Anatomy of the inner ear

• Introduction• Development• ANATOMY

– Bony labyrinth – vestibule - Cochlea - semicircular canals - Contents within bony labyrinth

– Membranous labyrinth - Cochlear duct - Organ of Corti - Utricle & Saccule - semicircular ducts - Sensory structures in maculae & cristae - Endolymphatic duct and sac

– Blood Supply of labyrinth

The Inner Ear

The inner ear – important organ for hearing and balance.

Consists of – Bony labyrinth Membranous labyrinth COCHLEA COCHLEAR DUCT VESTIBULE UTRICLE SEMICIRCULAR CANALS SACCULE

SEMICIRCULAR DUCTS ENDOLYMPHATIC DUCT & SAC

DEVELOPMENT• Develops independent of middle and external ear.• Phylogenetically, older semicircular canals and utricle (PARS SUPERIOR) precede development

of saccule and cochlear duct (PARS INFERIOR)

OTIC PLACODE( END OF 3RD WEEK) Lateral to hindbrain

FORMATION OF AUDITORY PIT ( 4 WEEKS)

OTOCYST (epithelial lining of membranous labyrinth derived)

TUBULAR DIVERTICULUM FROM MEDIAL SIDE ( ENDOLYMPHATIC DUCT & SAC)

SCCs ARISE FROM OTOCYST (end of 4TH WEEK)

6th WEEK – SCCs – lumina has formed Macula communis - superior and inferior segments Superior segment - macula of the utricle, ampullary crests of superior and lateral SCCs. Inferior segment - macula of saccule and ampullary crest of posterior SCC. Cochlear duct – extends from saccule, completes 1 turn during the course of the week. Completes 2.5 turns by 8 weeks

8 – 16 weeks – otic labyrinth approaches adult configuration. from 8th week – perilymphatic/bony labyrinth space formation begins. Completes by 20 weeks.

DEVELOPMENT

DEVELOPMENT

• Bony labyrinth – petrous bone adjoining the labyrinth – formed from cartilaginous otic capsule by endochondral ossification

• Cochlear modiolus – trabecular dermal bone• At 20 weeks, the cochlea and vestibular

labyrinth are fully formed.

• Located in petrous part of temporal bone.• Comprises of bony & membranous labyrinth.• BONY LABYRINTH COCHLEA VESTIBULE SEMICIRCULAR CANALS

ANATOMY

VESTIBULE

• Central chamber of the bony labyrinth.• Lies medial to tympanic cavity, postr to cochlea,

and antr. to SCCs• Anterior wall – opening leading into scala

vestibuli• Posterior part of vestibule – 5 openings of the

SCCs.• Lateral wall – oval window opening; closed by

footplate of stapes and its annular ligament

VESTIBULE• Medial wall – Spherical recess (for saccule) perforated by several small foramina – transmits the vestibular br. Of 8th Cranial n. to saccule• behind recess – crista vestibuli; antr end – vestibular pyramid • postero superior to crista vestibuli – Elliptical recess (for utricle)• Opening of vestibular aqueduct; contains endolymphatic duct.

SEMICIRCULAR CANALS

• 3 in number – lateral, posterior and superior• Lie in planes at right angles to each other.• Each SCC has ampullated end (opens independently into

vestibule) and non ampullated end (posterior and superior canals unite to form a common channel – CRUS COMMUNE).

• Superior SCC – vertical in orientation - transverse to long axis of petrous temporal bone, under arcuate eminence

- antr. Ampullated end - upper and lateral part of vestibule

SEMICIRCULAR CANALS• Posterior SCC – also vertical orientation - almost parallel with postr.

surface of petrous bone. - ampullated end – opens low in the

vestibule • Crus commune – opens into medial part of vestibule• Lateral SCC – directed horizontally backwards

and laterally. - anterior ampullated end - upper and lateral

angle of vestibule, above oval window. - non ampullated end opens below

orifice of crus commune.

COCHLEA• “COCHLOS” – Snail• Coiled for 2.75 turns around a central axis (Modiolus)• most anterior part of the labyrinth• 5 mm from base to apex; 9 mm across its base• Total length of cochlea – 35 mm

COCHLEA

• Apex (Cupula) – points towards antero -superior area of medial wall of tympanic cavity.• Base faces bottom of internal acoustic meatus – perforated by numerous apertures for cochlear n.• Basal coil of cochlea forms PROMONTORY in middle ear

COCHLEA

• openings at base of cochlea – RW (facing tympanic cavity, closed by

secondary tympanic membrane); Oval window (occupied by footplate of stapes)

aqueduct of cochlea – connects the scala tympani to the subarachnoid space

COCHLEA - MODIOLUS• Central cochlear pillar• Base lies near lateral end of internal acoustic meatus.• Transmits the cochlear n. through bony spiral lamina to membranous labyrinth• Rosenthal’s canal – contains spiral ganglion

COCHLEA• ledge of bone projects into bony canal like thread of the screw – osseus spiral lamina. • Osseus spiral lamina divides cochlea incompletely, gives attachment to Basilar membrane• Bony cochlea is divided into 3 compartments :• Scala vestibuli• Scala media (membranous labyrinth)• Scala tympani

COCHLEA

• Scala Vestibuli – closed by the footplate of stapes• Scala vestibuli and scala tympani become continuous

at the apex of the cochlea though helicotrema • Scala tympani – ends in round window.

CONTENTS OF BONY LABYRINTH

• Space between bony labyrinth and membranous labyrinth – filled with PERILYMPH.

• Perilymph – resembles Cerebrospinal Fluid/ Extracellular Fluid.

• Rich in Na+. • Drained via venules and middle ear mucosa.

MEMBRANOUS LABYRINTH

• Continuous system of ducts contained within the bony labyrinth.

• Contains endolymph (rich in K+). Produced by marginal cells in stria vascularis from perilymph at the cochlea and from dark cells in the cristae and maculae.

• Cochlear endolymph has positive electrical potential - +85mV• Consists of : cochlear duct,

utricle and saccule, the 3 semicircular ducts, and the endolymphatic duct and sac (Vestibular apparatus).• Primary absorption of endolymph – Endolymphatic Sac

COCHLEAR DUCT

• Cochlear Duct – membranous cochlea/ scala media• Bind coiled tube. Closed upper end attached

to Cupula.• Lower end of the cochlear duct connected to

saccule by ductus reuniens • Triangular in cross section.

COCHLEAR DUCT• Walls formed by:

• Bony spiral lamina and Basilar membrane• Reissner’s membrane• Stria vascularis

• Basilar membrane – supports the organ of Corti.

- forms part of roof of the scala tympani– Stretches from the bony spiral lamina to the crista

basilaris on outer wall of cochlea.– Length – 35 mm; width increases from base to apex.

0.21 mm at base to 0.36mm at apex.

COCHLEAR DUCT

• Reissner’s membrane – extends from bony spiral

lamina to outer wall of cochlear duct.

- separates the scala media from scala vestibuli.

- formed by 2 layers of squamous epithelium separated by basal lamina.

COCHLEAR DUCT

• Stria Vascularis – located on the outer wall of cochlear duct. Above the spiral eminence.

–Has special stratified epithelium continuing as dense intraepithelial capillary plexus.–Three cell types present:

• Marginal/chromophil cells• Intermediate/chromophobe cells• Basal cell

• stria vascularis mainly helps in secretion and maintainence of ionic composition of endolymph.

ORGAN OF CORTI• sense organ of hearing• Situated on the basilar membrane• Components are :1. Tunnel of Corti2. Inner and outer hair cells3. Supporting cells – Dieter’s cells, cells of Hensen, cells of Claudius.4. Tectorial membrane

TUNNEL OF CORTI

• Formed by the inner and outer rod cells / pillar cells.

• Inner rods – 6000; outer rods - 4000• The two rows of cells incline into each other and

come into contact above at the heads of the pillars• Tunnel of corti lies in between the inner and outer

rods and the basilar membrane below.• This space contains a fluid known as cortilymph.

INNER AND OUTER HAIR CELLS• Important receptor cells of hearing and transduce sound energy into

electrical energy.• Both hair cells contain hair bundles of regular microvilli (stereocilia) on

their flat apical ends.

INNER HAIR CELLS• 3500 in number• Flask shaped• Form a single row.• Stereocilia in inner hair cells are arranged in a smoothly curved line of 2

or 3 rows of stereocilia.• Richly supplied by afferent cochlear nerve fibres. • More resistant to damage by ototoxic drugs/ high intensity noise.

OUTER HAIR CELLS• 12000 in number.• Cylindrical shape• Outer hair cells are arranged into 3 – 4 rows.• Stereocilia arranged in a shallow V shape• Outer hair cells mainly receive efferent

innervation from olivary complex.• Concerned with modulating the function of

inner hair cells• Can be easily damaged by ototoxic drugs/ high

intensity noise.

SUPPORTING CELLS

• DEITER’S CELLS – External phalangeal cells• Lie between outer row of outer hair cells.• Expanded bases lie on the basilar membrane

and apical ends partially envelope bases of the hair cells with finger-like phalangeal process extending up diagonally between the hair cells to the reticular membrane.

• forms a plate like expansion to complete gaps between hair cell apices.

• Cells of Hensen – 5 to 6 rows of columnar supporting cells lying externally to the outer hair cell.

• Cells of Claudius – lie external to the cells of Hensen. Their surfaces bear microvilli.

• Near the base of the cochlea, another group of cells present among bases of the phalangeal cells – Boetcher’s cells.

• Reticular Lamina/membrane – delicate framework perforated with circular holes, which are occupied by apices of outer hair cells.

• Extends from the heads of external rods to outer row of outer hair cells. Completed by several rows of minute cuticular phalanges.

TECTORIAL MEMBRANE

• Consists of a gelatinous matrix with delicate fibres.• Overlies the organ of corti• Has a characteristic shape – underside nearly flat and

upper surface convex.• Thin on the modiolar side, where it is attached to the

vestibular lip of the limbus laminae spiralis.• Extends centrally as far as the vestibular membrane.• Shearing force between hair cells and tectorial

membrane produces the stimulus to the hair cells.

UTRICLE AND SACCULE UTRICLE• Irregular, oblong, dilated sac.• Occupies the posterosuperior region of the bony vestibule.• In contact with the elliptical recess and area inferior to it.• Number of labyrinthine ducts open into it

• Laterally, the ampulae of lateral and superior SCCs

• Medially, ampulla of posterior SCC, the crus communale, and posterior end of lateral SCC

• Utriculosaccular duct – connects utricle to saccule. - originates from the utricle’s

anteromedial surface and follows V- shaped course, the middle of which endolymphatic duct arises

Macula of utricle

• Specialised area of neurosensory epithelium• Largest of the vestibular sensory areas• Triangular or shield shaped in surface view.• lies horizontally with its long axis posteriorly

oriented.• Flat except for the anterior edge – gently folded onto

itself• Covering the epithelial surface - statoconial

membrane (gelatinous layer in which otoconia are embedded)

• Statoconial membrane - layer of extracellular material- can be divided into 3 layers: 1. external layer consisting of otoliths

(calcium carbonate) of varying sizes attached to basal layer. 2. basal gelatinous layer -

- superficial stratum - deeper honeycomb layer –

stereocilia & kinocilia of sensory cells are inserted in this layer

• Curved median ridge runs along the length of statoconial membrane – corresponds underlying sensory epithelium (Striola)

• Striola divides macula into pars interna medially and pars externa laterally.

• Macula of both Utricle and Saccule detect linear acceleration

• Slightly elongated Globular sac. • Lies in the spherical recess near the opening of the

cochlea’s scala vestibuli.• Connected to the utricle and endolymphatic duct by

utriculosaccular duct, and to cochlea by ductus reuniens.

• Saccular macula – elliptical structure; lies in a vertical plane on saccule’s side wall.

• Like macula of utricle, covered by statoconial membrane, and possesses striola, which divides the saccule into pars interna superiorly and pars externa inferiorly.

SACCULE

SEMICIRCULAR DUCTS• Detect angular acceleration• Follow the course of osseous canals• 1/4th the diameter of the semicircular canals in size.• Medial ends of the superior and lateral SCC fuse to

form a single common duct – CRUS COMMUNALE• Lateral end of each canal is dilated to form an

ampulla.• Membranous wall of each ampulla contains

transverse elevation – septum transversum.• On the septum there is a sensory area – Crista

ampullaris.

• Crista ampullaris»Saddle shaped ridge »Lies transversely along the duct»Broadly concave on its free edge along

most of its length.»Sectioned across this ridge, cristae of

superior and lateral SCCs have smoothly rounded corners, while that of posterior SCC is more angular.»Attached along free edge of the crista is a

vertical plate of gelatinous extracellular material – CUPULA. This projects far into the lumen of ampulla.

Sensory structures in Maculae and cristae

• Sensory epithelia of maculae and cristae consist of:

»Mechanoreceptive cells »Non sensory supporting cells»Afferent and efferent nerve endings of

vestibular nerve fibres by synapses at bases.• Sensory cells – 2 types :

• Type 1 vestibular sensory cells• Type 2 vestibular sensory cells

• Type 1 cells – bottle shaped with narrow neck.–Basal part of cell doesn’t reach basal lamina of

epithelium.– from apical surface, 30 – 50 stereocilia and

single kinocilium.–Afferent nerve endings – calyx– efferent nerve endings attach over the calyx

• Type 2 cells - Mostly cylindrical in shape.

- variable in size. - both afferent and efferent nerve endings attach over the sensory cell.

Orientation of sensory cells:Macula Utriculi – sensory cells are oriented

towards the striola, structurally and functionally.

Macula Sacculi – sensory cells are oriented away from the striola.

Cristae – cells are oriented along with their rows of stereocilia at right angles to the long axis of the semicircular duct.In lateral crista – kinocilia are on the side towards utricleIn superior and posterior crista – kinocilia are away from utricle.

ENDOLYMPHATIC DUCT & SAC

Endolymphatic Duct»Runs along the bony tunnel of bony

vestibular aqueduct.»Becomes dilated distally to form the

Endolymphatic sac

Endolymphatic sac»Terminal part of endolymphatic duct»Lies in between two layers of dura on the

posterior surface of petrous bone.

BLOOD SUPPLY OF LABYRINTH

Arterial blood supply• Mainly by the internal auditory/labyrinthine artery – br.

of AICA• Sometimes supplied by the basilar artery.• Divides into Antr. Vestibular and common cochlear

arteries.Venous drainage: 3 veins• Internal auditory vein,• Vein of cochlear aqueduct • Vein of vestibular aqueduct

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