ANATOMY OF INNER EAR

ANATOMY OF INNER EAR

Development of inner ear 

•     Develops from ectoderm in region

     of hindbrain

•     Thickening of ectoderm called auditory placode (day 22-23)

•      Invaginated to form otic pit

•      Separates from surface ectoderm

    to form otic vesicle

MEMBRANOUS LABYRINTH DEVELOPMENT

    22nd day – otic placode is seen.
5th week – Auricle starts forming.
6th week – The cochlea and semicircular canals starts forming.
8th week – The cochlea and semicircular canal assume their definitive external form.
10th week – Scala vestibuli and scala tympani appear.
7th month – External acoustic meatus gets canalized.
The ear is most sensitive to teratogens during the 4th and 9th weeks, and can be affected upto the 12th week.

•Location – Temporal bone

•LABYRINTH

•Parts - Bony labyrinth

               - Membranous labyrinth

          Osseous labyrinth

•Consists of cavities in petrous part of Temporal bone

•3 main parts-

 Vestibule

 3 bony SCC

 Bony cochlea

Vestibule

•Small ovoid bony chamber – 5mm.

•Lies between medial wall of middle ear & outer part of IAC

•Fenestra ovale or oval window - lies in lateral wall of vestibule, separated from ME by foot plate of stapes & annular ligament

•Aqueduct of vestibule lies in the posterior part of medial wall of vestibule - a small canal which passes backwards to the posterior surface of petrous bone, where it opens under the surface of dura.

MEDIAL WALL OF VESTIBULE

Posterior wall of vestibule contains 5 openings of SCC

Anterior wall contains an elliptical opening into the scala vestibuli of cochlea

At the ant part of medial wall lies a depression called spherical recess - perforated by several minute holes for passage of filaments of acoustic nerve to saccule

•Behind it lies an oblique ridge, crista vestibuli, the ant end of which is named pyramid of  vestibule.

•This ridge bifurcates below to enclose a small depression, cochlear recess, which carries cochlear nerve fibres to  base of cochlea.

•Above and behind the crest lies elliptical recess which contains utricle.

Bony SCC

3 bony SCC-Superior, Posterior, Lateral.

Open to post part of vestibule by 5 apertures

Superior and post SCC join posteriorly to form crus commune.

Semicircular canals

Superior SCC – 15 – 20mm

Posterior SCC – 18 –22mm

Lateral SCC – 12- 15mm