Anatomy of inner ear

The Inner EarThe Inner Ear

pgmedicalworld.com

The Inner Ear(Labrynth)

pgmedicalworld.com

The Inner Ear(Labrynth)

Bony labrynth

Membraneous labrynth

pgmedicalworld.com

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)

pgmedicalworld.com

Parts of the Bony labrynth

pgmedicalworld.com

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)

pgmedicalworld.com

Vestibule

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

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

pgmedicalworld.com

Vestibule-medial wall

pgmedicalworld.com

Semicircular canals

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

pgmedicalworld.com

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

pgmedicalworld.com

Superior SCC

It is 15-20 mm long

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

pgmedicalworld.com

Lateral SCC

pgmedicalworld.com

posterior SCC

pgmedicalworld.com

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.

pgmedicalworld.com

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

pgmedicalworld.com

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)

pgmedicalworld.com

pgmedicalworld.com

Rosenthal’s canal ?

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

pgmedicalworld.com

The cochlea uncoiled

pgmedicalworld.com

Scala vestibuli ?

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

pgmedicalworld.com

Scala tympani ?

This lowermost channel is closed by secondary TM of round window

pgmedicalworld.com

Scala media ?

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

pgmedicalworld.com

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.

pgmedicalworld.com

Membraneous labrynth

pgmedicalworld.com

Membranous labyrinth with the entire bony labyrinth stripped away.

pgmedicalworld.com

Parts of the Membraneous labrynth

Cochlear duct

Utricle

Saccule

Three semicircular canals

Endolymphatic duct and sac

pgmedicalworld.com

Cochlear duct

pgmedicalworld.com

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

pgmedicalworld.com

pgmedicalworld.com

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

pgmedicalworld.com

Cross-section of the cochlear duct

pgmedicalworld.com

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.

pgmedicalworld.com

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.

pgmedicalworld.com

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.

pgmedicalworld.com

Endolymphatic duct and sac

Ducts from utricle and saccule unites and form

utriculosaccular duct

Continues as endolymphatic duct that passes via the vestibular aqueduct

pgmedicalworld.com

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.

pgmedicalworld.com

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

pgmedicalworld.com

pgmedicalworld.com

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

pgmedicalworld.com

A closer look at the organ of Corti

pgmedicalworld.com

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.)

pgmedicalworld.com

pgmedicalworld.com

Reticular lamina

pgmedicalworld.com

Deiter’s cells

pgmedicalworld.com

Arrangement of hair cells

pgmedicalworld.com

Stereocilia

pgmedicalworld.com

Arrangement of stereocilia

pgmedicalworld.com

pgmedicalworld.com

Another view...

pgmedicalworld.com

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.

pgmedicalworld.com

Innervation of the organ of Corti

Nerve fibers

pgmedicalworld.com

Neuron review

From Gelfand (1998) pgmedicalworld.com

pgmedicalworld.com

The spiral ganglion

pgmedicalworld.com

Pattern of afferent innervation

pgmedicalworld.com

Pattern of afferent innervation

pgmedicalworld.com

There are differences among the

fibers innervating

an inner hair cell

pgmedicalworld.com

Pattern of efferent innervation

pgmedicalworld.com

Sources of efferent cochlear innervation

pgmedicalworld.com

Vestibular receptors

pgmedicalworld.com

Blood supply of labrynth

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

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

pgmedicalworld.com

Internal auditory canal About 1 cm long

Passes into petrous part of temporal bone in a lateral

direction

Lined by dura

pgmedicalworld.com

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.

pgmedicalworld.com

IAC - Contents

Vestibulocochlear Nerve

Facial nerve including nervus intermedius

Internal auditory artery and vein

pgmedicalworld.com

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

pgmedicalworld.com

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.

pgmedicalworld.com

THANK YOU

pgmedicalworld.com

pgmedicalworld.com

pgmedicalworld.com

Inner Ear Disorders: Prenatal Causes

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

pgmedicalworld.com

Anatomical Anomalies

Often seen asBonymalformationsExamples:Mondini (incomplete

cochlea)Enlarged Duct(shown here)

pgmedicalworld.com

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

pgmedicalworld.com

Age Effects

pgmedicalworld.com

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.

pgmedicalworld.com

Inner Ear Disorders

• Noise induced• Miniers disease• ototoxicity

pgmedicalworld.com