INNER EAR CONTAINS –
(a) ORGAN OF HEARING
(b) ORGAN OF BALANCE
PLACED INSIDE THE LABYRINTH , IN THE PETROUS
PART OF THE TEMPORAL BONE.
THEY ALL CONTAIN SENSORY EPITHELIA (SUPPORTING
AND MECHANOSENSORY CELLS ):-
1. ORGAN OF CORTI (IN COCHLEA) – HEARING ORGAN
2. UTRICLE AND SACCULE – STATIC BALANCE
3. CRISTAE OF THE SCC –DYNAMIC BALANCE.
EMBRYOLOGY OF INNER EAR
Initially membranous labyrinth is formed followed by
encasement by bony labyrinth.
Involves 3 main stages:
1. Development (4th-8th week)
2. Growth (8th to 16th week)
3. Ossification (16th to 24th week)
DEVELOPMENT OF INNER EAR
The membranous labyrinth is derived from a specialized of
surface ectoderm overlying developing HINDBRAIN.
This area of thickening is known as OTIC PLACODE.
Soon it depresses below the ectoderm to form OTIC PIT.
The pit becomes rounded to form OTIC VESICLE which
separates from surface ectoderm.
The otic vesicle is an oval structure in the beginning.
By differential growth of its various parts of its wall, it
gives rise to structures comprising MEMBRANOUS
Localized areas of epithelium of the membranous
labyrinth undergo differentiation to form specialized
sensory end organs of hearing and equilibrium:
1. Cristae of SCC
2. Maculae of utricle & saccule
3. Organ of Corti of cochlea.
They are innervated by peripheral process of
vestibulocochlear ganglion ( derived from neural
DEVELOPMENT OF ORGAN OF CORTI
Initially, epithelial cells of cochlear duct are alike.
They form two ridges :
Outer ridge- forms one row of inner hair cells or 3-4 rows of
outer hair cells.
Hair cells are the sensory cells of auditory system.
Tectorial membrane : a fibrilar gelatinous substance attached
to the spiral limbus that rest with its tip on the hair cells.
The sensory cells + Tectorial membrane = Organ of Corti.
The BONY LABYRINTH is formed by the
mesenchyme surrounding the membranous
This mesenchyme thus becomes condensed k/a
This condensation is soon converted into cartilage.
b/w the cartilage & membranous labyrinth there is a
layer of loose PERIOTIC TISSUE.
This periotic tissue disappears to create the space of
bony labyrinth and gets filled up by PERILYMPH &
the membranous one gets filled with ENDOLYMPH.
The periotic tissue around the utricle and saccule
also disappears to form a space k/a VESTIBULE.
They also disappears around the SC DUCTS to
form SCC forming to distinct spaces around the
cochlear duct on either side k/a S. TYMPANI &
The S. VESTIBULI communicated with the vestibule
while the S.TYMPANI grows towards the
TYMPANIC CAVITY from which it remains
separated by a membrane
The cartilaginous labyrinth subsequently ossifies to
form bony labyrinth.
Hard & outer bony shell
It consists of
a) Vestibule (utricle &
b) Semicircular canal
Gap between osseus and
membranous is filled with
perilymph (similar to ECF)
Placed inside bony labyrinth
It consists of :
a) Vestibular apparatus
b) Cochlear duct
o This is filled with
endolymph (similar to
OSSEOUS (BONY) MEMBRANOUS
It is the central part of the bony labyrinth.
It lies medial to the middle ear cavity & its lateral wall opens into m.ear at
the fenestra vestibuli which is closed by stapes footplate.
Ovoid in shape but flattened transversely (5mm*3mm).
SPHERICAL RECESS :present on medial wall contains saccule
perforated by many minutes holes (macula cribosa media)
transmitting fine branches of vestibular nerve.
VESTIBULAR CREST : present behind the recess. The anterior
end forms the vestibular pyramid. This divides to below to enclose
ELLIPTICAL RECESS: post sup to vestibular crest in the roof and
the medial wall of vestibule containing utricle.
The vestibular aqueduct opens below the elliptical recess.
In the post part of the vestibule are the 5 openings of the scc and
the anterior part leads in the scala vestibuli of cochlea.
There are 3 semicircular canals :
(a) Anterior (b)posterior (c) lateral
They all are compressed side to side and each forms approx
2/3 rd of a circle.
They all are unequal in length but similar in diameter i.e. 0.8mm
except at the AMPULLA which is twice the diameter.
Lie in plane at right angles.
Each have two ends ampullated and non ampulated
All three ampulated ends and the non ampulated end of lateral
SCC open independently into the vestibule.
The non ampulated end of anterior and posterior joins to form
crus commune (4mm long opening into medial part of vestibule)
Superior scc is situated transverse to the axis of pterous part of
Derived from greek word (cochlos for snail)
5mm from base to apex & 9mm around its base
2.5 to 2.75 turns around a central pyramid of bone = MODIOLOUS.
The base of MODIOLUS is directed towards IAM & transmits nerves to
PROMONTORY =Basal turn of cochlea produces a bulge in medial
wall of middle ear
Thin plate of bone winding spirally around MODIOLUS like a
thread of screw k/a BONY SPIRAL LAMINA.
ROSENTHAL’S CANAL: spiral ganglion are situated in this
canal which runs along the osseous spiral lamina
Bony cochlea is divivded into :
1. SCALA VESTIBULI : closed by stapes footplate and contains perilymph
2. SCALA TYMPANI : closed by secondary TM & contains perilymph.
Connected to sub archanoid space through Aqueduct of cochlea.
3. SCALA MEDIA : also known as COCHLEAR DUCT & contains endolymph.
Scala vestibuli & scala tympani connects to each
other at the apex of cochlea HELICOTREMA.
It is a blind coiled tube & made up of 3 walls :
1. BASILAR MEMBRANE
2. REISSINERS MEMBRANE
3. STRIA VASCULARIS
ORGAN OF CORTI
Discovered by Alfonso Giacomo Gaspare Corti.
Components of organ of Corti :
Hair cells : imp receptor of hearing & transduce sound energy
into electrical energy
Inner hair cells
Richly supplied by afferent
cochlear fibers & are more
important in transmision of
Outer hair cells
Arranged in 3-4 layers
Mainly receives efferent
innervation from olivary complex
& are connected with modulating
the function of inner hair cells.
TUNNEL OF CORTI : Formed by inner & outer rods &
contains fluid called as Cortilymph.
SUPPORTING CELLS : Dieters cells &cells of hansen.
TECTORIAL MEMBRANE : contains gelatinous matrix with
delicate fibers. The shearing force btw the hair cells & tectorial
membrane produces stimulus to hair cells.
UTRICLE & SACCULE
Utricle lies in the posterior part of the bony vestibule &
receives the 5 openings of scc
Connected to Saccule via Utricosaccular duct
The macula of the utricle lies mainly in the horizontal plane
on the inferior surface of the utricle and plays an important
role in determining orientation of head when the head is
upright (force like gravity)
The Macula of the SACCULE is located mainly in a vertical
plane & signals head orientation when the person is lying
The epithelial surface of each macula is characterized by hair cells with
numerous stercocilia & one prominent kinocilum. These cells are covered
by gelatinous structure known as OTOLITHIC or statoconial membrane
and embedded in it are several small crystals k/a otoconia (linear acc of
head will cause the otolithic memb to lag behind as a result of inertia
produced by its increased density & will lead to activation or inhibition of
the mechanosesitive hair cells)
COMPOSITION OF INNER EAR FLUID
PRESENT IN SCALA VESTIBULI SCALA TYMPANI SCALA MEDIA
(membranous labyrinth )
PRODUCTION SLIGHT DIFFERENCES
IN S.VESTIBULI – Plasma via endothelial
boundary of cochlear blood vessels.
In S. TYMPANI – derived from SUB
ARACHNOID SPACE via cochlear canaliculus.
Marginal cells of stria
vascularis & dark cell of
K+ 5mM 150mM
Na+ 150mM 2mM
Cl- 120mM 130mM
Ca+ 1.5mM 20uM
PROTEIN 200-400 mg/dl 126 mg/dl
The high potassium is
mechanosensory hair cell
GLUCOSE 85mg/dl 10-40 mg/dl
ENDOLYMPHATIC DUCT & SAC
Formed by the union of two ducts one forms the utricle
& another from the saccule.
Passes through the vestibular aqueduct
Terminal part is dilated to form Endolymphatic sac.
FUNCTIONAL ANATOMY OF COCHLEA
Basilar membrane is a fibrous membrane.
Made up of 20-30 thousand basilar fibers.
Length of basilar membrane increases from base
0.04mm to apex 0.5mm but diameter decreases leading
to 100 fold decrease of overall stiffness.
FUNCTIONAL ANATOMY CONTINUED..
Sound wave entering the oval window causes BM at the base of
cochlea to move in direction of round window
However due to elastic tension built in BM initiates a fluid wave that
travel along the BM which travel towards the Helicotrema
Each wave is weak at the onset but becomes stronger when it
reaches that portion of the BM that has a resonant frequency equal to
the respective sound frequency.
ORGAN OF CORTI generates the nerve impulse in respponse to
vibration of BM (which is mainly transmitted by inner hair cells)
Excitation of the hair cells : stercocilia project upward from the hair
cells & either touch or are embedded in the surface gel coating of
Bending in one direction causes depolarization & in other causes
INTERNAL ACOUSTIC MEATUS
About 1 cm longand passes into the petrous part of temporal bone in
a lateral direction.
IAM is separated from the internal ear by a vertical plate divided by
a transverse (Falciform) crest.
5 nerves pass through openings in vertical plate above and below
the transverse crest:
Facial & sup vestibular nerve : superior to the crest. Facial lies
anterior to the sup vestibular nerve and is seprated by bills barr
The sup vestibular area also contains nerve to utricle , ant & lat scc.
The nervus intermedius lies between facial motor root and the sup
vest nerve to which it may be adherent.
Cocohlear nerve and inferior vestibular nerve.
Inferior vestibular area also contains opening for saccular nerves and a
single hole k/a FORAMEN SINGULARAE which tranmits nerve to
VASCULAR SUPPLY :
ANTERIOR INFERIOR CEREBELLAR ARTERY
Cochlear branch (20%)
Post vestibular artery.
(SACCULE & POST CANAL
Main cochlear artery
Antreior vestibular artery.
(UTRICLE & LAT & SUP CANAL)
Divides at IAM
Veins draining the VESTIBULE and SCC
accompany the arteries.
Internal auditory vein
Vein of cochlear aqueduct
Vein of vestibular aqueduct
drain into inferior petrosal & sigmoid sinus
The vestibulocochlear (8th ) provides special
sensory innervations to the inner ear
After leaving IAM it divides into
Spirals towards cochlea where it give
branches throughout the coils of canal
It again divides into superior & inferior
with their own vestibular ganglion.
Fibers then branch off & form spiral
ganglia of corti in the modiolus of
Superior ganglion fibers innervate
ampulla of ant & lateral scc & the utricle
Nerve fibers of these ganglia travel
through the osseous spiral lamina &
their dendrites communicate with
stercocilia of organ of corti
Inferior ganglion fibers supply ampulla of
the post & saccule
Greys anatomy -41st edition
Dhingra’s – disease of ent