrane movement
  • lateral inhibition: sharpens selectivity

Phase locking enables encoding of low f by timing of action potentials


  • increased probability of firing during certain stages of cycle
  • dont fire at every cycle but firing triggers at same interval
  • when combined a greater frequency encoded

Relay via vestibulocochlear nerve to 1' auditory cortex and tonotopically organised cortical fields

Cochlear nucleus in medulla = 1st nucleus


each division contains neural map:

  • dorsal cochlear nucleus
  • anteroventral cochlear nucleus
  • posteroventral cochlear nucleus

Cell types vary with different firing patterns


  • spherical bushy cells have similar excitation pattern to auditory nerve fibre
  • variance in frequency tuning curve
  • first level with neural inhibiton; progressively increases from VCN to DCN

VCN axons project to neurons in sup. olivary complex


  • site of binaural convergence(input from both)
  • sensitive to phase differences from info received from each side

Medial superior olive receive excitatory input from ipsilateral CN and inhibitory input from contralateral CN


  • sensitive to interaural intensity differences
  • sound localisation importance

Sup. olive --> inf. colliculus in midbrain --> MGN of thalamus --> auditory receiving zones of temporal lobe

1' auditory cortex has tonotopic organisation


  • binaural interactions from brainstem transmitted to cortex
  • similar response pattern neurons clustered together
  • cortical neurons respond better to complex sounds compared to pure tones