the ear: hearing and balance

Copyright © 2010 Pearson Education, Inc.

THE EAR: HEARING AND BALANCE


The Ear: Hearing and Balance

• Three parts of the ear

• The External (outer) Ear

• Pinna- Composed of the Helix (rim) and lobule (earlobe)

• External acoustic canal (meatus)-short, curved tube leading to eardrum

• Lined with ceruminous glands

• Tympanic membrane- eardrum; CT boundary btwn. external and middle ear; vibrates in response to sound

Copyright © 2010 Pearson Education, Inc. Figure 15.25a

Externalacousticmeatus

Auricle(pinna)

(a) The three regions of the ear

Helix

Lobule

Pharyngotympanic(auditory) tube

Tympanicmembrane

Externalear

Middleear

Internal ear(labyrinth)


Middle Ear

• The Middle Ear (tympanic cavity)

• A small, air-filled, mucosa-lined cavity in the temporal bone; flanked laterally by the eardrum and medially by the oval and round windows

• Three bones (ossicles)- Incus, Malleus,Stapes


Middle Ear

• Two tiny skeletal muscles prevent damage due to large vibrations

• Tensor tympani-arises from wall of pharyngotympanic tube and inserts on the malleus

• Stapedius muscle-runs from posterior wall of middle ear to the stapes


• Pharyngotympanic (auditory) tubes

• Connects middle ear to nasopharynx

• Normally flattened closed; opens when yawning or swallowing

• Equalizes pressure in the middle ear cavity with the external air pressure

Middle Ear

Copyright © 2010 Pearson Education, Inc. Figure 15.26

Pharyngotym-panic tube

Tensortympanimuscle

Tympanicmembrane(medial view)

Stapes

Malleus

View

Superior

Anterior

Lateral

IncusEpitympanic

recess

Stapediusmuscle


• Consists of the bony (osseous) and membranous labyrinth

• Bony labyrinth

• Tortuous channels in the temporal bone

• Filled with perilymph

• Three regions:

• Vestibule-

• Contains two sacs

• Saccule-continuous w/ the cochlear duct

• Utricle-continuous w/ the semicircular ducts

• Cochlea

• Semicircular canals

Inner Ear


The Inner Ear

• Membranous Labyrinth

• Series of membranous sacs and ducts contained w/in the bony labyrinth

• Follows contours of bony labyrinth

• Filled with endolymph


Anterior

Semicircularducts insemicircularcanals

PosteriorLateral

Cristae ampullaresin the membranousampullae

Utricle investibule

Saccule investibule Stapes in

oval window

Temporalbone

Facial nerve

Vestibularnerve

Superior vestibular ganglion

Inferior vestibular ganglion

Cochlearnerve

Maculae

Spiral organ(of Corti)Cochlearductin cochlea

Roundwindow


The Maculae and Static Equilibrium• Maculae-

• Sensory receptors for static equilibrium (monitor the position of head in space, respond to linear acceleration)

• One in each saccule wall and one in each utricle wall

• Each maculae contains:

• ET hair cells and supporting cells

• Otolithic membrane- gel like membrane that overlies hair cells

• Otoliths- calcium carbonate crystals; increase weight and its inertia


Macula ofsaccule

Otoliths

Hair bundle

Kinocilium

StereociliaOtolithicmembrane

Vestibularnerve fibers

Hair cells

Supportingcells

Macula ofutricle


Steps of linear movement

• Hair cells are always releasing neurotransmitter

• When hair cells bend towards kinocilium they depolarize, neurotransmitter release is increased

• When hair cells bend away from kinocilium they hyperpolarize, slowing release of neurotransmitter

• This change in neurotransmitter release informs the brain of the changing of the position of the head in space


Otolithic membrane

Kinocilium

Stereocilia

ReceptorpotentialNerve impulsesgenerated investibular fiber

When hairs bend towardthe kinocilium, the hair cell depolarizes, exciting the nerve fiber, which generates more frequent action potentials.

When hairs bend awayfrom the kinocilium, the hair cell hyperpolarizes, inhibiting the nerve fiber, and decreasing the action potential frequency.

DepolarizationHyperpolarization


The Crista Ampullaris and Dynamic Equilibrium

• Dynamic Equilibrium

• Detected by:

• Crista Ampullaris –

• One in the ampulla of each semicircular canal

• Major stimuli are rotatory movements



• Semicircular Canals

• Three canals are located in each ear:

• Located in all three planes of space

• Anterior, posterior and lateral

• Endolymph-fills the semicircular ducts

• Ampulla- swellling at end of semicircular duct

• Crista Ampullaris

• Composed of hair cells and supporting cells

• Structure and function of the crista ampullaris is basically the same as the hair cells of cochlea and maculae

• Cupula – gelled mass that cilia of hair cells are embedded in

Copyright © 2010 Pearson Education, Inc. Figure 15.36a–b

Fibers of vestibular nerve

Hair bundle (kinociliumplus stereocilia)

Hair cell

Supportingcell

Membranouslabyrinth

Cristaampullaris

Cristaampullaris

Endolymph

Cupula

Cupula

(a) Anatomy of a crista ampullaris in a semicircular canal

(b) Scanning electron micrograph of a crista ampullaris (200x)


• Steps of Rotational Movement

• At rest the cupula stands upright

• During rotational acceleration, hair cells are bent, they depolarize and impulses reach the brain faster

• As movement slows, endolymph keeps moving, cilia are bent in opposite direction causing hyperpolarization and reduction of impulses to brain


Copyright © 2010 Pearson Education, Inc. Figure 15.36c

Fibers ofvestibularnerve

At rest, the cupula standsupright.

Section ofampulla,filled withendolymph

(c) Movement of the cupula during rotational acceleration and deceleration

Cupula Flow of endolymph

During rotational acceleration,endolymph moves inside thesemicircular canals in thedirection opposite the rotation(it lags behind due to inertia).Endolymph flow bends thecupula and excites the haircells.

As rotational movementslows, endolymph keepsmoving in the directionof the rotation, bendingthe cupula in theopposite direction fromacceleration andinhibiting the hair cells.


Equilibrium Pathway to the Brain

• Vestibular nerve-Impulses travel to the vestibular nuclei in the brain stem or the cerebellum

• Pathways are complex and poorly traced


Sound and the Cochlea

• Sound is detected by: the cochlea

• Cochlea

• The cochlea is A spiral, conical, bony chamber and contains the cochlear duct

• Cochlear duct- houses the spiral organ (of Corti)

• Divides cochlea into three chambers:

• Scala vestibuli-superior to cochlear duct (contains perilymph)

• Scala tympani-inferior to cochlear duct; terminates at round window (contains perilymph)

• Scala media (cochlear duct) -middle cavity; (contains endolymph)


(a) Helicotrema

Modiolus Cochlear nerve,division of thevestibulocochlearnerve (VIII)

Cochlear duct(scala media)

Spiral ganglion

Osseous spiral laminaVestibular membrane

Copyright © 2010 Pearson Education, Inc. Figure 15.28b

(b)

Cochlear duct(scala media;containsendolymph)

Tectorial membrane

Vestibular membrane

Scalavestibuli(containsperilymph)

Scala tympani(containsperilymph)

Basilarmembrane

Spiral organ(of Corti)

Spiralganglion

Osseous spiral lamina


• Oval window-an opening on the medial wall of the middle ear (foot of stapes rests at oval window)

• Round window-an opening on the medial wall of the middle ear (scala tympani terminates at round window)

• Vestibular membrane-roof of cochlear duct that separates the scala media from scala vestibuli

• Basilar membrane- fibrous floor of cochlear duct



• Organ of Corti

• Runs through center of cochlea

• Has hair cells and supporting cells

• Tectorial membrane- gel-like mass that cilia of hair cells are embedded in

• Bending of the cilia: excites hair cells


Copyright © 2010 Pearson Education, Inc. Figure 15.28c

(c)

Tectorial membrane Inner hair cell

Outer hair cells

Hairs (stereocilia) Afferent nervefibers

Basilarmembrane

Fibers ofcochlearnerve

Supporting cells


Sound Transmission

• Transmission of Sound to the Inner Ear

• Sound waves enter the external acoustic canal and cause tympanic membrane to vibrate

• Ossicles vibrate and amplify the pressure at the oval window

• Pressure waves move through perilymph of the scala vestibuli

• Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane


Scala tympani

Cochlear duct

Basilarmembrane

1 Sound waves vibratethe tympanic membrane. 2 Auditory ossicles vibrate.

Pressure is amplified.

3 Pressure waves created bythe stapes pushing on the oval window move through fluid in the scala vestibuli.

Sounds with frequenciesbelow hearing travel through the helicotrema and do not excite hair cells.

Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.

Malleus Incus

Auditory ossicles

Stapes

Ovalwindow

Scala vestibuli

Helicotrema

Cochlear nerve

32

1

Roundwindow

Tympanicmembrane

(a) Route of sound waves through the ear


• Resonance of the Basilar Membrane - fibers of the basilar membrane are “tuned” to a particular sound frequency

• Vibrations of the basilar membrane causes cilia of hair cells to bend

• Bending cilia towards kinocilium excites hair cells (increase neurotransmitter release)

• Bending cilia away from kinocilium inhibits hair cells (slow release of neurotransmitter)

Sound Transmission


• Impulses from the cochlea pass via the spiral ganglion to the cochlear nuclei of the medulla

• Eventually impulses are sent to the primary auditory cortex (temporal lobe)

Sound Transmission


Medial geniculatenucleus of thalamus

Primary auditorycortex in temporal lobeInferior colliculus

Spiral organ (of Corti)

Spiral ganglion of cochlear nerve

Vestibulocochlear nerve

Medulla

Midbrain

Cochlear nuclei

Vibrations

Vibrations


Deafness

• Hearing loss can be temporary or permanent

• Common causes:

• Middle ear infections

• Conduction deafness

• Can be caused by:

• Impacted earwax

• Ruptured eardrum

• Middle ear inflammations

• Otosclerosis


• Nerve Deafness

• Can be caused by:

• Gradual loss of hair cells throughout life

• Single explosively loud noise

• Prolonged exposure to loud noise

• Degeneration of cochlear nerve, tumors in auditory cortex, etc.

Deafness


Tinnitus

• Ringing or clicking sound in ears in the absence of auditory stimuli

• One of the first symptoms of cochlear degeneration

• Can be caused by middle ear inflammation


Meniere’s Syndrome

• Labyrinth disorder

• Affects all three parts of the internal ear

• Symptoms are repeated attacks of vertigo, nausea and vomiting

• Balance is severely disturbed and hearing is ultimately lost

the ear: hearing and balance

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