04/12/2023 1

ANATOMY OF

INNER EAR

MODERATOR:Dr.C.P.DASPRESENTER:RAVINDRA.D

04/12/2023 2

Development of inner ear

Starts within first few days( 22- 23 days)

Initially membranous labyrinth, followed by encasement by bony labyrinth.

Ectodermal thickening in hind brain.

Otic placode

Otic pit

Otocyst

Membranous labyrinth (by 25th week of GA)

04/12/2023 3

BONY LABRYNTH Mesenchyme enclosing the

otocyst becomes chondrified to form otic capsule

Ossification begins in around 16th week .

Certain channels remain within otic capsule like oval window where part of the otic capsule becomes the stapes footplate and the annular ligament.

04/12/2023 4

The inner ear is called as labyrinth, from the complexity of its shape.

It consists of two parts:1. The osseous

labyrinth: A series of cavities within the petrous part of the temporal bone.

2. The membranous labyrinth: A series of communicating membranous sacs and ducts, contained within the bony cavities.

04/12/2023 5

The Osseous Labyrinth (labyrinthus osseous)

The osseous labyrinth consists of three parts: The vestibule, semicircular canals, and cochlea. 

These are cavities hollowed out of the substance of the bone, and lined by periosteum.

They contain a clear fluid, the perilymph, in which the membranous labyrinth is situated.

04/12/2023 6

The Vestibule (vestibulum)

It is central part of the osseous labyrinth, and is situated medial to the tympanic cavity, behind the cochlea, and in front of the semicircular canals.

It is ovoid in shape, but flattened transversely.

Measures about 5 mm. from before backward, the same from above downward, and about 3 mm. across.

In its lateral or tympanic wall is the fenestra vestibuli, closed by the base of the stapes and annular ligament.

04/12/2023 7

On its medial wall, at the forepart, is a small circular depression, the recessus sphaericus, which is perforated, at its anterior and inferior part, by several minute holes (macula cribrosa media) for the passage of filaments of the acoustic nerve to the saccule.

Behind this depression is an oblique ridge, the crista vestibuli, the anterior end of which is named the pyramid of the vestibule.  

This ridge bifurcates below to enclose a small depression, the fossa cochlearis, which is perforated by a number of holes for the passage of filaments of the acoustic nerve which supply the vestibular end of the ductus cochlearis.

04/12/2023 8

04/12/2023 9

Hinder part of the medial wall is the orifice of the aqueductus vestibuli, which extends to the posterior surface of the petrous portion of the temporal bone.

On the upper wall or roof is a transversely oval depression, the recessus ellipticus, separated from the recessus sphaericus by the crista vestibuli.

The pyramid and adjoining part of the recessus ellipticus are perforated by a number of holes (macula cribrosa superior).

The apertures in the pyramid transmit the nerves to the utricle.

04/12/2023 10

2.The Bony Semicircular Canals (canales semicirculares ossei).

The bony semicircular canals are three in number, superior, posterior, and lateral,

These are situated above and behind the vestibule.

They are unequal in length, compressed from side to side, and each describes the greater part of a circle.

Each measures about 0.8 mm. in diameter, and presents a dilatation at one end, called the ampulla, which measures more than twice the diameter of the tube.

They open into the vestibule by five orifices, one of the apertures being common to two of the canals

04/12/2023 11

The superior semicircular canal 15 to 20 mm. in length.

vertical in direction, and is placed transversely to the long axis of the petrous portion of the temporal bone.

Its lateral extremity is ampullated, and opens into the upper part of the vestibule; the opposite end joins with the upper part of the posterior canal to form the crus commune

04/12/2023 12

The posterior semicircular canal

It is also vertical, is directed backward, nearly parallel to the posterior surface of the petrous bone.

It is the longest of the three, measuring from 18 to 22 mm.

It’s lower or ampullated end opens into the lower and back part of the vestibule, its upper into the crus commune.

04/12/2023 13

The lateral or horizontal canal Shortest of the three.

Measures from 12 to 15 mm.

Its ampullated end corresponds to the upper and lateral angle of the vestibule, just above the fenestra vestibuli, where it opens close to the ampullated end of the superior canal

It’s opposite end opens at the upper and back part of the vestibule.

04/12/2023 14

THE COCHLEA It bears some resemblance to a common snail-shell; it forms the

anterior part of the labyrinth. It is conical in form, and placed almost horizontally in front of the

vestibule. apex (cupula) is directed forwards and lateral, with a slight

inclination downward, toward the upper and front part of the labyrinthic wall of the tympanic cavity.

base corresponds with the bottom of the internal acoustic meatus, and is perforated by numerous apertures for the passage of the cochlear division of the acoustic nerve. It measures about 5 mm. from base to apex, and its breadth across the base is about 9 mm

04/12/2023 15

The modiolus The modiolus is the conical central

axis or pillar of the cochlea. Its base is broad, and appears at the

bottom of the internal acoustic meatus.

It is perforated by numerous orifices, which transmit filaments of the cochlear division of the acoustic nerve.

The nerves for the first turn and a half, pass through the foramina of the tractus spiralis foraminosus.

Those for the apical turn,pass through the foramen centrale.

The canals of the tractus spiralis foraminosus pass up through the modiolus and successively bend outward to reach the attached margin of the lamina spiralis ossea.

04/12/2023 16

The bony canal of the cochlea takes two turns and three-quarters around the modiolus.

It is about 30 mm. in length, and diminishes gradually in diameter from the base to the summit, where it terminates in the cupula, which forms the apex of the cochlea.

The beginning of this canal is about 3 mm. in diameter; it diverges from the modiolus toward the tympanic cavity and vestibule, and presents three openings.

04/12/2023 17

1st the fenestra cochlea, communicates with the tympanic cavity.In the fresh state this aperture is closed by the secondary tympanic membrane.

2nd of an elliptical form, opens into the vestibule.

3rd is the aperture of the aquaeductus cochleae, leading to a minute funnel-shaped canal, which opens on the inferior surface of the petrous part of the temporal bone and forms a communication between the subarachnoid cavity and the scala tympani.

04/12/2023 18

The osseous spiral lamina (lamina spiralis ossea)

Bony shelf or ledge which projects from the modiolus into the interior of the canal, and, like the canal, takes two-and three-quarter turns around the modiolus.

It reaches about half-way toward the outer wall of the tube, and partially divides its cavity into two passages or scalae, of which the upper is named the scala vestibuli, while the lower is termed the scala tympani. 

Helicotrema:through which the two scalae communicate with each other.

04/12/2023 19

The osseous labyrinth is lined by an exceedingly thin fibro-serous membrane.

It’s attached surface is rough and fibrous, and closely adherent to the bone.

It’s free surface is smooth and pale, covered with a layer of epithelium, and secretes a thin, limpid fluid, the perilymph.

04/12/2023 20

The Membranous Labyrinth (labyrinthus membranaceus)

The membranous labyrinth is lodged within the bony cavity, separated from the bony walls by a quantity of fluid, the perilymph.

In certain places it is fixed to the walls of the cavity.

The membranous labyrinth contains fluid, the endolymph, and on it’s walls the ramifications of the acoustic nerve are distributed.  

04/12/2023 21

PERILYMPH Resembles

extracellular fluid. Rich in Na+ ions. SOURCE:2 theories 1)filtrate of blood serum from capillaries of spiral ligament. 2)CSF reaching labyrinth via aqueduct of cochlea.

ENDOLYMPH: Resembles

intracellular fluid. Rich in K+ ions. SOURCE: 1) Stria vascularis 2) Dark cells of utricle & ampullated ends of semicircular canals.

04/12/2023 22

04/12/2023 23

Dark cells Dark cells are specialized non

sensory epithelial cells found on either side of the vestibular organs, and lining the endolymphatic space.

These dark-cell areas in the vestibular organ are structures involved in the production of potassium towards the endolymphatic fluid.

Dark cells take part in fluid homeostasis to preserve the unique high-potassium and low-sodium content of the endolymph and also maintain the calcium homeostasis of the inner ear.

04/12/2023 24

The membranous labyrinth consists of two membranous sacs, the utricle, and the saccule. 

1.THE UTRICLE The utricle, the larger of the two, is of an

oblong form, and occupies the upper and back part of the vestibule.

The portion which is lodged in the recess forms a sort of pouch or cul-de-sac, the floor and anterior wall of which are thickened, and form the macula, which receives the utricular filaments of the acoustic nerve.

The cavity of the utricle communicates behind with the semicircular ducts by five orifices.

04/12/2023 25

2.The Saccule It is globular in form and lies in the recessus sphaericus near the

opening of the scala vestibuli of the cochlea.

Its anterior part exhibits an oval thickening, the macula, to which are distributed the saccular filaments of the acoustic nerve.

Its cavity does not directly communicate with that of the utricle.

From the posterior wall a canal, the ductus endolymphaticus, is given off; this duct is joined by the ductus utriculosaccularis, and then passes along the aquaeductus vestibuli and ends in a blind pouch (saccus endolymphaticus) on the posterior surface of the petrous portion of the temporal bone, where it is in contact with the dura mater

From the lower part of the saccule a short tube, the canalis reuniens of Hensen, passes downward and opens into the ductus cochlearis near its vestibular extremity

04/12/2023 26

The walls of the utricle, saccule, and semicircular ducts consist of three layers

The outer layer is a loose and flocculent structure, apparently composed of ordinary fibrous tissue containing blood vessels and some pigment-cells.

The middle layer, thicker and more transparent, forms a homogeneous membrana propria, and presents on its internal surface, especially in the semicircular ducts, numerous papilliform projections.

The inner layer is formed of polygonal nucleated epithelial cells.

In the maculae of the utricle and saccule, and in the transverse septa of the ampullae of the semicircular ducts, the middle coat is thickened and the epithelium is columnar, and consists of supporting cells and hair cells. 

04/12/2023 27

Utricle and the saccule, respond to gravity and linear acceleration in horizontal (utricle) and vertical (saccule) directions.

The sense organs within the utricle and saccule are the maculae.

They occupy the concave spaces at the bottom of the utricle and the saccule and contain tiny pieces of calcium carbonate, called otoliths (ear stones) or otoconia (ear dust), which are embedded into a gelatinous membrane (otolithic membrane) into which the stereocilia of the maculae project.

04/12/2023 28

Since the otoliths are quite numerous in the otolithic membrane and they are heavier than the surrounding fluid, the membrane gets displaced towards the Earth during head tilting (due to gravity) and away from the source of motion during linear acceleration (due to inertia).

04/12/2023 29

The hair cells are aligned over the surface of the maculae in an orderly fanlike pattern so that the optimal direction response for each hair cell is related to its position on the macular surface.

Striola divides the utricular macula into the pars medialis (also known as pars interna) and pars lateralis (also know as pars externa).

04/12/2023 30

There are two types of hair cells in the semicircular canals and the vestibule.

Type I hair cells are flask-shaped cells while type II hair cells are cylinder-shaped cells.

Type I and type II hair cells are very similar in their structure and innervation to the inner hair cells and the outer hair cells of the organ of Corti, respectively.

04/12/2023 31

Each hair cell in the semicircular canals has 50 to 100 small stereocilia and a single larger cilium called a Kino cilium, which only exists in rudimentary form in the hair cells of the cochlea.

The stereocilia are arranged by length, with the longest stereocilia located close to the Kino cilium, and are all connected by tip links.

04/12/2023 32

Movement of the stereocilia hair bundle toward the kinocilium causes a depolarizing (excitatory) sensory response whereas movement away from the kinocilium causes a hyperpolarization (inhibitory) sensory response.

04/12/2023 33

The organ of Corti converts the mechanical vibrations of the basilar membrane into neural impulses that then travel through the auditory nerve and brainstem to the brain.

The fibers of the auditory nerves travel from the organ of Corti through a system of small perforations in the spiral lamina collectively called habenula perforata.

From habenula perforata, nerve fibers travel through a channel in the center of the modiolus (Rosenthal's canal), exit the base of the cochlea, and join vestibular nerve fibers to form the vestibulocochlear nerve.

04/12/2023 34

There are two types of hair cells in the organ of Corti: the inner hair cells (IHCs) and the outer hair cells (OHCs).

Each hair cell has a number of small hair-like projections called stereocilia (cilia) extending from the top of the cell.

The group of stereocilia at the top of a hair cell is called a stereocilia bundle. The stereocilia bundle of each hair cell is organized in several rows forming either a ““W”” or ““V”” pattern for OHCs and shallow ““U”” pattern for IHCs

04/12/2023 35

Stereocilia in each row have graduated heights (like stair steps) and their tips are connected together by thin fibers called tip links.

Each type of hair cell in the ear is connected to the nervous system by both afferent (ascending) and efferent (descending) nerve endings.

04/12/2023 36

characteristic Outer hair cell Inner hair cell

Number 12,000 3500

Location Farther from modiolus nearer

No. of rows 3-4 1

Shape of hair cells Cylindrical Flask shape

no. of rows of cilia 6-7 per cell 2-4 rows per cell

Steriocilia arrangement

W or v shape Shallow U shape

Length of steriocilia Long & thin Short & fat

Motility Motile nonmotile

04/12/2023 37

Outer hair cells Inner hair cells

Nerve supply Primarily efferent Mainly afferent

Development Develop late Develop earlier

Function Modulate function of inner hair cells.

Transmit auditory stimulus

Vulnerability Easily damaged. More resistant.

04/12/2023 38

The groups of IHCs and OHCs are separated by two rods (pillars) of Corti, which structurally support the organ of Corti.

The rods are attached at their tips and more widely separated at the base, forming a triangular shape called the tunnel of Corti.

The tunnel is filled with the cortilymph fluid that has similar properties to the perilymph fluid found in the bony labyrinth.

04/12/2023 39

The tops of the hair cells and supporting cells of the organ of Corti are tightly connected together at their tips to form a continuous layer called the reticular lamina.

The reticular lamina isolates all of the organ of Corti from the endolymph of the scala media except for stereocilia which project through the reticular lamina into the endolymph.

04/12/2023 40

The OHCs are held in position by the outer rod of Corti on one side and by Deiters cells on the other side.

Each Deiters cell holds an OHC at the bottom and through long projections called phalangeal processes from above.

The middle part of an OHC is not firmly supported and is surrounded by a perilymph-filled space called the space of Nuel

04/12/2023 41

Next to the Deiters cells, towards the outer end of the cochlea, there are several groups of supporting cells, called Hensen cells, Claudius cells, outer spiral sulcus cells, and Boettcher cells.

Lateral to these support cells is the Stria vascularis, a highly vascular organ attached to the outer surface of the scala media.

Stria vascularis recycles

potassium and produces endolymph for the scala media, thus maintaining the endocochlear potential of the inner ear.

04/12/2023 42

The stria vascularis is composed of three cell types:1.Marginal cells, which line the endolymphatic canal and have an essential role in ion exchange2. Intermediate cells, which are rich in the pigment melatonin3. Basal cells.

04/12/2023 43

Nerve supply:  The vestibular nerve supplies the utricle, the saccule, and the ampullæ

of the semicircular ducts.

On the trunk of the nerve, within the internal acoustic meatus, is a ganglion, the vestibular ganglion (ganglion of Scarpa); the fibers of the nerve arise from the cells of this ganglion.

On the distal side of the ganglion the nerve splits into a superior, an inferior, and a posterior branch. 

The filaments of the superior branchmacula of the utricle and in the ampullae of the superior and lateral semicircular ducts

inferior branchmacula of the saccule. The posterior branch runs through the foramen singulare at the

postero-inferior part of the bottom of the meatus and divides into filaments for the supply of the ampulla of the posterior semicircular duct.

04/12/2023 44

04/12/2023 45

Blood supply of labyrinth

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 cochlea

04/12/2023 46

04/12/2023 47

Venous drainage: Mainly by:

Internal auditory vein Vein of cochlear

aqueduct Vein of vestibular

aqueduct

These Drain into inferior petrosal and sigmoid sinuses.

04/12/2023 48

Next academic session:

Journal presentation on 07/02/2014 by:

Dr.K.Susritha

04/12/2023 49

Bibliography:

Scott&brown 7th edition, Grey’s anatomy 40th edition. Ludmann diseases of ear

04/12/2023 50

Thank u!!!