The Inner EarThe Inner Ear

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The Inner Ear(Labrynth)

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The Inner Ear(Labrynth)

Bony labrynth

Membraneous labrynth

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Bony labyrinth: Hard, bony outer shell.

Membranous labyrinth: Fully contained inside the bony labyrinth; like a convoluted-shaped water balloon stuff inside the bony labyrinth.

(from Minifie, Hixon, & Williams, 1973)

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Parts of the Bony labrynth

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labrynth

• Lateral wall of labrynth is formed by medial wall of middle ear

• Medial wall is formed by lateral limit of internal auditory canal (IAC)

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Vestibule

• Central chamber of labrynth (5 mm) • Lateral wall contains oval

window(fenestra vestibuli) – closed by footplate of stapes sorrounded by annular ligament.

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Vestibule-medial wall

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Semicircular canals

• Lateral(horizontal)• Posterior• Superior(anterior).

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Semicircular canals

• Occupies 2/3 rd of a circle.

• 0.8 mm in diameter.

• Lie in planes at right angles.

• Has ampullated (contain cristae) and non ampullated ends.

• All three ampullated ends and non ampullated ends of lateral SCC open independently and directly into vestibule.

• Involved in angular acceleration and balance

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Superior SCC

It is 15-20 mm long

Situated transverse to the axis of petrous part of temporal bone.

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Lateral SCC

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posterior SCC

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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.

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The Cochlea

• Snail shaped coiled tube• 2.5 to 2.75 turns round a central pyramid of bone

called modiolus.• 30 mm long• 5 mm from base to apex & 9 mm around its base

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Modiolus ?

Central pyramid of bone around which cochlea forms

The base of modiolus directed towards internal acoustic meatus

Transmits vessels and nerves to cochlea Apex lies medial to tensor tympani

musclepgmedicalworld.com

Osseous spiral lamina ?

A thin plate of bone winds spirally around modiolus like the thread of a screw .

This bony lamina gives attachment to the basilar membrane and divides the bony cochlea tube into three compartments.

1. Scala vestibuli2. Scala tympani3. Scala media (membraneous cochlea)

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Rosenthal’s canal ?

Spiral ganglions are situated in this canal which runs along the osseous spiral lamina.

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The cochlea uncoiled

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Scala vestibuli ?

This uppermost channel is continuous with vestibule and closed at oval window by stapes footplate

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Scala tympani ?

This lowermost channel is closed by secondary TM of round window

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Scala media ?

Bind coiled tube, connected to the saccule via ductus reunions.

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aqueduct of cochlea?

A bony bulge in the medial wall of middle ear , represents the basal coil of cochlea.

promontory ?

Scala tympani is connected with subarachnoid space via this.

It is thought to regulate perilymph & pressure in bony labrynth.

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Membraneous labrynth

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Membranous labyrinth with the entire bony labyrinth stripped away.

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Parts of the Membraneous labrynth

Cochlear duct

Utricle

Saccule

Three semicircular canals

Endolymphatic duct and sac

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Cochlear duct

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Cochlear duct-relations & boundaries

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

endolymph

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Notice the stria vascularis (also area vascularis) – The s.v. secretes endolymph.

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 spiral lamina.

modiolus spiral ligament

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Cross-section of the cochlear duct

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utricle 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.

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saccule

lies anterior to utricle opposite the stapes footplate in the

bony vestibule.

its sensory organ macula is concerned with linear

acceleration & decelaration.

Saccule is connected to the cochlea via a thin reunion duct.

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Semicircular ducts

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 &

decelaration.

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Endolymphatic duct and sac

Ducts from utricle and saccule unites and form

utriculosaccular duct

Continues as endolymphatic duct that passes via the vestibular aqueduct

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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.

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Inner ear fluids

perilymph – between bony and membraneous labrynth

Endolymph fills the entire membraneous labrynth

perilymph endolymph

Resembles ECF Resembles ICF

Rich in sodium ions Rich in pottasium ions

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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

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A closer look at the organ of Corti

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Detail of the Organ of Corti

(from Stevens,1951)

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 implants.)

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Reticular lamina

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Deiter’s cells

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Arrangement of hair cells

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Stereocilia

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Arrangement of stereocilia

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Another view...

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Cochlea

• The cochlea contains an array of highly specialized cells arranged in a highly specialized manner.

• There are structural differences between IHCs and OHCs that suggest that they differ in function

• The cochlea not only sends a message to the brain, but it may also receive messages from the brain via efferent innervation.

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Innervation of the organ of Corti

Nerve fibers

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Neuron review

From Gelfand (1998) pgmedicalworld.com

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The spiral ganglion

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Pattern of afferent innervation

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Pattern of afferent innervation

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There are differences among the

fibers innervating

an inner hair cell

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Pattern of efferent innervation

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Sources of efferent cochlear innervation

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Vestibular receptors

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Blood supply of labrynth

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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

Vestibulocochlear artery

1. Post vestibular artery-supplies saccule & post SCC

2. Cochlear branch –supplies to cochleapgmedicalworld.com

Venous drainage

Internal auditory vein

Vein of cochlear aquaduct

Vein of vestibular aquaduct

Drain into inferior petrosal and sigmoid sinuses

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Internal auditory canal About 1 cm long

Passes into petrous part of temporal bone in a lateral

direction

Lined by dura

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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

part

Upper part is again divided into ant &

post part by a vertical crest called

BILL’S BAR.

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IAC - Contents

Vestibulocochlear Nerve

Facial nerve including nervus intermedius

Internal auditory artery and vein

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Development of inner ear

Initially membraneous labrynth , followed by encasement by

bony labrynth.

Starts within first few days( 22- 23 days)

Ectodermal thickening in hind brain

Otic placode

Otic pit

OticystMembraneous labrynth(by 25 th week of GA)pgmedicalworld.com

Development of inner ear

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Development of inner ear

BONY LABRYNTH

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.

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THANK YOU

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Inner Ear Disorders: Prenatal Causes

• Genetic mutation/inheritance• Cytomegalovirus (CMV)• Rubella• Rh incompatibility

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Anatomical Anomalies

Often seen asBonymalformationsExamples:Mondini (incomplete

cochlea)Enlarged Duct(shown here)

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Mondini Aplasia

• AD• 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

interscalar septum• Tx: HA, cochlear implant

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Age Effects

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Noise Damage• Temporary Threshold Shift (TTS)• Permanent Threshold Shift (PTS)

• Duration, Timing and Intensity influence• Typical “Noise Notch” often seen between

____________ first.• Notch widens and deepens over time, with

hearing loss spreading to adjacent frequencies, and increasing in degree.

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Inner Ear Disorders

• Noise induced• Miniers disease• ototoxicity

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