Physiology of hearing Flashcards
Functions of hearing
Alerting to dangers
Localising objects
Recognition
Communication via speech
The nature of sound
Sounds are travelling pressure waves that propagate through air at 340m/s
Sounds have two important attributes: frequency (Hz) and intensity (dB)
dB
10xlog (sound intensity/ reference intensity)
The range of human hearing
Frequency range of ideal human hearing: approximately 20-20000 Hz
Adults progressively lose high frequencies
Intensity range of human varies over 14 orders of magnitude
Intensities >90dB can lead to permanent hearing damage
The peripheral auditory system
Outer ear
Middle ear
Cochlea
Auditory nerve
The middle ear
An impedance matching device: increase pressure ~ 45x by the ratio of tympanic membrane and oval window areas and to a lesser extent by the lever action of the middle ear ossicles
Prevents sound from being reflected back from the fluid filled cochlea
Otitis media
Infection or inflammation of middle ear (usually self limiting)
Common in children (often from upper respiratory tract infection)
Secretory form with effusion (glue ear, if chronic causes a conductive hearing loss)
Otosclerosis
Fusion of stapes with oval window
Maybe why Beethoven went deaf
Can be fixed by surgery
The inner ear
Cochlea isa . long, coiled, fluid filled tube
Different parts of tube are tuned to different frequencies
Basal end is tuned to high frequencies
Apical end is tuned to low frequencies
Sections of the cochlear duct
Scala vestibuli
Scala media
Scala tympani
Composition of cochlear fluids
Scala vestibuli and scala tympani contain perilymph, a normal extracellular fluid with high Na+ and low K+
Scala media contains endolymph, an unusual extracellular fluid rich in K+ and low in Na+ and an electrical potential of about 80mV
The organ of corti
Detects the sound induced motions of the basilar membrane
Contains two types of sensory hair cells
Apical membrane of hair cells bathed in endolymph
Basolateral membrane of hair cells bathed in perilymph
Two types of sensory hair cells
Inner hair cells innervated by afferent nerve fibres
Outer hair cells mainly innervated by efferent nerve fibres
Mechanotransduction in hair cells
Deflection of the hair bindle opens non selective cation channels (MET) at lower end of tip links
K+ enters and depolarises the hair cell, Ca2+ also enters and causes adaptation
Voltage gated Ca2+ cahnnels open, Ca2+ triggers vesicle release
Afferent nerve fibres are activated
Inner hair cells are sensory, outer hair cells are sensory motor cells
Electromotility of outer hair cells
Outer hair cells amplify basilar membrane motion
Depolarise- shorten; hyperpolarise- lengthen
Prestin, a modified anion exchanger in the basolateral membrane, is the OHC motor
Afferent innervation of the cochlea
Neurons in cochlear ganglion innervate hair cells and project axons to the brain via the auditory branch of the VIII nerve
Each hair cells is innervated by aons from 10-20 type I spiral neurons that signal the reception of sound over wide rang of intensities to the brain
Outer hair cells are innervated by type II spiral neurons that signal the reception of painfully loud sound that causes cochlear damage to the brain
Efferent innervation of the cochlea
Efferent fibres from the medial olive innervate the outer hair cells directly
Efferent fibres from the lateral olive synapse on the type I afferent fibres
Activation of efferent system modifies the sensitivity of the cochlea
Sensorineural hearing loss
Noise
Ageing
Ototoxic drugs
Genetic mutations
Noise causes
Physical effects on hair bundle structure
Mitochondrial damage, cytotoxic free radicals
Glutamate excitotoxicity
Ageing
30% of population over age 70 have significant hearing loss
Hair cells, stria vascularis, cochlear ganglion
Ototoxic drugs
Aminoglycoside antibiotics
Cisplatin
Loop diuretics
Salicylate
Solvents
Genetic mutations
High frequency 1:2000 of live births
Syndromic and non syndromic
.50 deafness genes identified, 80 additional loci
~50% of congenital deafness caused by mutations in gap junction genes
Cochlear implants
Surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf
Expensive: limited to the western world
Results often goof enough to recognise and comprehend speech
- maximum 24 channels to substitute for 15000 hair cells
- speech is reported to sound robotic
- music sounds awful
The cochlear nucleus
Parallel processing starts in cochlear nucleus
Auditory nerve fibres from cochlear ganglion innervate many types of neuron
Neurons extract information about level, onset and timing of sounds
The superior olivary complex
Two binaural cues are used to locaise sounds in space
Interaural level differences are differences are detected in the lateral superior olive
Interaural time differences are detected in the medial superior olive
The inferior colliculus
Obligatory station for all afferents
Laminar organisation in ICC, iso frequency sheets
Combines complex frequency and amplitude analysis of DCN with information on sound localisation from SOC
May encode complexity and locaisation f sounds
Auditory reflex centre, reflexive orientation to stimuli
The auditory cortex
Primary auditory cortex is located on upper surface of temporal lobe
Lesions in auditory cortex cause defects in: sound localisation, discrimination of temporal pattern, intelligibility of speech
Lesions in Broca’s and Wernicke’s areas also impair the production and comprehension of speech
