Hearing Science: Inner Ear Anatomy

Where are high frequency sounds received?

at the base of the cochlea

Where are low frequency sounds received?

at the apex of the cochlea

The cochlea turns around the...

modiolus

Where do the cochlear nerves connect?

the inner and outer hair cells

Where do the cochlear nerves go?

through the habenula perforata, and into the modiolus

What makes up the VIIIth nerve?

nerve fibers (when they come out of the modiolus, they are the VIIIth nerve)

What makes up the VIIIth nerve?

bipolar neurons

The VIIIth nerve is made up of bipolar neurons. Where does it start and end?

Base of hair cells, through the habenula perforata, into the modiolus (cell body is within/connects to cell body), becomes the VIIIth nerve, connects to cochlea nucleus

What happens to the nerve fibers?

the nerve fibers come out of base of hair cells, all go through habenula perforata, meet the cell bodies in the modiolus, forms the VIIIthe nerve, VIIIth nerve is connected to cochlea nucleus

What part of the cochlea is closest to the modiolus?

the medial aspect of the cochlea

Where is the tectorial membrane located?

on top of the stereocilia

What covers the tops of the hair cells?

reticular lamina (thin layer of connective tissue)

What is the modiolus closest to, inner or outer hair cells?

inner hair cells

Inner Hair Cell

-peak or flask shaped

-approx. 3,500

-single row

stereocilia not attached to the tectorial membrane

-stereocilia in cresent shape

-centralized nucleus

-organelles distributed throughout the cell body

More on the Inner Hair Cell

-95% of the afferent neurons

-many afferent neurons (inner radial fibers) connect to each IHC

-afferent neurons synapse with cell body, efferent neurons synapse with the afferent neurons

-no motility

Outer Hair Cell

-cylindrical shape

-approx. 12,000

-3 rows

-stereocilia attached under the tectorial membrane

-stereocilia in W or V shape

-nucleus found in base

-organelles found along the outer walls

More on the Outer Hair Cell

-5% of afferent neurons

-each afferent neuron (outer spiral fibers) connects to many OHCs

-efferent and afferent neurons synapse directly with cell body

-OHCs "stretch and shrink" this is called OHC motility

What is an afferent nerve fiber?

goes towards the brain

Afferent neurons synapse with...

inner hair cells

Efferent neurons synapse with...

afferent neurons

What synapses directly with the outer hair cells?

efferent and afferent neurons

What does each afferent neuron (outer spiral fibers) connect to?

many outer hair cells

Type I Fibers

associated with outer hair cells

-slow twitch

Type II Fibers

associated with inner hair cells

-fast twitch

Many Type I fibers synapse...

with one inner hair cell directly opposite their habenular opening

What makes the outer hair cells contract?

efferent nerve stimulation

Features of the Cochlea

-mechanical (vibratory motion)

-hydraulic (wave motion)

-chemo-electrical (nerve energy)

Membranous Labyrinth

filled with endolymph

What are the three major anatomical components of the inner ear?

semi-circular canals

vestibule (saccule and utricle)

cochlea

Semi-Circular Canals

sense movement of the head, both speed and direction

-three fluid-filled tubes

How many semi-circular canals are there?

three

Vestibule

utricle and saccule are used to detect the orientation of the head

Cochlea

closed, labyrinthine (maze-like) capsule, filled with fluid

-most anterior structure

-two 5/8th turns, large bundle of nerve cells enters the center (auditory branch of the VIIIth nerve)

Modiolus of the Cochlea

bony center of cochlea

-the bony canal turns around the modiolus

-"continuous left turn" (spiral staircase)

-walls are solid bone

The auditory nerve fibers from the hair cells...

enter the modiolus

Labyrinth of the Cochlea

-2 5/8th turns

-35 mm length base to apex

-ends at the helicotrema in the apex

Where is the base of the cochlea?

near the stapes footplate

Where is the apex of the cochlea?

other end of the bony labyrinth

Story through the Cochlea

-stapes footplate removed, you see oval window

-through oval window, you would be in the Scala vestibuli

-go through Scala vestibuli, you are in the helicotrema (at apex)

Helicotrema = where the scala vestibuli and scala tympani

-past helicotrema, you are now in lower level (scala tympani)

-down spiral staircase, to round window (the base)

Osseous Spiral Lamina

bony extension of the medial wall of the bony labyrinth

-runs continuously along the medial wall

-gets skinnier as it goes from the base to the apex

The width of the osseous spiral lamina _________ between the base and the apex of the cochlea.

decreases

Why is the change in width necessary?

-basilar membrane is narrow at the base because it is responsible for detecting high frequencies

-basilar membrane is wider at apex because it is responsible for detecting low frequencies

By the time the cochlea reaches its third turn, the osseous spiral lamina has nearly ___________.

disappeared

What are the three sections of the scala?

scala vestibuli

scala tympani

scala media

Scala Vestibuli

-bounded inferiorly by Reissner's membrane

-ends at helicotrema

-contains perilymph

Reissner's Membrane

-floor of scala vestibuli

-roof of scala media

Perilymph

fluid around scala media (fills scala vestibuli and scala tympani)

The scala vestibuli is bounded inferiorly by...

Reissner's membrane

Scala Tympani

-bounded superiorly by the basilar membrane

-ends at helicotrema

-contains perilymph

The width of the basilar membrane ___________ between base and apex of cochlea.

increases

Tonotopic

pertains to the way in which the primary auditory cortex is organized so that neurons that respond to particular frequencies are grouped together

The scala tympani is bounded superiorly by the...

basilar membrane

Scala Media

-bounded superiorly by Reissner's membrane

-bounded inferiorly by basilar membrane

-contains Organ of Corti

-contains endolymph

The scala media is bounded superiorly by...

Reissner's membrane

How do the scala vestibuli and scala tympani share perilymph?

at the helicotrema

What keeps the endolymph in the scala media?

reissner's membrane and basilar membrane

Gross Anatomy of Scala Media

-Reissner's Membrane

-Basilar Membrane

-Osseous Spiral Lamina

-Spiral Limbus

-Tectoral Membrane

-Organ of Corti

-Stria Vascularis

What are the attachment points for the basilar membrane?

lateral side: spiral ligament

medial side: osseoud spiral lamina

What kind of cells are on top of the osseous spiral lamina?

fibrous cells

Stria Vascularis

-occupies most of the lateral surface of the scala media

-major source of blood supply to all the structures of the scala media

-source of endolymph

What is another name for the stria vascularis?

endolymphatic pump

Exhausted Endolymph

chemical content changes and you need a fresh supply of ions for the nerve impulse

Tectorial Membrane

comprised mostly of water

-lighter density than endolymph

-seeks to float in a more salty endolymph environment

What ties down the tectorial membrane?

it is tied down at its medial (spiral limbus) and lateral (Hensen's cells) edges

Fine Features of the Scala Media

-inner sulcus supporting cells

-inner hair cells

-rods of corti

-deiter cells

-outer hair cells

-Hensen's cells

-cells of clause

Inner Sulcus Supporting Cells

support for organ of corti from medial side

Corti's Tunnel

gaps between pillar cells permit free circulation of liquids throughout the interior of organ of Corti

What liquids are present in the scala media?

endolymph

cortilymph

Endolymph

fluid within the labyrinth of the inner ear

-source; stria vascularis

-high concentration of K+

-relatively low concentration of Na+

-found: superior to reticular lamina

Cortilymph

the thick fluid located within the organ of Corti

-source: may be from perilymph, diffused through the basilar membrane

-similar in composition to perilymph

-found: interior of corti's organ

Who discovered Corti's Organ?

Alfonso Corti

Habenula Perforata

holes in the osseous spiral lamina where the nerve fibers enter the Organ of Corti

Reticular Lamina

under the tectoral membrane

Spaces of Nuel

spaces betwenn outer hair cells

Where are the phalanges of deiter cells found?

in the 2nd and 3rd row of deiter cells

Reticular lamina with...

outer hair cell cilia extending above the surface

Basilar membrane with...

bases of Deiter cells resting on it

The first row of Deiter cells do not have...

phalanges

-instead they have bulges that look like cotton balls stuck on their sides

-bulges are located near the pocket of Deiter cells where they hold the OHCs

Hair Cell Cilia

"hair"- extensions of cell membrane

Stereocilia

many per cell

-fine, feathery, paddle-like appearance

-highly organized in patterns

-rigid, interconnected with tip-links

-deep roots within hair cells

What are the light-grey colored areas between cilia?

tip-link inter-connectors

What happens when the cilia move?

the tip-link inter-connectors are stretched or shorten

-when stretched they open tiny

conduits for electron flow

What does the tapered handle-like appearance at the base of each cilia do?

acts as a hinge so that the cilia can swing back and forth

What acts as a support for sensory cells?

- Inner sulcus (medial side)

- Hansen's cells (lateral side)

- Reticular lamina (superior)

- Dieter cells (inferior to the outer hair cells)

- Corti's rods (between the IHCs and the OHCs)

Blood Supply to the Cochlea

-Arterial Course

-Venous Course

Arterial Course in the Scala Media

-the internal auditory artery arises from the meatal loop of the middle cerebral artery which usually sits on the cochlear nerve in the internal auditory canal.

-arteries penetrate into the cochlea via the modiolus.

-the spiral modiolar artery gives off radial branches to the lateral cochlear wall, including the stria vascularis

Venous Course in the Scala Media

-venous drainage of the cochlea occurs via the modiolus

-most mammals have a spiral modiolar vein

-no main vein is visible among the nerves in the internal auditory canal in humans

-the venous blood empties either directly into the inferior petrosal sinus or the internal jugular vein or travels through other venous sinuses

The inner ear is innervated by the...

VIIIth cranial nerve (auditory nerve)

What are the two branches of the VIIIth Cranial Nerve

auditory branch

vestibular branch

Auditory Branch

-nerve fibers connected to hair cells in the cochlea are routed through the habenula perforate to the modiolus

-the cell bodies of these nerve cells are called the spiral ganglion

Where are the auditory and vestibulaar branch found?

in the internal auditory canal (or meatus)

Ganglion

mass of nerve cell bodies

Afferent Nerve Fibers

send information from the hair cells (mostly the inner hair cells) to the brainstem and then to the brain

Efferent Nerve Fibers

send information from the brainstem and the brain to the

hair cells(mostly the outer hair cells)

Axons of both types of nerves reach the cochlea via the...

habenula perforata

How many neurons are in the afferent division of the auditory branch of the VIIIth nerve?

30,000

Radical Fibers (Type I)

-90 to 95 % of afferent nerve fibers

-about 20 of these fibers connect to a single inner hair cell

-bipolar neurons

-myelinated fibers with Nodes of Ranvier

-the neural spike jumps from node to node for fast conduction time

Spiral Nerve Fibers (Type II)

-about 5% of afferent nerve fibers

-connect to outer hair cells

-each nerve fiber connect to as many as 10 different outer hair cells

-pseudo-monopolar neurons

-not myelinated (slower conduction time)

The VIIIth nerve leaves the internal auditory canal and enters the brainstem at the...

cerebellopontine angle (where the

cerebellum and the pons meet)

Cerebellum

involved in the coordination of movement