INNER EAR

Inner Ear Anatomy

Introduction

Auditory transduction takes place in the inner ear

Transduction refers to the transformation of energy from one form to another. In the case of the auditory system,

acoustic (mechanical) energy is transformed to electrochemical energy

Inner Ear - Anatomy

Also known as the Labyrinth b’cos it is similar to a complex, winding cave.

Consists of two parts: the osseous or bony labyrinth, a

series of cavities within the petrous part of the temporal bone,

and the membranous labyrinth, a series of communicating membranous sacs and ducts, contained within the bony cavities.

Bony Labyrinth

Bony Labyrinth

Bony Labyrinth

Bony Labyrinth

Bony Labyrinth

Consists of three parts: the vestibule, semicircular canals, and cochlea.

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

Bony Labyrinth

The vestibule (similar to that in a house) is a oval cavity which is located between the cochlea and the semi-circular canals.

The vestibule has two windows - oval and round, which open into the middle ear.

Bony Labyrinth

The Bony Semicircular Canals – Are three in number, superior, posterior, and lateral, and are situated above and behind the vestibule.

They enclose the vestibular apparatus.Each semicircular canal has a swelling

at one end - Ampulla.

Bony LabyrinthThe cochlea bears some resemblance to a

common snail-shell. Earlier it was believed that the primary purpose

of the coiling was to save space. Recent research has however suggested a more

important function.

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

It is perforated in the base through which the VIII CN enters the cochlea.

Bony Labyrinth It consists of a conical

shaped central axis, the modiolus.

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

1 cm wide; 5 mm from base to apex

If straightened out, it is about 30 mm. in length and decreases in width from base to apex.

Membranous Labyrinth

It lies within the bony labyrinth and has the same shape.

It bathes in the perilymph (fluid of the bony labyrinth).

Contains the fluid- the endolymph.

Membranous Labyrinth

A humancochlea at 5 months gestation

Oval Window

Round Window

Membranous Labyrinth

Bony Labyrinth - Review

• Oval Window• Cochlea• Superior (anterior)

semicircular canal• Horizontal (lateral)

semicircular canal• Ampullae• Round window• Inferior (posterior)

Semicircular canal

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Membranous Labyrinth

The membranous vestibule and semicircular canals contain the vestibular organs.

The membranous vestibule (enclosed within the bony vestibule) consists of the Utricle and Saccule.

Membranous Labyrinth

Semicircular Canals

The semicircular canals help to maintain a sense of balance by responding to gravity and changes in angular acceleration of the head (up/down, forward/back, and side to side) .

The semicircular canals are arranged roughly at right angles with each other so that they represent all three planes in three-dimensional space.

Membranous Labyrinth

The utricle the larger of the two, is in contact with the membranous semi-circular canals.

The Saccule is in contact with the membranous cochlea by the ductus reuniens.

Both the Utricle and Saccule are responsible for sensing linear acceleration.

The sensory region of the utricle and saccule is known as macula.

The sensory regions of the semi-circular canals are located at the ampulla’s and are known as cupula.

Membranous Cochlea

The membranous cochlea lies within the snail-shaped bony cochlea.

The cochlea coils around the bony modiolus.

It consists of three ducts called Scalae Scala vestibuli, scala tympani & scala

media.

Membranous Cochlea

The scala media (also known as cochlear duct) lies between the scala vestibuli and scala tympani.

The scala vestibuli lies above the scala media and is separated by the Reissner’s membrane.

The scala tympani lies below the scala media & is separated by the basilar membrane.

Cross section of the whole cochlea

Scala media (1), scala vestibuli (2), and scala tympani (3).

The red arrow is from the oval window, the blue arrow points to the round window.

Within the modiolus (6), the spiral ganglion (4) and auditory nerve fibers (5) are seen.

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Membranous Cochlea

While both scala vestibuli and scala tympani are filled with perilymph, the scala media is filled with endolymph.

At the apex of the cochlea, the scala vestibuli and scala media are connected by an opening called the helicotrema.

The endolymph in the scala media is sealed from both the other scala’s.

Membranous Cochlea

Cochlear Scala

Perilymph and Endolymph

Membranous Cochlea

The scala vestibuli communicates with the middle ear by means of the oval window.

The scala tympani communicates with the middle ear by means of the round window.

Basilar Membrane

Is about 35mm long and varies in width from less than 0.1 mm at the basal turn to about 0.5mm at the apical turn.

This in reverse to that of the cochlear duct which decreases in width from base to apex.

Basilar Membrane

This varying width of the basilar membrane is important since it determines where different frequencies are represented. The stiffness of the basilar membrane

decreases by at least a factor of 100 from base to apex, in an approximately exponential fashion

The Basilar membrane separates the scala media from the scala tympani.

Basilar Membrane

The Basilar membrane is held by bony projections from the bony labyrinth.

It is held by the spiral lamina at its medial margin (ie, towards the modiolus) and the spiral ligament at its outer edge (ie towards the outer wall of the bony labyrinth).

Situated on the superior surface of the basilar membrane (ie, inside the scala media) is the organ of Corti.

Basilar Membrane

Cochlear partition

Base:

Narrowest

Stiffest

High Freq.

Apex:

Widest

Least stiff

Low Freq.

Denes&Pinson 1993

Animation

Inner Ear - Anatomy

Inner Ear - Anatomy

Inner Ear - Anatomy

Organ of corti (the real thing!)

Organ of Corti

Organ of corti

Fluids of the Inner Ear

Endolymph & Perilymph - Differ in chemical composition and in their DC (direct current) voltage.

The endolymph has a high potassium and a low sodium concentration.

Perilymph - low potassium but a high sodium content.

Inner Ear Fluids

Inner Ear Fluids

The Endolymph is similar to the fluid in the central nervous system (ie, cerebrospinal fluid).

The Perilymph is similar to extracellular fluids.

The Endolymph has a strong positive potential because of its high pottasium concentration (~ 80 millivolts) when compared to the perilymph of the scala tympani

Inner Ear Fluids

The Perilymph in the scala vestibuli demonstrates a positive, but much smaller potential of about 3 mV when compared to the perilymph of the scala tympani.

The remaining cochlear structures exhibit a negative DC potential.

Inner Ear Fluids

Thus, Endolymph of scala media - +80mV Perilymph of scala vestibuli - +3mV Perilymph of scala tympani - 0 mV Other cochlear structures - Negative

potential.

Innervation

The cochlea is innervated by the VIII CN.The human cochlea contains about

30,000 afferent (sensory) neurons and about 1,800 efferent (motor) neurons

Both inner and outer hair cells receive afferent and efferent innervations.

The inner hair cells receive the majority of the afferent innervations (about 95%).

Innervation of hair cells

Innervation Pattern of IHC & OHC

Innervation of hair cells

Each inner hair cell is innervated by about 20 afferent nerves (ie, cell:neuron ratio is about 1:20).

While a single afferent nerve innervates about 10 outer hair cells (ie, cell:neuron -10:1 ratio).

The outer hair cells, however, receive the majority of the efferent neurons.

Organ of Corti

Organ of Corti includes the inner and outer hair cells, and their supporting cells.

More central of the organ of Corti are two rows of rod-like bodies, the inner and outer rods (or pillars of Corti).

The inner & outer rods form the tunnel of Corti.

IHC and OHC

Organ of Corti – Hair cells

Situated on the basilar membrane are 3 to 5 rows of about 12,000 -15,000 outer hair cells and one row of about 3,000 inner hair cells.

The inner and outer hair cells are separated by the tunnel of corti.

The inner hair cells lie towards the medial end of the tunnel of corti.

Organ of CortiThe rows of the outer hair cells lie in

the outer or lateral end of the tunnel.The hair cells are about 0.01mm long

and 0.001mm in diameter.Located on the top of each hair cell

are hairlike projections called stereocilia.

Outer and Inner hair cells differ in their shape, manner of innervation and organization of stereocilia.

Floor of Scala media & Organ of Corti

1-Inner hair cell2-Outer hair cells3-Tunnel of Corti4-Basilar membrane5-Habenula perforata 6-Tectorial membrane7. Deiters Cell8. Inner Rod Cells9. Outer Rod Cells

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Organ of Corti- the real thing!

Organ of Corti - Hair cells

The inner hair cells are more round shaped more like the base of a vase.

The outer hair cells are longer and thinner and are more tubular (test tube) shaped.

While the inner hair cells stand alone next to the inner rod cells, the outer hair cells are supported and embedded in the cells of Deiters.

Hair Cells

Inner hair cell Outer hair cell

Organ of Corti - Hair cells

The stereocilia of the inner hair cells are organized in a single row,while that of the outer hair cells are arranged in a V or W-shaped pattern.

Sterocilia of Outer hair cells (rat)

Sterocilia of Inner Hair cells

Organ of Corti - Hair cells

The reticular lamina is a delicate membrane perforated by rounded holes which are occupied by the free ends of the outer hair cells. It extends from the heads of the outer rods of Corti to the external row of the outer hair cells.

Organ of Corti & the Reticular Lamina

Organ of Corti - Hair cells

The Tectorial membrane covers the sterocilia of the hair cells. It is a thin fibrous membrane that extends from the modiolus and barely touches the ends of the sterocilia of the outer and inner hair cells.

Tectorial Membrane

Tectorial Membrane