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. Notice that the oval window and round window open into the vestibule, at the base of the cochlea.
Maculae cribrosa media for inferor vestibular nerve Elliptical recess posteriorly ,lodges utricle Macula cribrosa superior (mike’s dot )-passage of ns of utricle and ampulla of superior and lateral SCC Vestibular crest and cochlear recess Aquaduct of vestibule below elliptical recess Anterior and posterosuperior
It is 12-15 mm long. Projects as rounded bulge in middle ear,aditus & antrum. Makes an angle of 30 with horizontal plane. Anterior end is ampulated and opened in upper part of vestibule. Posterior end is non ampullated & ends in lower part of vestibule below the orifice of crus cummune.
It is 18-22 mm long. Situated parallel and close to the posterior surface of petrous temporal bone lower end is ampulated and opened into lower part of vestibule Upper limp joins the crus cummune along with superior scc
Reissner’s membrane and the basilar membrane divide the the cochlea longitudinally into three 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 process
Notice that 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. The organ of Corti rests on the basilar membrane within scala media.
the length increases as it proceeds from the basal coil to apical coil , so higher frequency of sound are heard at the basal coil while lower tunes at the apical coil. Inner thin area is called zona arcuate while outer thick area called zona pectinata.
If you cut the cochlear tube cross sectionally, you’d see something like this. Scala vestibuli on top, scala tympani on the bottom. Scala media is a triangular duct 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.
Oblong and irregular Has anteriorly upward slope at an apparent angle of 30 It lies in posterior part of bony vestibule & recieves the five openings of three SCC Utricle(4.33 mm) is bigger than saccule (2.4 mm) & lies superior to saccule Utricle connected to saccule via utriculosaccular duct Its sensory organ macula is concerned with linear acceleration & decelaration.
Two types of cells in the organ of Corti are support cells and hair cells. The hair cells are the “receptor” cells-- the ones that transduce sound. Support cells such as the Deiter’s cells support hair cells. The tops of the hair cells and pillar cells form the reticular lamina, which isolates the hair cells’ stereocilia from their cell bodies. The tectorial membrane is loosely coupled to the reticular lamina. Endolymph & perilymph. There are 4 rows of hair cells, one on the inner (modiolar) side of the tunnel formed by the pillar cells-- these are the inner hair cells; and 3 one the outer side of the Tunnel of Corti, these are the outer hair cells. Notice that the Deiter’s cells support the Outer hair cells at their base, but that the outer hair cell walls are surrounded by fluid. The inner hair cell is surrounded by support cells.
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.
Deiter’s cell processes “fill in the gaps” between the tops of the outer hair cells to form the reticular lamina.
Outer hair cells, supported by Deiter’s cells, form “columns” between the basilar membrane and the reticular lamina.
Stereocilia on inner (left) and outer (right) hair cells. 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.
Nerve fibers exit the organ of Corti on the modiolar side.
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.
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.
Different types of nerve fibers innervate IHCs and OHCs. Type I fibers innervate IHCs; Type II neurons innervate OHCs.
Nearly all of the nerve fibers that carry messages from the ear to the brain innervate inner hair cells. Notice that the 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. and 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.
Neurons from the brainstem also contact hair cells. These neurons carry information from the brain to the ear and are called efferent neurons. The vast majority of efferents innervate OHCs, and the contacts on OHCs differ from those on IHCs. Efferents form large calyx-shaped contacts on the OHC cell body; efferents form small bouton-like contacts on the afferent nerve fibers that contact IHCs.
The nuclei shown here are in a part of the brainstem called the superior olivary complex. Fibers from both sides of the brain innervate both IHCs and OHCs, but the fibers innervating the two types of HC originate in different places. One recent study suggests that the SOC receives input from auditory cortex-- so fairly high level processing. The fiber tract containing the efferent fibers is known as the olivocochlear bundle (OCB). The tract from the same side of the brain is called the uncrossed OCB and the tract from the opposite side of the brain is called the crossed OCB.
• Occupies 2/3 rd of a circle.
• 0.8 mm in diameter.
• Lie in planes at right angles.
• Has ampullated (contain cristae) and non
• All three ampullated ends and non
ampullated ends of lateral SCC open
independently and directly into vestibule.
• Involved in angular acceleration and balance
It is 15-20 mm long
Situated transverse to the axis of petrous
part of temporal bone.
Crus cummune ?
the non ampullated ends of posterior and
superior canals join & form this (4 mm length)
Opens into medial part of vestibule.
So three SCC opens into vestibule by 5 openings.
• Snail shaped coiled tube
• 2.5 to 2.75 turns round a central pyramid of bone
• 30 mm long
• 5 mm from base to apex & 9 mm around its base
Central pyramid of bone around which
The base of modiolus directed towards
internal acoustic meatus
Transmits vessels and nerves to cochlea
Apex lies medial to tensor tympani
Osseous spiral lamina ?
A thin plate of bone winds spirally
around modiolus like the thread of a
This bony lamina gives attachment to
the basilar membrane and divides the
bony cochlea tube into three
1. Scala vestibuli
2. Scala tympani
3. Scala media (membraneous cochlea)
Scala vestibuli ?
This uppermost channel is continuous with
vestibule and closed at oval window by stapes
Scala tympani ?
This lowermost channel is closed by secondary
TM of round window
Scala media ?
Bind coiled tube, connected to the saccule via
aqueduct of cochlea?
A bony bulge in the medial wall of middle ear ,
represents the basal coil of cochlea.
Scala tympani is connected with subarachnoid
space via this.
It is thought to regulate perilymph & pressure in
Cochlear duct-relations &
1. Basilar membrane – base
It supports organ of corti
2. Reissners membrane
Seperates scala media from scala vestibuli
3. Stria vascularis
It contains vascular epithelium and secretes
Notice the stria vascularis (also area
vascularis) – The s.v. secretes
Notice also the spiral ligament,
which attaches the b.m. to the
bony wall of the cochlea, and
the limbus (or limbus spiralis),
a fibrous covering of the
modiolus spiral ligament
Cross-section of the cochlear
Oblong and irregular
Has anteriorly upward slope at an apparent angle of 30
It lies in posterior part of bony vestibule & recieves the five
openings of three SCC
Utricle(4.33 mm) is bigger than saccule (2.4 mm) & lies superior to
Utricle connected to saccule via utriculosaccular duct
Its sensory organ macula is concerned with linear acceleration &
lies anterior to utricle opposite the stapes footplate in the
its sensory organ macula is concerned with linear
acceleration & decelaration.
Saccule is connected to the cochlea via a thin reunion duct.
Three SC ducts , which open in the utricle correspond
exactly to the three bony canals
The ampullated end contains a thickened ridge of
neuroepithelium which is called crista ampullaris.
Crista ampullaris is concerned with angular acceleration &
Endolymphatic duct and sac
Ducts from utricle and saccule unites and form
Continues as endolymphatic duct that passes via the
The terminal part of the endolymphatic duct is dilated and forms
endolymphatic sac , which is situated between two layers of dura
on the posterior surface of petrous bone.
Inner ear fluids
perilymph – between bony and membraneous labrynth
Endolymph fills the entire membraneous labrynth
Resembles ECF Resembles ICF
Rich in sodium ions Rich in pottasium ions
Organ Of Corti
• The end organ of hearing
– Contains stereocilia & receptor hair cells
– 3 rows OHC, 1 row IHC
– Tectorial and Basilar Membranes
– Cochlear fluids
A closer look at the organ of Corti
Detail of the Organ of Corti
Any cut through the cochlea will show 1 inner hair cell (IHC) and 3 (sometimes 4) outer hair cells
(OHCs). This unit – 1 IHC and 3-4 OHCs is referred to as a hair cell channel. There are about 3000
channels in the human cochlea. (That number will become important later when we discuss cochlear
• The cochlea contains an array of highly
specialized cells arranged in a highly
• There are structural differences between
IHCs and OHCs that suggest that they differ
• The cochlea not only sends a message to
the brain, but it may also receive messages
from the brain via efferent innervation.
Innervation of the organ of Corti
From Gelfand (1998) pgmedicalworld.com
Blood supply of labrynth
Mainly by internal auditory artery (branch of AICA <branch
of basilar artery>)
Internal auditory artery divides into
1. Anterior vestibular artery
Supplies utricle ,superior & lateral SCC
2. Common cochlear artery
Main cochlear artery(80%)-supplies cochlea
1. Post vestibular artery-supplies saccule & post SCC
2. Cochlear branch –supplies to cochleapgmedicalworld.com
Internal auditory vein
Vein of cochlear aquaduct
Vein of vestibular aquaduct
Drain into inferior
petrosal and sigmoid
Internal auditory canal
About 1 cm long
Passes into petrous part of temporal bone in a lateral
Lined by dura
Internal auditory canal
At its lateral end (fundus) IAC is
closed by a vertical cribriform plate of
bone that seperates it from labrynth
A transverse crest divides this plate
into smaller upper and larger lower
Upper part is again divided into ant &
post part by a vertical crest called
IAC - Contents
Facial nerve including nervus intermedius
Internal auditory artery and vein
Development of inner ear
Initially membraneous labrynth , followed by encasement by
Starts within first few days( 22- 23 days)
Ectodermal thickening in hind brain
(by 25 th week of GA)pgmedicalworld.com
Development of inner ear
Mesenchyme enclosing the otocyst becomes chondrified
to form otic capsule
Ossification begins in around 16 th week
Certain channels remain within otic capsule like oval
window where part of the otic capsule becomes the stapes
footplate and the annular ligament.
Often seen as
• Most common cochlear abnormality
• Progressive or fluctuating HL
• risk of perilymphatic gusher and meningitis
from dilated cochlear aqueduct
• Dx: CT reveals single turned cochlea, no
• Tx: HA, cochlear implant
• Temporary Threshold Shift (TTS)
• Permanent Threshold Shift (PTS)
• Duration, Timing and Intensity influence
• Typical “Noise Notch” often seen between
• Notch widens and deepens over time, with
hearing loss spreading to adjacent
frequencies, and increasing in degree.