T2L12 physiology of hearing Flashcards
function of hearing
- alter to danger
- localising objects
- recognition
- communication
range of human hearing
20-20k Hz
- adults lose high frequencies
- intensity range varies 14 orders of magnitude
- more than 90db can lead to permanent damage
4 parts of peripheral auditory system
- outer ear
- middle ear
- cochlear
- auditory nerve
middle ear
increases pressure by 45x
- prevents sound being relflected back into fluid filled cochlear
s7
otitis media
infection/inflammation of middle ear
- usually self limiting
- kiddies mainly (from upper resp tract infection)
glue ear- a secretory form which has effusion
chronic can cause hearing loss, may need draining
otosclerosis
- fusion of stapes with oval window
- fixed with surgery
inner ear
s9
- chochlea is long fluid filled tube
- different parts tuned to different frequencies
basal end = high frequency
apical end = low frequency
see also s10
cochlear fluid composition
scala vestibuli and scala tympani contain PERILYMPH - normal extracellular fluid - high na low k
scala media contains ENDOLYMH- rich in k and low in na, electric potential of +80mV
the hairs bathe in this
orgna of corti
- detects sound induced motions of basilar membrane
2 sensory hairs:
- inner hair cells - afferent nerve fibres
- outer hair cells- efferent nerve fibres
only 15,000 in each human cochlear and cant be regenerated.
mechanotransduction in hair cells
s13
moving hairs open non selective cation channels, mechano-electrical transducer (MET) channels, at lower end of tip links
k+ enters and depolarises hair cell, driven by its electrochemical gradient. ca also triggers adaptation
- voltage gated ca2+ channels open, Ca triggers vesicle release
- afferent nerve fibres activated
inner hair cells = sensory
outer hair cells = sensori motor cells
electromobility of outer hair cells
- outer hair cells amplify basilar membrane motion
- depolarise = shorten
- hyperpolarise - lengthen
- prestin in basolateral membrane is OHC membrane
OUTER HAIRS ARE ELECTROMOTILE
afferent innervation of cochlea
Neurons in cochlear (spiral) ganglion innervate hair cells and project axons to the brain via the auditory branch of the VIIIth nerve
Each inner hair cell is innervated by axons from 10-20 Type I spiral neurons that signal the reception of sound over a wide range 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
s16
efferent innervation of cochlear
efferent fibres from medial olive innervate the outer hair cells directly
efferent fibres from lateral olive synapse on type I afferent fibres (modifies sensitivity of cochlear)
s17
PERIPHERAL AUDIOTRY SYSTEM OVERVIEW
s18
Outer ear: collects sounds and funnels them on tympanic membrane
Middle ear: transmits vibrations of tympanum to oval window of cochlea,
increases pressure 45x
Cochlea: tonotopically organised; apical end responds to low frequencies,
basal end to high frequencies
Organ of Corti: contains sensory hair cells that detect vibrations of
the basilar membrane and convert them into electrical signals
Inner hair cells are purely sensory
Outer hair cells are sensorimotor, amplify basilar membrane motion
Cochlear ganglion: transmits afferent information to brain via VIIIth nerve
Type I neurons innervate inner hair cells
Type II neurons innervate outer hair cells
Efferent innervation of afferent synapses and outer hair cells modifies
cochlear responses
sensorineural hearing loss
- noise
- mitochondrial damage to hair cells
- glutamate excitotoxicity - ageing
- 30% people over 70 have hearing loss - ototoxic drugs
- eg antibiotics, loop diuretics - genetic mutation
- syndromic and non syndromic
- 1:2000
targets of deafness genes in cochlear 6
- tight junctions
- gap junctions
- afferent synapse
- transduction complex
- stria vascularis
- tectorial membrane
cochlear implants
- surgically implanted
- expensive
- good results but music sounds awful and speech robotic
components of central auditory system6
primary auditory cortex
medial geniculate body
inferior colliculus
nuclei of lateral leminiscus
superior olivary complex
cochlear nucleus
s22
cochlear nucleus
- parallel processing starts in cochlear nucleus
- auditoruy nerve fibres from chochlear ganglion innervate many times of neuron
- neurons extract info about level, onset and timing of sounds
superior olivary complex
2 binaural cues used to localise sounds in space:
1. interaural level differences detected in the lateral superior olive (LSO)
- intraural time differences are detected by medial superior olive (MSO)
inferior colliculus
- obligatory synaptic station for all afferents
Laminar organisation in ICC, iso-frequency sheets
Combines complex frequency and amplitude analysis of DCN
with information on sound localization from SOC
May encode complexity and localization of sounds
Auditory reflex centre; reflexive orientation to stimuli
s25
auditory cortex s26
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 (motor aphasia) and Wernicke’s (sensory aphasia) areas also impair the production and comprehension of speech
overview outer auditory system
Cochlear Nucleus (CN) – parallel processing starts here
Superior Olivary Complex (SOC) – uses inter-aural time differences and inter-aural intensity differences for localization
Inferior Colliculus (IC) – combines spatial analysis from SOC with information from dorsal CN, and directs auditory reflexes
Auditory Cortex (AC) – many functions including the analysis of complex sounds (eg speech) and sound localization
