Ear

• Auditory system - organ of hearing• Vestibular system – for balance• Divisions of ear – External, Middle, Internal

– External & Middle parts – for the collection & conduction of sound waves to internal ear

– Internal ear- convert sound energy into electrical impulses

Ear - Embryology

Ear - Embryology

• Ear develops from – Surface ectoderm, – Components of 1st & 2nd pharyngeal arches, – Mesenchyme

External Ear• Vestigial in humans• Role in sound localization &

amplification• parts:

– Auricle: or pinna, made of skin, hair follicles, sweat & sebaceous glands, Elastic cartilage

– External Acoustic Meatus (EAM):

• lateral ⅓ - elastic cartilage, lining is same as pinna except ceruminous glands in place of sweat glands

• Medial ⅔ - within Temporal bone, with lining of thinner skin, fewer hair & glands

Clinical : Cerumen or earwax ( secretions of ceruminous and sebaceous glands) if accumulates in excess ear ache and deafness

Middle Ear• Located in air filled space of temporal bone

Tympanic cavity• Boundaries

– laterally : TM– Medially: Internal Ear– Anteriorly : ET– Posteriorly : Mastoid with air cells

• Function: convert sound vibrations to mechanical vibrations and send to internal ear

• Openings– Oval window – vestibular– Round window – cochlear

• Structures– Tympanic membrane– Ossicular chain– Muscles (tensor tympani & Stapedius)

Middle Ear

• Tympanic membrane– Outer stratified

Squamous epithelium– Middle connective tissue– Inner simple cuboidal

epithelium

Clinical : perforation of TM leads to deafness

Middle Ear

• Ossicular chain– Malleus –attaches to TM– Incus -links Malleus with

stapes– Stapes with its foot

process attaches oval window

Clinical : calcification at foot plate of Stapes Ankylosis (otosclerosis) causes

deafness

Internal Ear

• Location– Petrous part of temporal bone

• Divisions– Bony labyrinth – Membranous labyrinth

• Spaces – Endolymphatic space: within membranous labyrinth,

contains endolymph ( similar to intracellular fluid)– Perilymphatic space: between bony and membranous

labryinth, contains perilymph ( similar to ECF)– Cortilymphatic space: with organ of corti, contains

cortilymph ( like ECF)

Internal Ear

• Bony labryinth– Vestibule with utricle

and saccule– Semicircular canals– cochlea

Internal Ear

• membranous labryinth– Cochlear labryinth with

cochlear duct connected to saccule

– Vestibular labryinth having semicircular canals(3) & utricle & Saccule

Membranous labryinth

• Hair cells : – specialized cells of

membranous labryinth– Form a hair bundle with

stereocilia and tallest kinocilium

– Mechanoelctric transduction in stereocilia lead to influx of K+ gated ion channels opening of voltage gated Ca++ channels release of neurotransmitter generation of action potential in afferent nerve endings

Hair cells

Vestibular Sensors

Gravity & Linear Movement

Angular movement of head

Sound Sensors

Spiral organ of corti

Outer hair cell

FACIAL NERVE

VIIIth NERVE

COCHLEA

VESTIBULE

AURICLE

EAR CANAL

MIDDLE EAR

Semicircular CANALS

EAR OVERALL

Auditory/Eustachian TUBE

Nasopharynx

Cranial cavity

CARTILAGE

Meninges

EAR DRUM

EAR

pinna

elasticcartilage

Temporal bone

Ear ossicles

semicircularcanals vestibule

Cochlea

eustachian tube

round window

oval window

Tympanic membrane

External auditory meatus

lobe

CRANIAL CAVITY

EUSTACHIAN TUBE

VESTIBULE

SEMICIRCULAR CANALS

MIDDLE EAR

COCHLEA

EXTERNAL CANAL

MASTOID AIR CELLS

TEMPORAL BONE

AQUEDUCTS

BONY CAVITIES

One not shown

The ‘HAIRS’ of HAIR CELLS

The electron microscope revealed that each ‘hair’ consists of one kinocilium at the side of an array of many non-motile sensory stereocilia. (These stereocilia are not the absorptive kind found in the male repoductive tract.)

A cell - View from on topCilium

Stereocilia

Viewed from the side, the stereocilia differ regularly in height, becoming shorter going away from the tall kinocilium

They are more numerous than shown (70 per cell), and are attached by links near their tips

Viewed from the side, the stereocilia vary regularly in height, becoming shorter going away from the tall kinocilium

HAIR-CELL DIRECTIONAL SENSITIVITY

Vesicles & tubules

Sensitive to bending:

Kinocilium

Stereocilia

Plate for attachment of actin cores of stereocilia

Tip links between stereocilia

Synapse

Towards kinocilium causes cell depolarization, and increased afferent fiber firing

Bending away from kinocilium causes cell hyperpolarization, and decreased afferent fiber firing

Afferent axon

CUPULA

CANAL FLUID

HAIR CELLS

The bending of the hairs sometimes is co-ordinated and amplified by imbedding the hairs in a gelatinous body called the cupula

CUPULA

HAIR-CELL SIGNAL TRANSDUCTION

How does bending towards kinocilium cause cell depolarization, and increased afferent fiber firing?

Kinocilium

Vesicles & tubules

Stereocilia

Tip links between stereocilia

Synapse

Transduction channels for cations, e.g., Ca2 +, K+

are opened by the bending

The entering cations depolarize the cell

which increase s transmitter release at the base,

raising the firing rate in the axon

Supporting, basal, mantle, etc cells

Electron microscopy also revealed that the ‘supporting’ cells were of various different kinds

Nerve fibers and synapses were both afferent, and coming from the CNS as effferent (controlling)

Afferent synapses were of more than one kind, as are the hair cells

Although there was a fundamental pattern, species differences were widepsread in ‘hairs, sensory cells, ‘supporting’ cells, and almost all aspects of the receptor structures

Certain of the supporting cells secrete the gelatinous (glycoprotein) cupula

VESTIBULAR APPARATUS I

The fluid in the bags - endolymph - has a special ionic composition to allow for efficient depolarization, when the hair-cell stereocilia are deflected.

Spaces form in the skull’s temporal bone on each side for three differently oriented CANALS communicating with a larger space - VESTIBULE - to hold a system of fluid-filled bags & tubes

Each canal, and the hair cells positioned within it, provide nervous signals responsive to movement of the head in a particular way.

The three mutually perpendicular canals on each side can thus inform on any angularly accelerated (rotary) head movement

VESTIBULAR APPARATUS II Semicircular canal & duct

BONE SEMICIRCULAR CANAL containing

SEMICIRCULAR DUCT containing

Perilymph

Endolymph

Always an initial source of confusion - the semicircular space in the bone is the CANAL

Inside, and attached to the wall, is the smaller membranous tube - the DUCT

The rest of the space in the canal is taken by a loose arachnoid-like tissue, occupied by CSF-like perilymph

The duct is filled with endolymph, high in K+, & made elsewhere

When the head moves in the plane of the canal, the endolymph lags a little in relation to the canal’s & duct’s movement

VESTIBULAR APPARATUS III Duct’s Ampulla & Christa

At one end of the canal, where it opens into the bony vestibule, the duct swells out, then constricts, creating the ampulla

BONE

SEMICIRCULAR CANAL

SEMICIRCULAR DUCT

Perilymph

AMPULLA

Raised ridge - CRISTA - with hair cells & gelatinous cupula

Opening into utricle

Endolymph

CUPULA

ENDOLYMPH

VESTIBULAR APPARATUS IV Duct & Christa Activity

As th head moves so , the endolymph in this duct lags

BONE

SEMICIRCULAR CANAL

SEMICIRCULAR DUCT

Perilymph

Endolymph

CUPULA

along with the cupula

ENDOLYMPH

But moving with the head are the tissues, including the hair cells

So the hair cells are bent by the dragging cupula

causing opening or closing of the cation channels, with change in hair-cell polarization & synaptic drive

to the christa axons

Ampulla of superior semicircular duct

start of superior semicircular duct

UTRICLE

SACCULE

MACULA of Utricle

MACULA of Saccule

Saccular Duct

Utricular Duct

VESTIBULAR APPARATUS V Saccule & Utricle

SACCULE

MACULA of Saccule

UTRICLE

SACCULE

MACULA of Utricle

Saccular Duct

Utricular Duct

VESTIBULAR APPARATUS VI Saccule versus Utricle

Both contain endolymph & are connected via the U & S ducts

Both utricle & saccule contain a macula with hair cells

Both maculae are covered with a gelatinous otolithic membrane

The utricle is much larger

The maculae are oriented differently

The utricle has the six openings for the 3 semicircular ducts

but

Saccule’s near vertical; Utricle’s near horizontal

VESTIBULAR APPARATUS VII Macula Structure

Crystalline OTOCONIA on gelatinous

OTOLITHIC MEMBRANE

HAIR CELLS

Basement membrane

AXONS of vestibular ganglion neurons

Supporting cells

Being in pairs, and in different orientations, the maculae can sense the head’s position and its linear movement

The OTOCONIA of calcium salts and protein contribute to the effect of gravity on the hair cells, providing a vestibular drive to eventually keep ‘postural’ skeletal muscles active in maintaining one’s posture

OTOLITHIC MEMBRANE

Connective tissue

Endolymph

VESTIBULAR GANGLION

Bipolar neurons VESTIBULAR

NERVE

start of superior semicircular duct

Ampulla of superior semicircular duct

SACCULE

MACULA of Utricle

MACULA of Saccule

VESTIBULAR APPARATUS VIII Vestibular nerve & Ganglion

UTRICLE

CRISTA The vestibular ganglion & nerve lie in the bony internal acoustic meatus

Also, within the bone, spaces must be found for the air vibrations to be conveyed to the cochlea; while air pressure has to be equilibrated across the ear drum

The cochlea has to have its own coiled space in the bone

We have seen that: the semicircular ducts require three canals in each temporal bone; the utricle and ampullae, & the saccule, need a vestibule in the bone; and the vestibular ganglion & nerve need a passageway (meatus) to reach the brainstem.

TEMPORAL BONY SPACES

Finally, passages (aqueducts) are needed to keep the two fluids - perilymph and endolymph - in balance

The intricate result is best depicted initially as a crude diagram for learning parts and relations

CRANIAL CAVITY

EUSTACHIAN TUBE

VESTIBULE

SEMICIRCULAR CANALS

MIDDLE EAR

COCHLEA

EXTERNAL CANAL

MASTOID AIR CELLS

TEMPORAL BONE

AQUEDUCTS

BONY CAVITIES

One not shown

EUSTACHIAN TUBE

VESTIBULE

SEMICIRCULAR CANALS

MIDDLE EAR

COCHLEA

EXTERNAL CANAL

TEMPORAL BONE

AQUEDUCTS

UTRICLE & SACCULE

UTRICLE

SACCULE

CRANIAL CAVITY

SEMICIRCULAR CANALS

MIDDLE EAR

COCHLEA

AQUEDUCTS

FLUID CONNECTIONS I

UTRICLE

SACCULE

BRAIN

CSFPERILYMPH

PERILYMPH

ENDOLYMPH SAC

ENDOLYMPHDUCT

PERILYMPH DUCT

Perilymph & Brain’s CSF are in continuity via the Perilymphatic Duct

SEMICIRCULAR CANALS

COCHLEA

AQUEDUCTS

FLUID CONNECTIONS II

UTRICLE

SACCULE

BRAIN

CSFPERILYMPH

PERILYMPH

ENDOLYMPH SAC

ENDOLYMPHDUCT

PERILYMPH DUCT

Endolymph fills the utricle, saccule, semicircular ducts, and scala media of the cochlea, with several small connecting tubes for continuity

Also, endolymph passes up the endolymphatic duct to a sac in the dura, from whence excess fluid can filter into the CSF

SEMICIRCULAR DUCTS

AQUEDUCTS

ENDOLYMPH SYSTEM

BRAIN

CSF

ENDOLYMPH SAC

COCHLEA

UTRICLE

SACCULE

ENDOLYMPHDUCT

Utricular DuctSaccular Duct COCHLEAR

DUCT Scala mediaDuctus reuniens

AMPULLA

SEMICIRCULAR DUCTS & AMPULLAE

ENDOLYMPH SYSTEM II

UTRICLE

SACCULE

ENDOLYMPHATIC DUCT & SACUtricular Duct

Saccular Duct

COCHLEAR DUCT (Scala media)

Ductus reuniens

}

The signals are turned into nerve-cell electrical activity by mechanoreception for sensing fluid movement

EAR, HEARING & BALANCE

In the inner ear are the organs for the senses of hearing and balance - the cochlea and the vestibular apparatus

The outer and middle ear get airborne sound to the inner ear.

W Beresford

COCHLEAR APPARATUS I

The cochlear duct inside contains endolymph , with a special ionic composition to allow for efficient depolarization, when the hair-cell stereocilia are deflected.

Spaces form in the skull’s temporal bone on each side for three differently oriented CANALS communicating with a larger space - VESTIBULE - to hold a system of fluid-filled bags & tubes

The deflection arises from membrane deflections, ultimately derived from air vibrations outside the head

Also, coming off the vestibule is the snail-like bony cochlea with 21/2 turns

Also, within the bone, spaces must be found for the air vibrations to be conveyed to the cochlea; while air pressure has to be equilibrated across the ear drum

The cochlea has to have its own coiled space in the bone

We have seen that: the semicircular ducts require three canals in each temporal bone; the utricle and ampullae, & the saccule, need a vestibule in the bone; and the vestibular ganglion & nerve need a passageway (meatus) to reach the brainstem. (Other nerves pass by.)

TEMPORAL BONY SPACES

Finally, passages (aqueducts) are needed to keep the three fluids - perilymph, endolymph, & CSF - in balance

The intricate result is best depicted initially as a diagram for learning parts and relations, but first a more anatomical overview of the whole system

COCHLEA II

BRAIN

COCHLEA

UTRICLE

SACCULE

Note the TWO chambers for perilymph with the Scala media in between

Two chambers connect

COCHLEAR DUCT or Scala media

Ductus reuniens

COCHLEAR DUCT or Scala media

BONE

Basilar membrane

Reissner’smembrane

ORGAN of CORTI

Scala vestibuli

Scala tympani

BONE

COCHLEA III One turn - Compartments

PERILYMPH

PERILYMPH

Osseous spiral lamina

COCHLEA IV Bony Modiolus

HELICOTREMA where Scalae vestibuli & tympani connect

Scala vestibuli

Scala tympani

COCHLEAR DUCT or Scala media

MOD I OLUS

The cochlea spirals around a bony core - the Modiolus

Note that although, in a section, we see five profiles, the structures spiral continously e.g.,

OSSEOUS SPIRAL LAMINA

COCHLEA IV Spiral ganglion & Modiolus

The modiolus is very spongy bone , filled with nerve fibers becoming the cochlear nerve

ORGAN of CORTI

SPIRAL GANGLION

Also, the VIIIth nerve has incoming efferent fibers to influence the outer hair cells in the Organ of Corti

’efferent’ - from brain-stem neurons

Axons to Inner hair cells derive from spiral- ganglion cell bodies

SPIRAL LIGAMENT

BONE

Basilar membrane

Scala vestibuli

Scala tympani

BONE

COCHLEA VI Basilar membrane I

PERILYMPH

PERILYMPH

Osseous spiral lamina

STRIA VASCULARIS

The basilar membrane is tensed between the osseous spiral lamina & the spiral ligament

makes endolymph

BONE

Basilar membrane

ORGAN of CORTI

Scala vestibuli

Scala tympani

BONE

PERILYMPH

PERILYMPH

COCHLEA VII Basilar membrane II

It vibrates well to low frequency sounds at its apex

Vibrations from oval window of vestibule

The basilar membrane is vibrated by fluid pressures in the Scala typani

The spiralling hides that the basilar membrane is LONG

Its WIDTH & STIFFNESS alter regularly along its length, so that

The high-frequency response is at the base

COCHLEAR DUCT or Scala media

The particular component frequencies of a ‘sound’ produce a pattern of vibrations along the basilar membrane,

detectable by the inner hair cells attached to the active regions of the

Scala tympani

Basilar membrane

INNER HAIR CELL

TECTORIAL MEMBRANE with attached

ENDOLYMPH

innervated by axon from spiral-ganglion neuron

Tectorial membrane & Inner Hair Cell

SPIRAL LIMBUS

Support for Reissner’s membrane & Tectorial membrane

TECTORIAL MEMBRANE is gelatinous, like the cupula, but is attached at one side, aside from its hair-cell connections

Organ of Corti - cell typesCrista & Macula -- “Electron microscopy also revealed that the ‘supporting’ cells were of various different kinds”. Far more true for the Organ of Corti, and detectable already in the 19th century, hence some eponyms

Basilar membrane

OUTER HAIR CELLSTECTORIAL MEMBRANE

INNER & OUTER PILLAR CELLS

SPIRAL LIMBUSINNER HAIR CELL

INNER & OUTER PHALANGEAL CELLS DEITER’S

TECTORIAL CELLS

HENSEN & CLAUDIUS CELLS

Stria vascularis & K+ recycling I

Basilar membrane

OUTER HAIR CELLS

INNER & OUTER PILLAR CELLS

INNER HAIR CELL

OUTER PHALANGEAL CELLS DEITER’S

HENSEN & CLAUDIUS CELLS

FIBROBLASTS

STRIA CELLS

K+

The Kcc4 channel gets the K+ into the Deiter’s cells, whence it goes via gap junctions to theStria for pumping into the endolymph

Stria vascularis II

The Stria vascularis was so named because, quite unusually, capillaries are found amongst the three kind of epithelial cells

Basilar membrane

HENSEN & CLAUDIUS CELLS

STRIA CELLS

Also, within the bone, spaces must be found for the air vibrations to be conveyed to the cochlea; while air pressure has to be equilibrated across the ear drum

The cochlea has to have its own coiled space in the bone

We’ll return to the schematic of the whole auditory system for:

SOUND CONDUCTION TO THE INNER EAR

The membrane-sealed openings - oval & round windows - from vestibule to middle ear, allowing transmission of pressures, but keeping in the perilymph

The tympanic membrane (ear drum) separating outer auditory meatus from the middle ear

CRANIAL CAVITY

EUSTACHIAN

TUBE - OPEN

LIMITS

VESTIBULE

SEMICIRCULAR CANALS

MIDDLE EAR

COCHLEA

EXTERNAL CANAL

- OPEN

MASTOID AIR CELLS TEMPORAL BONE

AQUEDUCTS

OVAL WINDOW

ROUND WINDOW EAR DRUM

CRANIAL CAVITY

EUSTACHIAN TUBE

VESTIBULE

SEMICIRCULAR CANALS

MIDDLE EAR

COCHLEA

EXTERNAL CANAL

MASTOID AIR CELLS TEMPORAL BONE

LININGS

MENINGES

PERIOSTEUM

‘SKIN’‘AIRWAY’MUCOSA

AQUEDUCTSPERIOSTEUM

CRANIAL CAVITY

EUSTACHIAN TUBE

VESTIBULE SEMICIRCULAR CANALS

MIDDLE EAR

COCHLEA

EXTERNAL AUDITORY CANAL

MASTOID AIR CELLS

OTIC DUCT

LININGS of BONY SPACES

MENINGES

PERIOSTEUM

‘SKIN’

‘AIRWAY’MUCOSA

EUSTACHIAN TUBE

VESTIBULE

SEMICIRCULAR CANALS

COCHLEA

EXTERNAL CANAL

MASTOID AIR CELLS

AQUEDUCTS

EAR DRUM

OVAL WINDOW

ROUND WINDOW

MIDDLE EAR

AUDITORYOSSICLES

STAPES

MALLEUSINCUS

MALLEUS

INCUS

STAPES

AUDITORY OSSICLES II

MALLEUS

INCUS

STAPES

EXTERNAL CANAL EAR DRUM

OVAL WINDOW

ROUND WINDOW

MIDDLE EAR

MALLEUSINCUS

STAPES

The malleus (hammer) is vibrated by air impinging on the tympanic membrane (ear-drum). Malleus movements drive the incus (anvil), which in its turn moves the stapes (stirrup) in and out of the oval window, so pulsating the fluid (perilymph) in the vestibule. The bony chain & geometry amplify the air’s initial force.

VESTIBULE

To relieve fluid pressures in the vestibule

AUDITORY OSSICLES II

MALLEUS

INCUS

STAPES

EXTERNAL CANAL EAR DRUM

OVAL WINDOW

ROUND WINDOW

MIDDLE EAR

MALLEUSINCUS

STAPES

The malleus (hammer) is vibrated by air impinging on the tympanic membrane (ear-drum). Malleus movements drive the incus (anvil), which in its turn moves the stapes (stirrup) in and out of the oval window, so pulsating the fluid (perilymph) in the vestibule. The bony chain & geometry amplify the air’s initial force (& match impedance)

VESTIBULE

To relieve fluid pressures in the vestibule

AUDITORY OSSICLES III

The malleus (hammer) is vibrated by air impinging on the tympanic membrane (ear-drum). Malleus movements drive the incus (anvil), which in its turn moves the stapes (stirrup) in and out of the oval window, so pulsating the fluid (perilymph) in the vestibule. The bony chain & geometry amplify the air’s initial force.

OVAL WINDOW

MALLEUS

INCUS

STAPES

EAR DRUM

Stapedius muscle & Facial nerve

INCUS

STAPES

Tympanic cavity/ Middle ear

VESTIBULE

FACIAL NERVE

Stapedius muscle

Other long spaces in the bone house the Facial nerve &

the Stapedius muscle

whose contraction hinders the movement of the so

protecting the ear from loud sounds

along with Tensor tympani‘s action (Next slide)The two responses constitute Sound attenuation reflex

Oval window

COCHLEA

VESTIBULE

AURICLE

EAR CANAL

MIDDLE EAR

TENSOR TYMPANI

Auditory TUBE

Tensor tympani muscle TT tendon

Malleus

Tensor tympani muscle has its bony tunnel parallel to Eustachian tube’s

TT contraction limits Malleus movement for protection from loud sounds

V th

NERVE

EAR PATHOLOGY

FACIAL NERVE

VIIIth NERVE

COCHLEA

V

AURICLE

EAR CANAL

MIDDLE EAR

Auditory/ Eustachian TUBE

Nasopharynx

CARTILAGE

EAR DRUM

Angle tumor -Neuroma of VIIIth N - bad balance /hearing

Lost Hair cells - loud noises, age, streptomycin, neomycin, cisplatin

Blocked tube Perforated ear-drum -infection, blast injury

Excess endolymph - hydrops

Otitis media - middle ear infection; Cholesteatoma - kerat strat squam ep

Ankylosed ossicles

Wax, foreign bodies in canal

Meningitis, abscess

Overgrowth of bone - Otosclerosis

EAR PATHOLOGY II

Lost/damaged Hair cells from - loud noises, age; ototoxic agents - streptomycin, neomycin (aminoglycoside antibiotics), cisplatin (anticancer agent)

Congenital deafness - One of a number of defects in genes can impair the development of the inner ear, or the differentiation and functioning of hair cells

Hypothyroidism and iodine deficiency in pregnancy can result in defective development of the fetus’ Organ of Corti

EAR PATHOLOGY III

Angle tumor -Neuroma of VIIIth N - bad balance /hearing

Lost Hair cells - loud noises, age, streptomycin,

Blocked Eustachian tube

Perforated ear-drum -infection, blast

Excess endolymph - hydrops

Otitis media - middle ear infection

Ankylosed ossicles

Wax, foreign bodies in canal

Meningitis, abscess

Overgrowth of bone - Otosclerosis

Congenital deafness - defects in genes