ultra stuctures of cochlea & its central connections

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ULTRASTRUCTURE OF COCHLEA & ITS CENTRAL CONNECTIONSDR. RAJU MANDAL JUNIOR RESIDENT DEPT. OF E.N.T M.C.H WHAT IS COCHLEA ?The inner ear is contained in the petrous apex of the temporal bone, and is encased in a bony structure called the osseous or bony labyrinthThe labyrinth consists of three continuous sections: Vestibule-Cochlea-Semicircular canals-

WHAT IS COCHLEA ?Snail shaped coiled tube(cochlos is greek for snail)2.5 to 2.75 turns round a central axix called modiolus.30 mm long

PARTS OF COCHLEAOSSEUS LABYRINTH.MEMBRANOUS LABIRYNTH- It has 3 canals>Scala media(cochlearduct)Scala vestibuliScala tympani

Scala media- separated froms scala vestibuli by Reissners membrane & from scala tympani by basilar membrane. contains endolymph.Scala vestibule - connected to middle ear via oval window. contains perilymphScala tympani- connected to middle ear by round window contains perilymph


ORGAN OF CORTI It acts as sensor `(auditory receptor organ)of cochlea.= inner hair cell outer hair cellsupporting cellbasilar membrnetectotial membrane

BASILAR MEMBRANETravelling wave moves from base to apex along the basilar membFor a pure tone stimulus,the travelling wave reaches a maximum at characteristic place along basilar memb. & then decays.Location of maximum depends on frequency of stimulus(TONOTOPIC ORGANIZATION OF COCHLEA)

Characteristic frequency at specific location depends on molecular structure/passive system & active system.

Anatomical change from base to apex includes increase width of basilar memb. & size of O.H.C

Base is tuned for frequency as high as 2oKHz & apex for as low as 20 Hz

Sensitivity to noise/ototoxic insult is more at base

Passive cochlear mechanismActive cochlear mechanismMECHANO-ELECTRICAL Of I.H.CEnergy cosnuming(prestin)Of O.H.CSusceptible to hypoxia or noise induced insults.Activation of it causes altered cochlear outputINNER HAIR CELLSTUCTURES-3500 flask-shaped inner hair cell lined up in single row.

have hair bundles of highly organised actin filled 3-4 stereocilia

Sterocilia graded in height-most lateral row being tallest & most medial row being shortest.

Sterocilia has dense rootlet that penetrate into apical cuticular plate.Mechano-electric al transduction apparatus present at tip of sterocilium which contains mechanically gated cation channel.

FUNCTIONS- MECHANO-ELECTRIC TRANSDUCTION> movement of stapes > displacement of cochlear fluid in scala vestibuli> incompressibility of perilymph causes movement of basilar memb.

mechanical deflection of hair cells stereocilliay bundle towards tallest row of stereocilia

Increase mechanical tension in transduction apparatus

Confrontational change in the transduction potein

Increase channel open & depolarzation of cell

B. ADAPTATIONRapid closure of transduction protein by ca ion binding

Fast adatationSliding of myosin based motor which associated with transduction protein

Slow adaptation

OUTER HAIR CELLSSTUCTURES-Three rows of outer hair cellv shaped arrangementCylindrical shapeContain steriocilia bundle like inner hair cell but it touches tectorial membrane.

FUNCTIONS- AMPLIFICATION > necessary for detection of sound at low sound pressure. It has two mechanism=A. Electromotility- electrical stimulus when depolarize ,then OHC contract & elongates (by Prestin motor protein) when hyperpolarised OHC exert mechanical force that feeds back into movement of basilar membrane further moves basilar membrane moves sterocilia in excitatory direction Amplification.B. Active hair bundle movement-

SUPPORTING CELLSDeiters cell-phalangeal supporting cell of OHCHensens cel(outer marginal cell)Claudius cellBorder cell of held

TECTORIAL MEMBRANEExtracellular structure that overlies both IHC & OHCBut only tallest sterocilia of OHC embedded into it FUNCTION: Previously thought as simple liver which helps in moves up & down of basilar memb.

Recently _thought as resonant gel & helps to increase frequency selectivity of cochlea. STRIA VASCULARISHighly vascularised, multi-layered tissue that is a part of lateral wall of the scala media3 layers-Marginal (tight junction)IntermediateBasal layer(tight junction)

Extracellular space b/w marginal & basal layer- intrastrial compartment



Due to such cochlear homeostasis the endocochlear potential of endolymph formed(+85 mv)With resting membrane potencial of-45 mv creates 130 mv of K gradientwhich is one of the main driving force of K entry into cell.DISEASE DUE TO ALTERED COCHLEAR HOMEOSTASIS

SPIRAL GANGLION & INNERVATIONIt located in Rosenthals canal within modulus of cochlea.


Afferent fibres-upto cochler nucleus in brain stem-Type 1 ganglion neuron- (95%)-thick ,myelinated, innervate only single I.H.Ctype2 (5%)ganglionic neuron-thin ,unmyelinated, hae multiple branches to multiple O.H.C.Efferent fibres- From superior olivary nucleusvia brainstemto both I.H.C & O.H.Callow central nervous system to modulate the operations of cochlea.CENTRAL CONNECTIONS OF AUDITORY SYSTEM

Sound driven activity enters in brain by way of auditory nerve, it is transformed by no less than 12 types of projection in 7 major processing centers before converging in the auditory thalamus.COCHLEAR NUCLEUSProminent bulge in lateral surface of brainstemPARTS-Dorsalless prominentpotential site for generation tinnitustonotopically organised with low frequency ventrolateraly & high frequency dorsomedially.project directly to I.C.

VENTRAL...> Antero-ventral> Initial processor of auditory nerve inforation.Bushy cell- send large callibre axon to b/l olivary complex & helps in sound localisation.Post-ventral> Stellete neuron & Octopus & fusiform cell- send fine multiple axon to Inf. colliculus & help in encoding frequency , spectral & sound intensity.

SUPERIOR OLIVAY COMPLEXIn caudal aspect of pons.It is first central auditory center to receive binaural innervation.Two part-MSO & LSOSound localisation by inter-aural time delay by MSO & inter-aural intensity difference by LSO.Also helps in sound detection & compound sound processing. Projections of S.O.C

ASCENDING DESCENDING lateral lemniscusnucleus of lateral lemnicusinferior colliculusOLIVO-COCHLEAR BUNDLEinfluence cochlear sensitivity & tunning through modulation of OHCInfluence aspect of hearing in noise.

LATERAL LEMNISCUSBy which medullary & pontine auditory nerve fibre reach to inferior colliculus.Two part- ventral & dorsal

Helps in sound localisation & processing.

Component of acoustic startle reflex pathway( with ventral cochlear nucleus )> form 3 to 5 wave in ABR.receive differential innervation from ipsilateral & contralateral cochlear nucleus & S.O.C subdivisions.INFERIOR COLLICULUS Present in midbrainHelps in sound localisation, frequency determination, integration of auditory with non-auditory system.3 parts-Central nucleus-projections come from directly/ indirectly , mon/binauralCortex- Projections to primary & secondary auditory cortex through M.G.B.Paracentral nucleus Central nucleus is layered into isofrequency bandsAlong each band, the cells have same dendritic fields & respond best to approximately same frequency

Higher frequency in midline & lower frequency towards outsideProduce tonotopic map

MEDIAL GENICULATE BODYPresent in thalamus3 parts-Ventral-(3 subnuclei)-projection to auditory cortex.Helps in tonotopic organisation.Dorsal-(1o sub nuclei)Medial-Have both auditory & non-auditory connections.plays role in arousal & attending in auditory stimuli.AUDITORY CORTEXDeep within sylvian fissure of superior surface of of the temporal lobe.Consists of multi-layered tonotopically organised regionPrimary(A1)-brodman area 41secondary(A2)-broad area 42Anterior auditory field area(AAF/A)Ventral auditory field area(V)Posterior auditory field area(P)Numerous cortical association areas surround the primary auditory cortex-Wernicke's area (left side)/ area 22-neural substrate for receptive language Just posterior to area 22, in the inferior parietal lobe, are the angular gyrus and supramarginal gyrus (areas 39 and 40)-integrate auditory, somatosensory, and visual informationBroca's area/ area 44 and 45-expressive language, and the perception of musical syntax.Functions-Discrimation of sound sourceLocalisation of soundRecognition of voiceAuditory memoryTHANKYOU


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