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Decibel intensity range 10^12 : 1 -> strength relative to reference value (20 microPa) -> close to human hearing threshold at 3 kHz
Decibel sound pressure level equation dB SPL = 20 log10 (Pa/20 microPa)
0 dB meaning Sound has same Pa as reference signal (log(1) = 0)
-ve dB meaning Specified sound less than reference
Human hearing range Infrasound 20 Hz -> ultrasound 20 kHz
Role of pinna Sound localisation -> modifies sound spectra in space dependent manner via head related transfer function (HRTF)
HRTF Head related transfer function -> notches above 5 kHz are direction dependent -> 1st notch cues sound localisation on vertical plane -> sound elevated to +45 degrees -> increase spectral notch frequency -> sound at -45 -> decrease frequency
Pinna plasticity Binaural molds perturbed original spectral-shape cues but localisation relearned w/in few weeks -> didn't interfere w/ original spectral cues before/after removal -> plasticity
Function of middle ear Impedance matching, protection against loud sounds, anti-masking at high sound levels
Components of outer ear Pinna, EAM
Role of EAM Gain of transfer -> amplifies low/less penetrating sounds of human voice -> peak gain around 4 kHz -> 13dB gain -> human speech
Function of outer ear Sound localisation (pinna), low frequency amplification (EAM)
Impedance matching Tympanic membrane (60 sq mm) -> stapes footplate (3.2 sq mm) -> increase force applied to overcome impedance differences and prevent reflection of sound waves instead of transmission
Protection against loud sounds Acoustic reflex -> loud sound -> stapedius mainly stiffens ossicular chain (pulls stapes away from oval window), tensor tympani stiffens ossicular chain (loads tympanic membrane by pulling malleus medial -> tense tympanic membrane -> dampen vibration)
Tensor tympani innervation Tensor tympani branch of Vc -> supplied by motor fibres and therefore doesn't receive trigeminal ganglion fibers
Stapedius innervation Nerve to stapedius branch of VII
Conductive hearing loss Middle ear cavity fluid, BC > AC -> -ve Rinne's test -> defective ear hears Weber tuning fork louder -> low frequency hearing loss of >30 dB
Sensorineural hearing loss Cochlea/auditory pathway damage, AC > BC -> +ve Rinne's test -> defective ear hears Weber tuning fork quieter
Rinne's test Vibrating tuning fork held on mastoid process then next to EAM -> measure air vs bone conduction
Weber test Vibrating tuning fork held on skull in midline -> see which ear is defective
Weber conductive hearing loss Defective ear hears tuning fork louder -> middle ear ossicle problems dampen ambient room noise -> well functioning inner ear picks up sound via bone conduction
Weber sensorineural hearing loss Normal ear hears tuning fork louder -> same bone conduction (equidistant) so only neural problems (no input conversion)
Oval window Opening from stapes footplate (middle ear) into cochlea (inner ear) -> found in base of scala vestibuli
Round window Inferoposterior to oval window (base of scala tympani) -> vibrates at opposite phase to oval window vibrations/bulges outwards -> ensures BM hair cells will be stimulated
Inner hair cells numbers 1 row of 3,000 hair cells -> 1 IHC connects to 10-20 type I afferent fibres -> 90% of CN VIII fibres synapse w/ IHC
Outer hair cells numbers 3 rows of 11,000 hair cells -> 1 type II afferent fibre connects to multiple OHC -> unmyelinated -> 10% of CN VIII fibres synapse w/ OHC
Olivocochlear neuron location and projections W/in superior olivary complex -> medial neurons project to OHC, lateral neurons project to primary afferents beneath IHC
BM band pass filter When shifting frequency up/down from most sensitive point (18 kHz) -> sensitivity decreases -> need higher stimulation frequency for same point on BM to vibrate
OHC hearing loss numbers 40% OHC function loss -> 30 dB threshold loss, 100% OHC function loww -> 55 dB threshold loss
OHC change shape Cell depolarisation -> length decreases -> increase tension on tectorial/BM -> augment vibrations (+ve feedback via -ve damping)
Stria vascularis Capillary loop pumping K+ out of perilymph (SV/T - 7mM) to keep endolymph (SM - 154 mM) K+ high -> large driving force for K+ influx into hair cells when ion channels pulled open -> +80mV (endolymph) to 0 mV (perilymph)
Presbycusis Hearing thresholds elevate w/ increasing age -> loss of endocochlear potential (lateral wall degeneration w/ age -> can't hear high frequency noises
Division of type I auditory nerve fibres Multiple afferents synapse w/ 1 IHC -> low/high threshold fibres -> differential termination in cochlear nucleus
How to discriminate intensity differences over wide range of sound levels Louder stimuli -> more pronounced BM mvmt -> recruit more active fibres
Low threshold fibres Type 1 auditory nerve fibres -> originate from IHC modiolar side (OHC side) -> 15-40 dB SPL -> most sensitive -> high spontaneous firing rate
High threshold fibres Type 1 auditory nerve fibres -> originate from IHC pillar side (CN VIII side) -> 50-90 dB SPL -> least sensitive -> low spontaneous firing rate
Lateral OC neuron fibre functions Protection from loud sounds -> threshold increases after loud tone exposure Efferent anti-masking -> less adaptation with loud background -> increase in tone-induced rate at high levels -> compress AP firing rate output
Interaural time differences Sound travels further to reach one ear -> delay lines in MSO - medial superior olive -> particular neuron receives both right/left AP simultaneously
Interaural phase differences Same frequency waves but shifted slightly -> localisation cue for low frequency (<1.4 kHz) continuous waves -> limited by size of head (ambiguous phase)
Interaural intensity/level differences High frequency waves quieter in ear further away -> 20 dB difference at 90/270 degree azimuths -> insignificant below 1 kHz, LSO has excitatory input from ipsilateral VCN and inhibitory input from ipsilateral MNTB (excitatory contralateral VCN)
MNTB Medial nucleus of trapezoid body -> gives inhibitory input to ipsilateral LOS, excited by contralateral VCN
VCN Ventral cochlear nucleus -> gives excitatory input to ipsilateral LOS, excitatory input to contralateral MNTB (for inhibition)
Created by: vykleung
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