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Auditory system
Organ of hearing
Auditory systems is an engineering
masterpiece
Size of a pea
Detects vibrations as small as the size of an
atom
Responds 1000 times faster than the visual
photoreceptors
Much of human communications is mediated
by the auditory system, including music
Role of Audition in Behavior
Language
Localization of a sound source in the absence of
visual cues
Species-specific vocalizations
Prosody – emotional content of language
Appreciation of music
Periodic condensation and rarefaction of air
molecules are detected by the auditory system
Complex sounds can be broken down into the
fundamental frequency components
Sound = pressure waves generated by
vibrating air molecules
Waveform (amplitude/time)
Phase
Amplitude (dB)
Frequency (Hz) – spectrum 20 Hz to 20 kHz
Flow of information
External and middle ears collect sound waves and
amplify their pressure
Sound energy in the air is transmitted to the fluidfilled cochlea of the inner ear
In inner ear a series of biomechanical processes
break up the signal into simpler sinusoidal
components
Frequency, amplitude, phase of the original signal
are all faithfully transduced by the sensory cells
and encoded by the electrical activity of nerves
Several levels of central processing in CNS
Major components of the ear
External (outer) ear, auris externa
Middle ear, auris media
auricle (pinna)
external acoustic meatus (ear canal)
tympanic membrane (ear drum)
tympanic cavity
auditory (eustachian) tube
auditory ossicles
Internal (inner) ear, auris interna - auditory and
vestibular portions
osseous labyrinth
membranous labyrinth
Ear overview – 3 components
Major components of the ear (Lat. auris)
External ear – auricle (pinna)
Tuberculum
Tragus
Antihelix
Helix
Antitragus
Lobulus
Thin skin with fine hairs
Elastic fibrocartilage
Lobule of auricle
Auricular tubercle (of
Darwin)
Ligaments of auricle extrinsic (connect the auricle
with the temporal bone) and
intrinsic (connect individual
auricular cartilages)
External ear – muscles
M. auricularis sup.
M. helicis major
M. auricularis ant.
M. helicis minor
M.antitragicus
M.tragicus
Extrinsic auricular
muscles - auriculares
anterior, superior and
posterior
Intrinsic auricular
muscles - helicis major
and minor, tragicus,
antitragicus,
transversus auriculae
and obliquus auriculae
Innervation - CN VII
External ear – blood supply
External ear – sensory innervation
External ear – external acoustic meatus
Cartilaginous part –
outer (~8 mm long),
Osseous part – inner
(~16 mm long),
Thin skin; the thicker
cerumen-producing
ear canal
Skin has fine hairs,
tragi
Sebaceous glands in
the hair follicles
Ceruminous glands
ear wax, cerumen
Conductive hearing loss
Occlusion of the auditory
canal by cerumen
The tip of a water-filled syringe is
placed just inside the ear canal, and
a stream of warm water is instilled
into the canal to remove earwax.
Middle ear
Function - sound
conduction
apparatus
Components
tympanic membrane
tympanic cavity
auditory (eustachian)
tube
auditory ossicles
Tympanic membrane
Separates the tympanic cavity from
the external acoustic meatus
Diameter 10-11 mm longest/8-9
mm shortest
Parts
pars flaccida (Schrapnell’s membrane)
pars tensa → umbo
Layers
cuticular layer - continuous with the
thin skin of the meatus (keratinized)
fibrous layer – absent in pars flaccida
radiate fibers
circular fibers
mucous layer - part of the mucosa of
the tympanic cavity (single layer of
flat cells, no ciliated cells
Otoscopic
appearance of
the right
tympanic
membrane
Tympanic membrane
Otoscope
The inner surface of the membrane is convex and the point of greatest convexity is termed the umbo
Conductive hearing loss
Damage to the tympanic
membrane
perforation of the tympanic
membrane
this ear drum has a 20%
perforation in the posterior
portion of this right ear drum
this hole would usually be
repaired with a graft placed
under the ear drum...an
operation called a
tympanoplasty
Mainly innervated by the auriculotemporal nerve, and appears to perceive only pain
Conductive hearing loss
Otitis media
inflammation of the middle ear
cleft
immobile tympanic membrane,
which can be dull, opaque, red,
bulging, or even show pus
through it
a demonstrable conductive
hearing loss
Tympanic cavity – 1.5 cm3, air-filled
Epitympanic recess (the
attic) - above the level of the
membrane, which contains
the upper half of the malleus
and most of the incus
Tympanic cavity proper
(mesotympanum) opposite the tympanic
membrane
Hypotympanum
(hypotympanic recess) –
connected to the eustachian
tube
Tympanic cavity – 6 walls
Lateral wall – paries membranaceus
Medial wall – paries labyrinthicus
Superior wall, roof – paries tegmentalis
tegmen tympani → otogenic meningitis
Inferior wall, floor – paries jugularis
membrana tympani et recessus epitympanicus
canaliculus tympanicus
Anterior wall – paries caroticus
Posterior wall – paries mastoideus
antrum mastoideum
Tympanic cavity
Course of facial nerve in the tympanic cavity
medial
Chorda tympani
lateral
Course of chorda tympani nerve
Brain is in proximity to the cells of the
temporal bone → infection propagation
Auditory (Eustachian, pharyngotympanic) tube
Connects middle ear with
nasopharynx
Ventilates middle ear and air
cells
Equalizes pressure between
middle ear & atmosphere
Drains middle ear spaces
Creates a barrier to
ascending infection (more
horizontal and shorter in
infants)
cartilaginous part - – 24 mm
bony part - – 12 mm
Middle ear - ossicles
Malleus (Lat., hammer) - the
largest, 8-9 mm long
Incus (Lat., anvil)
head, caput mallei
neck, collum mallei
handle, manubrium mallei
anterior and lateral processes
body, corpus incudis
long process, crus longum
lenticular process
short process, crus breve
Stapes (Lat., stirrup)
head, caput stapedis
limbs (crura) – anterior & posterior
base, basis stapedis
Middle ear – ligaments & joints
body of incus
long
limb
Conductive hearing loss
Otosclerosis
a bony ankylosis (knee)
knits the bone of the
middle ear to the stapes,
preventing normal
transmission of sound from
the eardrum into the inner
ear
Importance of middle ear for hearing
External ear
airborne environment
Low impedance
Middle Ear
Avoid reflection
Air-fluid boundary
Inner ear
aqueous environment
High impedance
Because of the marked difference in elasticity and density between
air and fluid, almost 99% of acoustic energy is reflected back at the
air-fluid interface between the middle ear and inner ear
Middle ear ensures transmission of the sound energy across the
air–fluid boundary by boosting the pressure measured at the
tympanic membrane almost 200-fold by the time it reaches the
inner ear
Middle ear - muscles
Stapes - stapedius muscle – CN VII
Malleus - tensor tympani muscle – CN V
Reflexively activated by loud noise
Protection against loud noise damage
Stapedius & tensor tympani muscles
tendon of tensor tympani
m. tensor tympani
m. stapedius
tendon of stapedius
Stapedius & tensor tympani muscles
tendon of tensor tympani
tendon of stapedius
The stapedius reflex
Hyperacusis
Definition
abnormal sensitivity to everyday sound levels
or noises, often sensitivity to higher pitched
sounds, in the presence of essentially normal
hearing
Cause
damage to the nerve to the stapedius or the
stapedius muscle, thereby causing faulty
amplification of sounds
Meneire’s
Note:
innervated by
VII, IX, and X
cerumen,
foreign bodies
otitis media,
otosclerosis
Meniere’s disease
Inner ear
Located within the petrous portion of the temporal
bone
Contains 2 systems of canals or cavities
osseous (bony) labyrinth - protective “capsule”
membranous labyrinth (within bony) - “working” part
Location of labyrinth within the petrous portion of the temporal bone
anterior
medial
lateral
posterior
Bony vs membranous labyrinth
Bony labyrinth
Membranous labyrinth
Semicircular canals
Semicircular ducts
Vestibulum
Sacculus (anterior cavity) → cochlear duct
Utriculus (posterior cavity) → semicircular ducts
Cochlea
Cochlear duct
Perilymph
Endolymph
bony
Components of the labyrinth
membranous
sensory
Perilymph vs endolymph
Bony labyrinth – perilymph
similar in ionic composition to cerebrospinal fluid and
extracellular fluid of other tissues, but contains little protein
emerges from the microvasculature of the periosteum and is
drained by a perilymphatic duct into the adjoining
subarachnoid space
suspends and supports the closed membranous labyrinth,
protecting it from the hard wall of the bony labyrinth
Membranous labyrinth – endolymph
high K+ (150 mM) and low Na+ (16 mM) similar to
intracellular fluid
generated largely by capillaries in the stria vascularis in the
wall of the cochlear duct
drained from the vestibule into venous sinuses of the dura
mater by the small endolymphatic duct
Inner ear fluids
Na+
K+
mEq/l
mEq/l
MM
g/l
Perilymph
140
5
0.68
200
Endolymph
5
150 0.023
30
Ca++ Protein
Plasma
145
4
2.6
4300
CSF
140
3
1.2
30
The membranous labyrinth includes the sense
organs for equilibrium & hearing
Organs for the sense of equilibrium and balance
Saccule → macula
Utricle → macula
Semicircular ducts → crista ampullaris in ampulla
Organ for the sense of hearing
Cochlea
Liquor of inner ear
Endolymphatic duct
Perilymphatic duct
Components of the labyrinth
Cochlea (L., snail)
35 mm in length
Makes 2⅓ two-and-one-half turns around a
bony core – modiolus
Modiolus contains blood vessels and surrounds
the cell bodies and processes of the acoustic
branch of CN VIII in the cochlear (spiral)
ganglion
Modiolus (Lat.,
Modiolus & cochlear nerve
Spaces of Cochlea
Scala vestibuli - perilymph
Scala media (cochlear duct)
- endolymph
Scala tympani - perilymph
Scala vestibuli and scala tympani
are in reality one long tube,
beginning at the oval window
and ending at the round window.
They communicate at the apex
of the cochlea via an opening
known as the helicotrema.
2 membranes separate scala media from the
other compartments
(vestibular)
basement membrane
with simple squamous
epithelium on each side
Organ of hearing
thick basal lamina
zona arcuata
zona pectinata
Although the cochlea narrows from base to apex, the basilar membrane
widens toward the apex. The helicotrema is the continuity between the
scala vestibuli and the scala tympani.
Cross-section through cochlea
= Scala media
Stria vascularis
Epithelium responsible for
production and maintenance
of the endolymph for the
entire membranous labyrinth
Encloses a network of
capillaries
Cells with many deep basal
infoldings of their plasma
membranes → numerous
mitochondria
Fluid and K+ ions pumped
from the capillaries by the
epithelial cells are released in
the cochlear duct
Reissner’s (vestibular membrane)
Stria vascularis
Spiral prominence
Stria vascularis & disease
CONGENITAL
HEARING LOSS
Derived from ectoderm,
neural crest cells and
mesenchyme
Failure of NC migration
results in absence of
endolymph production
Hair cells don’t survive
Spiral organ (organ of Corti)
Organ of Corti = sensory + supporting cells
(~150 µm wide)
The site of mechanochemical transduction
Hair cells
~16,000/cochlea
Kandel, Schwartz, Jessell; Principles of Neural Science, 4th ed.
Hair cells – auditory receptor cells
Outer hair cells (OHC) – in 3 rows
Inner hair cells (IHC) – in 1 row
Hair cells – 1 Kinocilium + 30
stereocilia
K
Functions of kinocilia are unclear, they disappear
shortly after birth in mammals
Kandel, Schwartz, Jessell; Principles of Neural Science, 4th ed.
Links between stereocilia
Current Opinion in Neurobiology Volume 12, Issue 4 , 1 August 2002, Pages 380-386
Stereocilia contain actin filaments
Supporting cells of the organ of Corti
Pillar cells - filled with
tonofibrils; converge to form
the fluid-filled tunnel of Corti
Phalangeal (Deiters') cells –
support IHC & OHC
Hensen’s cells – columnar
cells, outer to the outer
phalangeal cells; outer border
of the organ of Corti
Border cells (inner side) &
Claudius cells (outer side) –
form external borders of the
organ of Corti
Tectorial membrane
Gelatinous structure with embedded filamentous elements. It extends over the
free surface of the organ of Corti. The hairs of the hair cells are attached to it.
Relations between phalangeal & hair cells
Outer phalangeal cells have processes → OHC
Inner phalangeal cells do not have processes → IHC
Bony
cochlea
vestibulum
H – helicotrema
M – modiolus
SG – spiral ganglion
VN – n.vestibulocochlearis
FN – facial nerve
OC – organ of Corti
SV – scala vestibuli
ST – scala tympani
CD – cochlear duct
AO – auditory ossicle
А – ampulla of a
semicircular canal
CA – crista ampullaris
Bony
cochlea
limbus
spiralis
scala
vestibuli
tectorial
membrane
vestibular
membrane
cochlear
duct
stria
vascularis
Lamina spiralis
ossea
ganglion
spirale
Organ of Corti
prominentia
spiralis
Fibers of
cochlear
nerve
scala
tympani
basilar
membrane
Lig. spirale
epithelium
Bony
cochlea
Organ of Corti
sulcus
spiralis
ext.
cuniculus
Cells of
Claudius
Cells of
Hansen
Outer hair medius
cells
(Nuel)
sulcus
spiralis
int.
tectorial
membrane
Inner hair
cell
Border
cell
cuniculus
ext.
limbus
spiralis
Internal
sulcus cells
cuniculus
med.
(Nuel)
Fibers of
cochlear
nerve
Tunnel of
Corti
Lig. spirale
Cells of
Boettcher
basilar
membrane
Outer
Outer
pillar
phalangeal cell
cells
(Deiters)
Inner
pillar
cell
Inner
phalangeal
cells
Lamina
spiralis
ossea
Basilar membrane
is displaced by
energy (sound)
waves from the
stapes
Basilar membrane
Basilar membrane displacement is converted
into electrical signals by the organ of Corti
Mechanoelectrical transduction
Changes in sound waves
Basilar membrane vibrations
Ion Channels open
Depolarization
Action potential
Mechanoelectrical transduction & hair cells
Drug-induced damage to hair cells
Some
antibiotics damage OHCs
Hearing abnormalities produced by
antibiotics is a consequence of
damage to the cochlear amplifier
Hair cells are innervated by the spiral ganglion
30,000 bipolar neurons
Receptors of sensory systems - primary
sensory neurons
pseudounipolar
part of PNS
(except jaw
proprioception)
bipolar
part of CNS
Nerve supply to hair cells
90% of spiral ganglion
neurons (type I cells)
innervate IHC
10% of spiral ganglion
neurons (type II cells)
innervate OHC
each fiber diverges to
innervate many OHC
each IHC receives
contacts from about 10
fibers; each fiber contacts
only 1 IHC
Efferent fibers to hair cells originate in
the contralateral superior olive in the
pons and and terminate on OHC and
the afferent terminal boutons
innervating IHC. Efferent fibers have
an inhibitory effect on auditory stimuli.
Types of hearing loss
Conductive hearing loss - various problems in
the middle ear which can reduce conduction of
vibrations by the chain of ossicles from the
tympanic membrane to the oval window
Sensorineural hearing loss – is due to defects
in any structure or cell from the cochlea to
auditory centers of the brain, but commonly
involves loss of hair cells or nerve degeneration
(can be congenital or acquired)
Patients with severe sensorineural deafness
may be helped by cochlear implants
prosthetic electrode array
Basilar membrane is the mechanical analyzer
of sound - tonotopy
A traveling wave is shown at a given instant along the cochlea, which has been uncoiled.
The graphs profile the amplitude of the traveling wave along the basilar membrane for
different frequencies and show that the position where the traveling wave reaching its
maximum amplitude varies directly with the frequency of stimulation.
Auditory system
Spiral Ganglion
Cranial Nerve VIII
Dorsal Cochlear
Nuc
Cochlear Nucleus
Superior Olivary Nuc
Lateral Lemniscus
Ventral
Cochlear Nuc
Origin:
Course:
Termination:
Laterality:
Spiral ganglion
CNVIII, auditory pathway, auditory cortex
Auditory Cortex (Heschl’s gyrus) BA 41,42
Bilateral
Inferior Colliculus
Medial Geniculate
Auditory Cortex
Auditory system
Superior Olivary
Nuc
Lateral
Lemniscus
Origin:
Course:
Termination:
Laterality:
Spiral ganglion
CNVIII, auditory pathway, auditory cortex
Auditory Cortex (Heschl’s gyrus) BA 41,42
Bilateral
Superior olivary complex → sound localization
MSO = medial superior olive
Purves, et al, Neuroscience, 3rd ed.
Auditory system
Lateral
Lemniscus
Origin:
Course:
Termination:
Laterality:
Spiral ganglion
CNVIII, auditory pathway, auditory cortex
Auditory Cortex (Heschl’s gyrus) BA 41,42
Bilateral
Auditory system
Inferior
Colliculus
Lateral
Lemniscus
Origin:
Course:
Termination:
Laterality:
Spiral ganglion
CNVIII, auditory pathway, auditory cortex
Auditory Cortex (Heschl’s gyrus) BA 41,42
Bilateral
Auditory system
Brachium of
Inferior
Colliculus
Origin:
Course:
Termination:
Laterality:
Spiral ganglion
CNVIII, auditory pathway, auditory cortex
Auditory Cortex (Heschl’s gyrus) BA 41,42
Bilateral
Auditory system
Medial
Geniculate
Nucleus
Origin:
Course:
Termination:
Laterality:
Spiral ganglion
CNVIII, auditory pathway, auditory cortex
Auditory Cortex (Heschl’s gyrus) BA 41,42
Bilateral
Auditory system – area A1 (A-I)
Transverse gyri of Heschl
Central auditory
pathways bilaterality
Purves, et al, Neuroscience,
3rd ed.
Auditory system - overview
Modality:
Modality: Auditory
AuditorySensation
Sensation(Hearing)
(Hearing)
Receptor:
Receptor:Organ
Organof
ofCorti
Cortiof
ofCochlear
CochlearDuct
Duct
Cranial
-cochlear
CranialNerve:
Nerve:VIII
VIII-cochlear
1st
1st Neuron:
Neuron:Spiral
SpiralGanglion
Ganglion
2nd
2ndNeuron:
Neuron:Cochlear
CochlearNucleus,
Nucleus,Ventral
Ventral&&Dorsal
Dorsal
dorsal,
dorsal,intermediate
intermediateand
andventral
ventralacoustic
acousticstriae
striae
trapezoid
trapezoidbody
body
lateral
laterallemniscus
lemniscus
3rd
3rd Neuron:
Neuron:Inferior
InferiorColliculus
Colliculus
brachium
brachiumof
ofinferior
inferiorcolliculus
colliculus
4th
4th Neuron:
Neuron:Medial
MedialGeniculate
GeniculateNucleus
Nucleus(MG)
(MG)
auditory
auditoryradiation
radiation
Termination:
Termination:Primary
PrimaryAuditory
AuditoryArea
Area(A
(AI)I)
Brodmann
Brodmannarea
area41,
41,42
42
Auditory Pathways
A. spiral ganglion
B. dorsal and ventral
cochlear nucleus
C. inferior colliculus
D. medial geniculate nucleus
E. Primary auditory area
F. superior olivary nucleus
G. nucleus of
lateral lemniscus
1. acoustic striae &
trapezoid body
2. lateral lemniscus
3. brachium of inferior
colliculus
4. auditory radiation
5. commissure of
inferior colliculus
VIIIc. cochlear division
Auditory pathways – 3D
Primary auditory cortex – A I
The Auditory Cortex
Purves, et al, Neuroscience, 3rd ed.
Tonotopic organization is maintained along
the entire auditory pathway
Tonotopic organization is maintained along
the entire auditory pathway
Purves, et al, Neuroscience, 3rd ed.
Auditory cortex
Tonotopic (cochleotopic) organization
Binaural properties of cortical neurons
Wernicke’s area, next to A1, is for the
comprehension of speech
Probably as we move into auditory association
areas - neurons that respond to complex
sounds and species-specific vocalizations
(Grandma’s voice…)
Lesion localizing
BECAUSE OF HEAVY CALLOSAL
CONNECTIONS, A UNILATERAL LESION OF
THE AUDITORY CORTEX PATHWAYS DISTAL
TO THE COCHLEAR NUCLEI WILL RESULT IN
VIRTUALLY NO LOSS OF HEARING
THE PATIENT WILL MOST LIKELY
EXPERIENCE AN IMPAIRMENT IN THE
ABILITY TO LOCALIZE THE DIRECTION AND
DISTANCE OF SOUNDS
Sound Localization – 2 Cues
Time cues – Low frequency sounds (below 3
KHz) are localized due to interaural time
differences, capitalizing on the phase-locking
abilities of primary afferent fibers. Achieved by the
medial superior olive (MSO).
Intensity cues – High frequency sounds (above 3
KHz) are localized due to interaural intensity
differences. The head creates an acoustic shadow.
Achieved by the lateral superior olive (LSO).
INTERAURAL TIME
DELAY AS A CUE TO
THE LOCATION OF
SOUND
(a) Sound waves coming
from the right side reach
the right ear first, and
there is a large interaural
delay before the sound
propagates to the left ear.
(b) If the sound comes from
straight ahead, there is no
interaural delay. Delays
for three directions are
shown.
Interaural time differences - MSO
Illustration of how the MSO computes the location of a
sound by interaural time differences. A given MSO
neuron responds most strongly when the two inputs
arrive simultaneously, as occurs when the contralateral
and ipsilateral inputs precisely compensate (via their
different lengths) for differences in the time of arrival
of a sound at the two ears. They systematic (and
inverse) variation in the delay lengths of the two inputs
creates a map of sound location: in this model E would
be most sensitive to sound located to the left, and A to
sound from the right; C would respond best to sounds
coming from directly in front of the listener.
Higher-order cortical auditory areas
“Where” pathway
“What” pathway
“What” & “Where” are processed separately
Inner ear development
The rudiments of the
internal ears appear
shortly after those of the
eyes as two patches of
thickened surface
epithelium, the otic
placodes, lateral to the
hindbrain
Carnegie stage 12 – ~day 27
Late 4th – early 5th week
Otic pit
By the time the
anterior neuropore
closes, the first and
second pharyngeal
arches are evident
The regions between
the pharyngeal arches
are termed pharyngeal
clefts. The
indentation just dorsal
to the second
pharyngeal cleft is the
developing inner ear,
the otic pit.
Otic pit
Later ear development
Carnegie stage 21 – ~day 53
Derivatives of the pharyngeal pouches
Some of the neural crest cells in each
of the arches become cartilage
Otic vesicle & pinna development
CRL
External ear development