DR. RAJU MANDAL JUNIOR RESIDENTDEPT. 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 labyrinth

The labyrinth consists of three continuous sections:

1. Vestibule-

2. Cochlea-

3. 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 COCHLEAA. OSSEUS LABYRINTH.

B. MEMBRANOUS LABIRYNTH-

o It has 3 canals>

1. Scala media(cochlear duct)

2. Scala vestibuli

3. Scala tympani

Scala media- separated fromsscala vestibuli by Reissner’smembrane & from scalatympani 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

THREE FUNCTIONAL UNITS OF COCHLEA-

ORGAN OF CORTI-

STRIA VASCULARIS-

SPIRAL GANGLION-

ORGAN OF CORTI It acts as ‘sensor `(auditory

receptor organ)of cochlea.=

inner hair cell

outer hair cell

supporting cell

basilar membrne

tectotial membrane

BASILAR MEMBRANETravelling wave moves from base to apex along the basilar memb

For a pure tone stimulus,thetravelling 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 mechanism

Active cochlear mechanism

MECHANO-ELECTRICAL

Of I.H.C

Energy cosnuming(prestin)

Of O.H.C

Susceptible to hypoxia or noise induced insults.

Activation of it causes altered cochlear output

INNER HAIR CELL STUCTURES-

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-

A. 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 cell’s 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. ADAPTATION Rapid closure of

transduction protein by ca⁺⁺ ion binding

“Fast” adatation

Sliding of myosin based motor which associated with transduction protein

“Slow” adaptation

OUTER HAIR CELLSSTUCTURES-

Three rows of outer hair cell

“v” shaped arrangement

Cylindrical shape

Contain 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 CELLS

1. Deiters cell-phalangeal supporting cell of OHC

2. Hensen’s cel(outer marginal cell)

3. Claudius cell

4. Border cell of held

TECTORIAL MEMBRANE• Extracellular structure that overlies both IHC & OHC

• But 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 VASCULARIS Highly vascularised, multi-

layered tissue that is a part of lateral wall of the scala media

3 layers-

1. Marginal (tight junction)

2. Intermediate

3. Basal layer(tight junction)

Extracellular space b/w marginal & basal layer- intrastrialcompartment

COCHLEAR HOMEOSTASIS

COCHLEAR HOMEOSTASISK> enter hair cell by M.E.T Ch >released

through basolateral surface of H.C into extracellular

perilymph>

Type 2 & 1 fibrocyte of spiral ligament takes up K

By KCN QN/E1 marginal cell release K in Scala

media.

By NKCCL Ch ,K enter basal layer of Stia vascularis.

ByKCNJ10 ch released to intastrial space .From which

it actively pumped and contraported to marginal

layer

FUNCTIONDue to such cochlear homeostasis the endocochlearpotential of endolymphformed(+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 & INNERVATION

It located in Rosenthal’s canal within modulus of cochlea.

CONTAINS

Afferent fibres-upto cochlernucleus in brain stem-

A. Type 1 ganglion neuron-(95%)-thick ,myelinated, innervate only single I.H.C

B. type2 (5%)ganglionic neuron-thin ,unmyelinated, hae multiple branches to multiple O.H.C.

Efferent fibres-

From superior olivary nucleus

via brainstem

to both I.H.C & O.H.C

allow 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 NUCLEUS Prominent bulge in lateral surface of

brainstem

PARTS-

• Dorsalless prominent

potential site for generation tinnitus

tonotopically organised with low frequency ventrolateraly & high frequency dorsomedially.

project directly to I.C.

VENTRAL...>

a) Antero-ventral>

Initial processor of auditory

nerve inforation.

Bushy cell- send large callibre axon to

b/l olivary complex & helps in sound

localisation.

b) Post-ventral> Stellete neuron & Octopus &

fusiform cell- send fine multiple axon to Inf.

colliculus & help in encoding frequency ,

spectral & sound intensity.

SUPERIOR OLIVAY COMPLEX• In caudal aspect of pons.

• It is first central auditory center to receive binaural innervation.

• Two part-MSO & LSO

• Sound 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 lemniscus

nucleus of lateral

lemnicus

inferior

colliculus

OLIVO-COCHLEAR BUNDLE

influence cochlear sensitivity & tunningthrough modulation of OHC

Influence aspect of hearing in noise.

LATERAL LEMNISCUS• By 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 midbrain

• Helps in sound localisation, frequency determination, integration of auditory with non-auditory system.

• 3 parts-

1. Central nucleus-projections come from directly/ indirectly , mon/binaural

2. Cortex- Projections to primary & secondary auditory cortex through M.G.B.

3. Paracentral nucleus

Central nucleus is layered into isofrequency bands

Along each band, the cells have same dendritic fields & respond best to approximately same frequency

Higher frequency in midline & lower frequency towards outside

Produce tonotopic map

MEDIAL GENICULATE BODY Present in thalamus

3 parts-

A. Ventral-(3 subnuclei)-projection to auditory cortex.Helps in tonotopic organisation.

B. Dorsal-(1o sub nuclei)

C. Medial-

Have both auditory & non-auditory connections.playsrole in arousal & attending in auditory stimuli.

AUDITORY CORTEX Deep within sylvian fissure of superior surface of of the

temporal lobe.

Consists of multi-layered tonotopically organisedregion

1. Primary(A1)-brodman area 41

2. secondary(A2)-broad area 42

3. Anterior auditory field area(AAF/A)

4. Ventral auditory field area(V)

5. Posterior auditory field area(P)

Numerous cortical association areas surround the primary

auditory cortex-

1. Wernicke's area (left side)/ area 22-neural substrate for

receptive language

2. 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

information

3. Broca's area/ area 44 and 45-expressive language, and the

perception of musical syntax.

Functions-

1. Discrimation of sound source

2. Localisation of sound

3. Recognition of voice

4. Auditory memory