Labyrinths contain Three parts Three parts Semicircular canals Semicircular canals Vestibular system Vestibular system Vestibule Vestibule Cochlea Auditory system Cochlea Auditory system

Vestibular system Function: Balance and equilibrium Function: Balance and equilibrium Sensory cells: Hair cells Sensory cells: Hair cells

Auditory system: Cochlea Name derived from snail-like shape Name derived from snail-like shape Tube of decreasing diameter coiled around itself Tube of decreasing diameter coiled around itself Coiled around a central bony canal called modiolus Coiled around a central bony canal called modiolus Broad base, narrow apex Broad base, narrow apex Length: About 35 mm Length: About 35 mm In human beings: 2 and 5/8 coils In human beings: 2 and 5/8 coils

Isolated cochlear turns from the inner ear of fetal sheep: Photo courtesy of Gerhardt, K.

Cochlear macrostructure Partially divided throughout its length by a thin spiral shelf of bone called osseous spiral lamina Partially divided throughout its length by a thin spiral shelf of bone called osseous spiral lamina At the outer (lateral) wall of the cochlea: Spiral ligament At the outer (lateral) wall of the cochlea: Spiral ligament

Between the osseous spiral lamina and the spiral ligament: Basilar membrane Between the osseous spiral lamina and the spiral ligament: Basilar membrane Runs all the way along the length of the cochlea, except for a small opening at the apex called the helicotrema Runs all the way along the length of the cochlea, except for a small opening at the apex called the helicotrema Reissner’s membrane projects diagonally from spiral lamina to outer bony wall of cochlea Reissner’s membrane projects diagonally from spiral lamina to outer bony wall of cochlea Joins the basilar membrane at the helicotrema Joins the basilar membrane at the helicotrema

Between the osseous spiral lamina and the spiral ligament: Basilar membrane Between the osseous spiral lamina and the spiral ligament: Basilar membrane Runs all the way along the length of the cochlea, except for a small opening at the apex called the helicotrema Runs all the way along the length of the cochlea, except for a small opening at the apex called the helicotrema Reissner’s membrane projects diagonally from spiral lamina to outer bony wall of cochlea Reissner’s membrane projects diagonally from spiral lamina to outer bony wall of cochlea Joins the basilar membrane at the helicotrema Joins the basilar membrane at the helicotrema

Cross-section of the cochlea Basilar membrane and Reissner’s membrane divide the cochlear canal into three ducts: Basilar membrane and Reissner’s membrane divide the cochlear canal into three ducts: Scala vestibuli Scala vestibuli Scala tympani Scala tympani Scala media Scala media

Scala Vestibuli Above Reissner’s membrane Above Reissner’s membrane Extends from oval window in the vestibule to the helicotrema Extends from oval window in the vestibule to the helicotrema Contains perilymph Contains perilymph

Scala Tympani Below the basilar membrane Below the basilar membrane Extends from round window to the helicotrema Extends from round window to the helicotrema Contains perilymph Contains perilymph

Scala media/cochlear sac Bound below by basilar membrane Bound below by basilar membrane Bound above by Reissner’s membrane Bound above by Reissner’s membrane Bound on the outer side by Stria vascularis Bound on the outer side by Stria vascularis Contains endolymph Contains endolymph

Dimensions of the cochlear partitions Cochlea: Narrower toward the apex Cochlea: Narrower toward the apex Basilar membrane: Narrower at the base and wider toward the apical end Basilar membrane: Narrower at the base and wider toward the apical end Apical end: Flaccid and under no tension Apical end: Flaccid and under no tension Base end: Stiff and under a small amount of tension Base end: Stiff and under a small amount of tension

Cochlear microstructure Organ of Corti Organ of Corti Lies on the basilar membrane in the scala media Lies on the basilar membrane in the scala media Contains many different types of specialized cells Contains many different types of specialized cells

Organ of Corti

Pillar cells or rods of Corti Provide structure and support Provide structure and support Inner and outer pillar cells Inner and outer pillar cells Form a tunnel called tunnel of Corti Form a tunnel called tunnel of Corti Contains fluid called cortilymph Contains fluid called cortilymph

Hair cells On the outer side of the outer pillar/rod cells: Outer hair cells (OHC) On the outer side of the outer pillar/rod cells: Outer hair cells (OHC) (Away from modiolus) On the inner side of the inner pillar/rod cells: Inner hair cells (IHC) On the inner side of the inner pillar/rod cells: Inner hair cells (IHC) (Toward the modiolus)

Supporting cells OHC supported by cells of Deiter and Hensen OHC supported by cells of Deiter and Hensen IHC supported by border cells of the inner sulcus IHC supported by border cells of the inner sulcus

Reticular lamina Formed partly by the phalangeal processes of the Deiter’s cells.

OHC Around in number Around in number Test-tube shaped Test-tube shaped Three-five rows Three-five rows

IHC Around 3400 in number Around 3400 in number Flask shaped Flask shaped Single row Single row

Stereocilia Hairlike projections Hairlike projections Project from the top of IHC and OHC Project from the top of IHC and OHC Graded in length Graded in length Have cross-bridges called tip-links Have cross-bridges called tip-links

Stereocilia of OHC and IHC OHC: OHC: 6-7 rows per OHC 6-7 rows per OHC Each row has a W shaped arrangement Each row has a W shaped arrangement IHC: IHC: 2-4 rows per IHC 2-4 rows per IHC Each row in a shallow U- shaped arrangement Each row in a shallow U- shaped arrangement

Tectorial membrane Gelatinous transparent membrane projecting from spiral lamina Gelatinous transparent membrane projecting from spiral lamina Attaches loosely on outer edge to the Deiter’s and Hensen’s cells. Attaches loosely on outer edge to the Deiter’s and Hensen’s cells. Longest stereocilia of OHC embedded in inferior surface of tectorial membrane Longest stereocilia of OHC embedded in inferior surface of tectorial membrane Stereocilia of IHC not embedded in tectorial membrane Stereocilia of IHC not embedded in tectorial membrane