Anatomy of the Inner Ear

By Dr. Aditya Tiwari

Brain

MAJOR DIVISIONS OF THE EAR

Ectoderm = Membranous labyrinth Mesoderm = Bony labyrinth

BY THE 3RD WEEK :- a)Surface ectoderm Otic Placode b)Otic Placode Invaginates --> Otic pit

BY THE 4TH WEEK :- Otic Pit edges fuse to become otocyst.

INNER EAREMBRYOLOGICAL DEVELOPMENT

Otocyst has 2 portion : - a)Dorsal Utricular portion b)Ventral Saccular portion

Utricular portion :- 3 diverticula for Semicircular Canals

Saccular portion :- a)Tubular diverticulum (cochlear duct) grows in spiral fashion to become membranous cochlea

b)The organ of Corti differentiates from cells along the wall of the cochlear duct

BY THE 6TH WEEK :-

Neuroectoderm --> Spinal and Vestibular ganglia and corresponding sensory nerves

Mesoderm around otocyst soon forms a cartilaginous otic capsule.

Ossifies by 25 weeks

Bony labyrinth ossifies from 14 ossification centres Starts at 16th week of intrauterine life & ends by 20-21

weeksof gestation.

Vacuoles containing the perilymph develop within the Otic capsule.

The vacuoles enlarge and unite to form the perilymphatic space.

Perilymphatic space divides into the scala tympani and the scala vestibuli.

The cartilaginous otic capsule ossifies to form the bony labyrinth of the inner ear (mesoderm).

The embryologic source & the time of development of external & middle ear

are independent of the inner ear development.

Therefore, malformed & non functional inner ear can have normal

external & middle ears and vice versa.

Two compartments:- а) Bony labyrinth and b) Membraneous labyrinth.

BONY LABYRINTH :- Complex cavity in dense bone (Pars Petrosa)

Parts of the bony labyrinth: - a) Vestibulum. b) Semicircular canals. C) Cochlea.

ANATOMY OF INNER EAR

So, the inner ear consists of a “membranous” labyrinth encased in an “osseous” labyrinth.

The vestibule and semicircular canals are concerned with vestibular function (balance).

The cochlea is concerned with hearing. The cochlea is a coiled tube.

It should be noticed that the oval window and round window open into the vestibule, at the

base of the cochlea.

Two walls: External and internal.

External wall has a)Fenestra Vestibuli.

Internal wall has: a) Recessus Ellipticus b) Recessus Sphericus c) Recessus Cochlearis d)Maculae cribrosae superior, medius, inferior

VESTIBULUM

Diagramatic representation of Vestibulum

EXTERNAL WALL INTERNAL WALL

Lateral wall oval windowclosed by stapes footplate & surronded by annular ligament.

Medial wall :- a)Sperical recess ant. Situatedlodge sacculeperforaton of macula cribrosa media provides passage for fibres of inf vestibular nerve. b)Elliptical recesspost. Situatedlodge utricle perforaton of macula cribrosa superior(MIKE’S DOT) provides passage for nerve fibres that supply to uticle & ampulla of sup & lateral SSC

Boundries of vestibule

c)Vestibular crests & cochlear recess. d)Opening of aqueduct of vestibule

Enlarged vestibular aqueduct asso. with SNHL,Pendred syndrome & anatomical defect of cochlear

modiolus

a) Fenestra vestibuli.b) Fenestra cochleae.c) Openings (5) of the semicircular canals d) Aqueductus vestibuli

Openings into Vestibulum

They are 3 in numbers :- a) Anterior, b) Posterior and c) Lateral.

Have ampulla and crus. A) Canalis Semicircularis Superior/Anterior – Frontal. - Eminentia arcuata on pars petrosa of os temporale

SEMICIRCULAR CANALS

B) Canalis Semicircularis Lateralis – Horizontal. - Eminentia canalis semicircularis lateralis on the medial wall of tymypanic cavity.

C) Canalis semicircularis posterior –Sagittal

Solid Angle

Area of bony Labyrinth between 3 SSC.

The cochlea is the auditory portion of the inner ear.

The cochlea (plural is cochleae) is a spiralled, hollow, conical chamber of bone in the bony labyrinth

The cochlea resembles a snail shell and spirals for about 2 3/4 turns around a bony column

It is coiled around the modiolus.

COCHLEA

It forms a cone approximately 9 millimetres (0.35 inch) in diameter at its base and 5 millimetres in height.

When stretched out, the tube is approximately 30 millimetres in length; it is widest—2 millimetres—at the point where the basal coil opens into the vestibule and tapers until it ends blindly at the apex.

Area bounded by the Bony labyrinth anterioly, Sigmoid sinus posteriorly & the dura or superior petrosal sinus superiorly.

Trautmann’s Triangle

In cochlea, waves propagate from the base (near the middle ear and the oval window) to the apex (the top or center of the spiral).

Reissner’s membrane and the basilar membrane divide the the cochlea longitudinally into three canals called as scalae.

These three scalae are : - a)Scala Vestibuli b)Scala Tympani c)Scala Media

Diagramatic representation of scalae

Movement of the the basilar membrane by pressure changes induced by stapes footplate motion at the oval window is a critical step in the transduction in the middle.

The process of transduction occurs in the structures within scala media, sitting on the basilar membrane -- these structures comprise the organ of Corti. The side of the duct where the nerve fibers exit (left in this picture) is the “inner” or “modiolar” side of the duct. The opposite side is the “outer” side.

.

Scala media is more or less triangular, formed by Reissner’s membrane, basilar membrane and the structure called the Stria vascularis.

The fluid that fills scala tympani and scala vestibuli is called perilymph; the fluid that fills scala media is called endolymph. Both communicate with each other at the the apex of cochlea

through an opening called Helicotroma.

The organ of Corti rests on the basilar membrane within scala media

Perilymph VS Endolymph

**** Perilymph and endolymph participate in a unidirectional flow that is interrupted in Ménière's disease.****

1) Resembles ECF.2) Present in scala

tympani and scala vestibuli.

3) Major cation is Na.4) Has potential of 0

mV. 5) Also c/a Cotunnius'

liquid.

1. Resembles ICF.2. Present in scala

media.

3. Major cation is K. 4. Has potential of

80mV.5. Also c/a Scarpa's

liquid.

ORGAN OF CORTI Organ of Corti also called as “End organ of hearing”.

It is found only in mammals and provided with hair cells or auditory sensory cells.

It evolved from the basilar papilla found in all tetrapods

It is highly specialized structures that respond to fluid-borne vibrations in the cochlea with a shearing vector in the hairs of some cochlear hair cells.

It contains between 15,000-20,000 auditory nerve

receptors.

Each receptor has its own hair cell. The shear on the hairs opens non-selective

transduction ion channels that are permeable to K and Ca , leading to hair cell plasma membrane depolarization, activation of voltage-dependent Ca channels at the synaptic basolateral pole of the cells

It triggers vesicle autopsy and liberation of

glutamate neurotransmitter to the synaptic cleft and electrical signaling to the auditory cortex via spiral ganglion neurons.

Diagramatic representation of Organ of Corti

Two types of cells in the organ of Corti are

a) Support cells and b) Hair cells.

The Hair cells are the “Receptor” cells cells that transduce sound.

Support cells such as the Deiter’s cells support hair cells

Boettcher's cells Present only in the lower turn of the cochlea Lie on the basilar membrane beneath Claudius' cells Supporting cells for the auditory hair cells in the organ of Corti.

Claudius' cells Located above Boettcher's cells Supporting cells for the auditory hair cells in the organ of Corti Contain a variety of aquaporin water channels involved in ion transport play a role in sealing off endolymphatic spaces

SPECIAL TYPES OF CELL S IN ORGAN OF CORTI

Hensen’s cells High columnar cells that are directly adjacent to the third row of Deiters’ cells.

Hensen's stripe Section of the tectorial membrane above the inner hair cell.

Nuel's spaces Fluid filled spaces between the outer pillar cells and adjacent hair cells Also the spaces between the outer hair cells.

Hardesty's membrane Layer of the tectoria closest to the reticular lamina and overlying the outer hair cell region.

Rosenthal canal Spiral ganglion are situated in Rosenthal canal, which runs along osseous spiral lamina

There are two types of hair cells a)Outer Hair Cells (OHC) from sound to nerve signal b)Inner Hair Cells (IHC)acoustical pre-amplifiers

HAIR CELLS

There are 4 rows of hair cells, one on the inner (modiolar) side of the tunnel of Corti formed by the pillar cells-- these are the INNER HAIR CELLS .

3 one the outer side of the Tunnel of Corti, these are the OUTER HAIR CELLS.

The Deiter’s cells support the Outer hair cells at their base, but the outer hair cell walls are surrounded by fluid.

The inner hair cell is surrounded by support cells.

The Outer hair cells mechanically amplify low-level sound that enters the cochlea.

The amplification may be powered by

movement of their hair bundles, or by an electrically driven motility of their cell bodies.

The Inner hair cells transform the sound

vibrations in the fluids of the cochlea into electrical signals that are then relayed via the auditory nerve to the auditory brainstem and to the auditory cortex.

IHC Vs OHC1) Approx 3500 in no.2) Lies in one row.3) Flask shaped.4) Mainly afferent nerve

fibres supply.5) Early development.6) Transmit auditory

stimuli.7) More resistant to

ototoxic drugs.8) More resistant to high

intensity noice.9) No generation of

otoacoustic emissions.

1) Approx 12,000 in no.2) Lies in 3-4 rows.3) Cylindrical shaped.4) Mainly efferent nerve fibres

supply.5) Late development.6) Modulate function of inner

hair cells.7) More sensetive & easily

damaged by ototoxic drugs.8) More sensetive & easily

damaged by high intensity noice.

9) Generates otoacoustic emissions.

RETICULAR LAMINA The reticular lamina is a solid surface at the

tops of the hair cells, so the tops of the hair cells are in endolymph and the bottom of the hair cells are in perilymph

Stereocilia are lined up in the Organ of Corti within the cochlea of the inner ear.

In hearing, stereocilia transform the

mechanical energy of sound waves into electrical signals for the hair cells, which ultimately leads to an excitation of the auditory nerve.

STERCOCILIA

Stereocilia are composed of cytoplasm with embedded bundles of cross-linked actin filaments.

The actin filaments anchor to the terminal web and the top of the cell membrane and are arranged in grade of height.

When the stapes causes sound waves in the endolymphatic fluid in the cochlea, the stereocilia are deflected in a shearing motion, which results in the mentioned electrical signal for the hair cell

MECHANISM OF ACTION OF STERCOCILIA

Stereocilia are arranged in curved or v-shaped rows that face toward the modiolus.

Each row of stereocilia is taller than the next. The tip of each stereocilium is linked to the side of the stereocilium behind it by a tip link.

Arrangement of Stereocilia

KINOCILIUM Kinocilia are found on the apical surface of

hair cells and are involved in both the morphogenesis of the hair bundle and mechanotransduction, Vibrations (either by movement or sound waves) causes displacement of the hair bundle, resulting in depolarization or hyperpolarization of the hair cells.

The depolarization of the hair cells in both instances causes signal transduction via neurotransmitter release.

One kinocilium is the longest cilium located on the hair cell next to 40-70 sterocilia.

During movement of the body, the hair cell is depolarized when the sterocilia move

Deiter’s cell processes “fill in the gaps” between the tops of the outer hair cells to form the reticular lamina.

It holds the base of the hair cell in a cup-shaped depression. From each Deiters’ cell a projection extends upward to the stiff membrane, the reticular lamina, that covers the organ of Corti.

DIETER’S CELL

Acellular gels in the cochlea of the inner ear along with basilar membrane (BM).

The TM is located above the sulcus spiralis internus

and the spiral organ of Corti and extends along the longitudinal length of the cochlea parallel to the BM.

Divided into three zones:- a) Limbal , b) Middle and c) Marginal

TECTORIAL MEMBRANE

LIMBAL ZONE Thinnest and overlies the auditory teeth of Huschke with its inside edge attached to the spiral limbus.

MARGINAL ZONE a) Thickest and is divided from the middle zone by Hensen's Stripe.

b) Overlies the sensory inner hair cells and electrically-motile outer hair cells of the organ of Corti c) During acoustic stimulation, stimulates the inner hair cells through fluid coupling, and the outer hair cells via direct connection to their tallest stereocilia.

Seperates scala media and scala tympani.

The Basilar membrane is widest (0.42–0.65 mm) and least stiff at the apex of the cochlea, and narrowest (0.08–0.16 mm) and most stiff at the base.

BASILAR MEMBRANE

FUNCTIONS OF BASILAR MEMBRANE :- a) Perilymph/endolymph seperation. b) Base of sensory cells c) Frequency dispertion

High-frequency sounds localize near the base of the cochlea (near the round and oval windows), while low-frequency sounds localize near the apex.

Zona arcuata inner thin area Zona pectinata Outer thick area

The membranous labyrinth contains fluid called endolymph.

The walls of the membranous labyrinth are lined with distributions of the acoustic nerve, also known as the vestibularcochlear nerve.

It consists of :- a) Cochlear duct b) Utricle c) Saccule d) 3 semicircular ducts e)Endolymphatic duct & sac

MEMBRANOUS LABYRINTH

The cochlear duct (or scala media) is an endolymph filled cavity inside the cochlea, located in between the scala tympani and the scala vestibuli, separated by the basilar membrane and Reissner's membrane (the vestibular membrane) respectively.

Scala media houses the organ of Corti.

Cochlear Duct

Oblong, irregular, ant. Upward slope approx at 30 degree.

4.33mm2, recieves 5 opening of 3 semicircular canal, lies in posterior part of bony vestibule.

Connected with saccule via utriculosaccular duct. Its sensory epithelium c/a MACULE concerned with linear acceleration and dicceleration.

UTRICLE

UTRICLE

FIRST PART OF MEMB. LABYRINTHTO DEVP. DURING INTRA -UTERINE LIFE.

Consists of three layers ;-

a) Bottom layer sensory hair cellsconsists of 40 to 70 steriocilia and a kinocilium,

b) Top layercalcium carbonate crystals called statoconia or otolithsrelatively

heavy, providing weight to the membrane as well as inertiagreater sense of gravity and motion. C)macula acustica utriculereceives the utricular filaments of the acoustic nerve.

Globularlies in the recessus sphæricus near the opening of the scala vestibuli of the cochlea.

The Anterior oval thickening, the macula acustica sacculi saccular filaments of the vestibular branch of the vestibulocochlear nerve.

Posterior wall Ductus endolymphaticus.

Lower part short tube, the CANALIS REUNIENS OF HENSEN, passes downward and opens into the ductus cochlearis near its vestibular extremity.

SACCULE

Open in the utricle, corrosponds to three semicircular canal.

Ampulated end has thickened ridge of neuroepithelium c/a CRISTAE AMPULARIS

SEMICIRCULAR DUCT

From the posterior wall of the saccule a canal, the endolymphatic duct, is given off.

This duct is joined by the ductus utriculosaccularis, and then passes along the aquaeductus vestibuli and ends in a blind pouch (saccus endolymphaticus) on the posterior surface of the petrous portion of the temporal bone, where it is in contact with the dura matER.

ENDOLYMPHATIC DUCT AND SAC

In the auditory nerve, the dendrites contact the hair cells.

The cell bodies form what is called the spiral ganglion, and the axons form the auditory nerve that connects the ear to the brainstem.

The “contact” points between the dendrites and the hair cells or between the axons of one neuron and the dendrites of another are called synapses.

Synapses have specialized structures and substances that allow communication between receptors and neurons or between neurons.

Endolymphatic sac is thought to regulate pressure of membranous

labyrinth .

Endolymphatic sac is decompressed, drained or shunt in Meniere’s disease

This line is surgical landmark for endolymphatic sac. It passes through

horizantal SSC bisecting posterior SSC. The endolymphatic sac that appears as thickening of post cranial fossa dura is situated inferior to

Donaldson’s line.

Donaldson’s Line

In the auditory nerve, the dendrites contact the hair cells.

The cell bodies form what is called the spiral ganglion, and the axons form the auditory nerve that connects the ear to the brainstem.

The “contact” points between the dendrites and the hair cells or between the axons of one neuron and the dendrites of another are called synapses.

Synapses have specialized structures and substances that allow communication between receptors and neurons or between neurons.

NEURONAL REVIEW

When a neuron’s intracellular electrical potential is changed enough by release of neurotransmitter at a synapse(or in some other way), an abrupt change in electrical potential, an “action potential”, occurs.

Action potentials are transmitted along the axon to another synapse, where neurotransmitter is released and an action potential may be generated in the neuron on the other side of the synapse (postsynaptic neuron).

ACTION POTENTIAL

SPIRAL GANGLION

The cell bodies of the neurons that form the auditory nerve are located within the cochlear modiolus. The collection of cell bodies is called the spiral ganglion.

Pattern of Afferent innervation

Different types of nerve fibers innervate IHCs and OHCs. Type I fibers innervate IHCs; Type II neurons innervate OHCs.

Pattern of afferent innervation

Many nerve fibers that contact one inner hair cells do not branch to other inner hair cells.

Each IHC has its own “private” set of fibers.

The Type II nerve fibers innervate many OHCs.

The OHCs they innervate are basal to the point at which the nerve fiber enters the cochlea. Thin fibers attach toward modiolar side, thick fibers toward outer side of IHC.

Thin fibers attach toward modiolar side, Thick fibers toward outer side of IHC.

SENSORY EPITHELIAL HAIR CELLS

They are of two types : - a) Type 1 cells b) Type 2 cells.

TYPE 1 CELLS

1. Found only in birds & mammals2. Flask shaped3. Correspond to IHC of Organ of Corti4. Has single large cup like nerve

terminal surrounding the base

TYPE 2 CELLS

5. Cylindrical shaped6. Correspond to OHC of Organ of Corti7. Has multiple nerve terminal at the

base.

They are of two types :- a)Cristae b)Macule. CRISTAE Lies in ampullated end of semicircular duct.

Responds to angular acceleration.

Cristae of lateral SSC polarisation towards utricle

Cristae of sup.& post. SSC polarisation away from utricle

VESTIBULAR RECEPTORS

The cupula is the gelatinous component of the crista ampullaris that extends from the crista to the roof of the ampullae.

Providing the sense of spatial orientation.

When the head rotates, the endolymph filling the semicircular ducts initially lags behind due to inertia. As a result, the cupula is deflected opposite the direction of head movement.

As fluid rushes by the cupula, the hair cells stimulated transmit the corresponding signal to the brain through the CN VIII.

CUPULA

NOTE : - Fig A a) In stationary head position, the cupulae are located on the medial aspect of the semicircular canals.

b)In this position, the kinocilia rest on the posterior aspect of the cupula. Fig B As the head rotates,the cupula is deflected opposite to the direction of head movement.

They lie in otolith organs i.e. utricle and saccule. STRIOLA Narrow curved line in centre, dividing macula in 2

areas In Utricle situated in floor in horizantal plane in dilated sup.

portion of utricle Has approx 33,000 hair cells, large in size

In Saccule situated in medial wall in verticle plane Has approx 18,000 hair cells

In Utricle Perceive changes in longitudinal acceleration as well as effects of gravity.

MACULAE

A) Sensory neuroepithelium B) Otolith membrane Sensory Neuroepithelium Made up of type 1 & type 2 cells, similar to cristae

hair cells

Type 1 cells Higher in conc. in striola region

Kinocilia face striola in utricular macula & away from striola in saccular macula.

Offers CNS wide range of neural information.

PARTS OF MACULAE

Every hair cell in sensory beds of this consist of 40-70 stereocilia and a kinocilium.

The sterocilia and kinocilium are embedded in the otolithic membrane and are essential in the function of the otolith organs.

The hair cells are deflected by structures called otoconia.

OTOLITH MEMBRANE

Otoconia are crystals of Ca carbonate and make the otolithic membrane heavier than the structures and fluids surrounding it.

The otoconia are composite crystallites that overlie the macular sensory epithelium of the gravity receptors and are required for optimal stimulus input of linear acceleration and gravity.

Biomineralization of otoliths and otoconia results mainly from the release of soluble Ca ions, which is in turn precipitated as Ca carbonate crystals.

VESTIBLAR RECEPTORS & DIRECTION OR PLANE OF HEAD MOVEMENT

Horizantal SSC

Sup. & post. SSC

Otolith organs (utricle & saccule)

Horizantal head turning(angular acceleration

Piching the front to back & side to side

Linear head movements(vertical & horizantal),tilting & gravity

Basilar artery Anterior inferior cerebellar artery Internal auditory artery Labyrinthine artery.

Labyrinthine Comman cochlear artery artery Ant. Vestibular artery (to utricle & lat. and sup. Canals)

Comman cochlear Vestibulocochlear artery artery Main cochlear artery (to cochlea 80%)

Vestibulocochlear artery Cochlear artery ( 20% cochlea) Post. Vestibular artery (to saccule & post. Canal)

BLOOD SUPPLY OF LABYRINTH

Via internal auditory canal, vein of cochlear aqueduct & vein of vestibular aqueduct

Inferior petrosal & sigmoid sinuses

VENOUS DRAINAGE OF LABYRINTH

drain into

Cochlea does not have any collateral arterial circulation.

Convert mechanical sound waves to neural impulses that can be recognized by the brain for:-

a)Hearing

b)Balance

FUNCTION OF INNER EAR

HEARING PATHWAY

1) STATIC BALANCE

2) DYNAMIC BALANCE

BALANCE

STATIC BALANCE

Maculae react to gravitational forces and participate in maintaining the static balance.

DYNAMIC BALANCE

Cristae ampullares react to rotatory movements and paticipate in

dynamic balance.

Birds have an auditory system similar to that of mammals, including a cochlea.

Reptiles, amphibians & fish do not have cochleas but hear with simpler auditory organs or vestibular organs, which generally detect lower-frequency sounds than the cochlea.

COMPARATIVE ANATOMY