Transcript Slide 1

21 The Auditory System
Chadi Darwich
April 15 2009
Outline
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Intro
Sound Processing, Deafness
3 parts of the Ear
Weber & Rene Tests
Central Auditory pathways & Vas supply
Brainstem Nuclei
– Lateral Lemniscus and Inferior colliculus
• Auditory Cortex
Intro
• The auditory apparatus is adapted for receiving sound
waves at the tympanic membrane and transmitting
auditory signals to the central nervous system.
• Injury to elements of the peripheral apparatus, such as
the ear ossicles, may result in conductive deafness.
• Damage to the cochlea or the cochlear portion of the
eighth cranial nerve may result in sensorineural (nerve)
deafness.
• Injury to the central auditory pathways cause central
deafness which is usually combined with other signs
and symptoms. Central lesions seldom result in
complete deafness in one ear.
Intro
• The frequency of audible sounds is measured in cycles
per second, or hertz (Hz)
• The normal frequency range for human hearing is 50 to
16,000 Hz.
• Most human speech takes place in the range of 100 to
8,000 Hz, and the most sensitive part of the range is
between 1,000 and 3,000 Hz
• Intensity of sound is related to the perception of
loudness and is usually measured in decibels (dB).
• Intensity is also related to a measure of sound pressure
level at the tympanic membrane. A sound that has 10
times the power of a just-audible sound is said to have a
20-dB sound pressure level.
• Normal conversational levels of sound are about 50 dB.
Processing of Sound
• Sound waves are captured by the external ear
(pinna) and channeled through the external
auditory meatus to the tympanic membrane
• Sounds are transmitted across the middle ear or
tympanic cavity from the tympanic membrane to
the fluid-filled inner ear by a chain of three bony
ossicles: the malleus, incus, and stapes
– Diseases such as otosclerosis and otitis media result
in conductive hearing loss by affecting the efficiency
of the ossicle movement
Conductive Deafness
• Conductive deafness is a deficit related to an obstructed,
or altered, transformation of sound to the tympanic
membrane and/or through the ossicle chain of the middle
ear.
– Damage to the pinna results in a failure of sound waves to be
properly conducted to the auditory meatus.
– Infection involving the auditory canal (otitis externa),
inflammation or trauma to the tympanic membrane, or even the
excessive accumulation of cerumen (wax) in the auditory canal
are other causes of conduction deafness.
• The deficit experienced by the patient may range from
decreased hearing to total deafness in the affected ear.
Inner Ear
• The membranous cochlea, the coiled portion of the inner
ear, is encased in the osseous cochlea and consists of
three spiraling chambers.
• Inner hair cells form a single line spiraling from base to
apex, and the outer hair cells form three parallel lines
that follow the same course. Once damaged, human hair
cells do not regenerate
• The central processes of the spiral ganglion cells form
the cochlear portion of the vestibulocochlear nerve
(cranial nerve VIII).
Sensorineural Deafness
• Results from damage to the cochlea or to the cochlear
root of the vestibulocochlear nerve.
• The causes of sensorineural deafness may include
repeated exposure to loud noises, treatment with certain
antibiotics, infections such as rubella, mumps, or
bacterial meningitis, and tumors at different levels of the
neuraxis.
• Trauma in the form of skull fracture may also result in
sensorineural deafness.
• The deficits experienced by the patient are deafness in
the ear on the affected side, varying degrees of tinnitus if
the cochlea is damaged, and additional signs and
symptoms indicative of damage to the adjacent
vestibular root.
Weber and Rinne Tests
• Air conduction: a vibrating tuning fork,
usually with a 512-Hz frequency, is held
about 2.5 cm from the opening of the
auditory canal. Normally sound waves
generated pass through the external and
middle ears
– Disease or damage in these areas would
result in decreased or lost hearing in this ear
Weber and Rinne Tests
• Bone conduction: The vibrating tuning
fork is placed directly on the skull.
Perceiving these vibrations as sounds
means that the sound (vibration) is
transmitted directly to the cochlea of the
inner ear and bypasses the external ear
and the middle ear
Weber and Rinne Tests
• Rinne test (bone + air conduction)
• The tuning fork is placed against the
mastoid process.
– The normal patient perceives the sound in the
ear on that side by bone conduction
– after the sound is no longer perceived the
tuning fork is immediately moved to the
auditory canal and the sound is again heard
by air conduction.
Weber and Rinne Tests
• Negative Rinne test: The sound is
perceived by bone conduction but not by
air conduction (middle ear
disease/deafness)
• Positive Rinne test the sound is
perceived by air conduction but not bone
conduction (sensorineural deafness
cochlea or cochlear nerve damage).
Weber and Rinne Tests
• Weber test
• The tuning fork is placed on the midline of
the skull or forehead.
• In a patient with normal hearing the sound
(vibration) is perceived about equally in
both ears.
Weber and Rinne Tests
• A patient with sensorineural deafness
(e.g., cochlear damage) would perceive
the sound of the tuning fork in the normal
ear
• A patient with conduction deafness
(canal middle ear obstruction) would
perceive the sound of the tuning fork in the
ear on the side of the damage.
Central Auditory Pathways
• All fibers in the cochlear nerve synapse in
the cochlear nuclei and the cochlear info
ascends to the auditory cortex
• Information is distributed through multiple
parallel pathways that ultimately converge
in the inferior colliculus.
Central Auditory Pathways
• The hierarchy of auditory nuclei involved in
these parallel pathways includes
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cochlear nuclei
nuclei of the superior olive and trapezoid body
nuclei of the lateral lemniscus
inferior colliculus.
• Specific fiber bundles that convey this
information from one level to the next are
– the trapezoid body
– acoustic stria
– lateral lemniscus.
Central Auditory Pathways
• From the midbrain, auditory information is
conveyed:
inferior colliculus
Brachuim
medial geniculate nucleus of the thalamus
through the sublenticular limb of the internal capsule
auditory cortex
Vascular Supply
• The internal auditory (labyrinthine) artery, a branch
AICA, supplies the inner ear and the cochlear nuclei
• Short circumferential branches of the basilar artery
supply the superior olivary complex and lateral
lemniscus
• The superior cerebellar and quadrigeminal arteries
supply the inferior colliculus
• Thalamogeniculate arteries supply the medial
geniculate bodies
• M2 segment of the MCA supply the primary auditory
and association cortices
Cochlear Nuclei
• The posterior cochlear nucleus (dorsal cochlear nucleus)
and the anterior cochlear nucleus (ventral cochlear
nucleus) are located lateral and posterior to the restiform
body and are partially on the surface of the brainstem at
the pontomedullary junction.
• The posterior cochlear nucleus drapes over the restiform
body just inferior to the pontomedullary junction.
• The anterior cochlear nucleus extends rostral to the
posterior cochlear nucleus, where it may be covered by
the flocculus and by caudal fascicles of the middle
cerebellar peduncle.
Cochlear Nuclei
• Cochlear nerve fibers end in the cochlear nuclei
on the ipsilateral side
• They enter the brainstem at the CPA angle, they
divide into ascending and descending bundles.
• Ascending bundle fibers synapse in the anterior
part of the anterior cochlear nucleus
• Descending bundle fibers synapse in the
posterior part of the anterior cochlear nucleus
and in the posterior cochlear nucleus
Cochlear Nuclei
• In the cochlear nuclei, each afferent nerve
fiber makes specialized synaptic contacts
with several different cell types so that the
resulting order produces distinct tonotopic
maps in each division
• The frequency-related lines are organized
so that low frequencies are represented
laterally and high frequencies medially
Cochlear Nuclei
• Specific cell types of the cochlear nuclei, in turn,
give rise to parallel but separate ascending
pathways in the auditory system
• Monaural information is conveyed to the inferior
colliculus
• Binaural processing is conveyed to the superior
olivary complex.
Posterior cochlear nucleus
• Many of the cells in the posterior cochlear
nucleus contribute to complex local circuits
and are not easily correlated with distinct
ascending channels.
• A major output of the posterior cochlear
nucleus is via a direct pyramidal cell
projection to the contralateral inferior
colliculus
Anterior cochlear nucleus
• Anterior cochlear nucleus is distinguished
by the presence of anatomically and
physiologically distinct output cell types:
– Bushy cells: binaural information
– Multipolar cells: direct monaural pathway
– Octopus cells: indirect monaural pathway
Anterior cochlear nucleus
• Bushy cells: Their axons travel in the trapezoid body.
They are the central origin of ascending channels and
are useful for sound localization
• Multipolar cells are sensitive to changes in sound
pressure levelthey convey information to the
contralateral inferior colliculus about the intensity of the
sound
• Octopus cells: Their axons synapse mainly in the
contralateral anterior nucleus of the lateral lemniscus.
They convey information that is useful in analyzing brief
components of speech sounds
Superior Olivary Complex
• Located near the facial motor nucleus in the
caudal pons
• It is the first site in the brainstem where
information from both ears converges.
• It is a binaural processing.
• It is essential for accurate sound localization and
the formation of a neural map of the
contralateral auditory hemifield.
Superior Olivary Complex
• Components
– The medial superior olivary nucleus (MSO), the principal nucleus
in the human superior olivary complex
– The lateral superior olivary nucleus (LSO), is not distinct and
contains fewer cells. Summation of excitatory and inhibitory
inputs to LSO neurons causes detection of interaural intensity
differences, which provide spatial cues caused by shadowing of
sounds by the path from the contralateral side of the head
– The trapezoid body is a bundle of myelinated fibers passing
anterior to the superior olivary complex
Superior Olivary Complex
• Axons in the trapezoid body arising from ipsilateral
spherical bushy cells make excitatory synapses with the
laterally directed dendrites.
• Axons in the trapezoid body from the contralateral
spherical bushy cells make excitatory synapses with the
medially directed dendrites
• The excitatory neurotransmitter is probably glu or asp.
Local inhibitory circuits use glycine as a
neurotransmitter.
Superior Olivary Complex
• The MSO projections from the travel largely in
the ipsilateral side in the lateral lemniscus and
synapse in the central nucleus of the inferior
colliculus
• The LSO projections always end in the posterior
nucleus of the lateral lemniscus that projects to
the contralateral inferior colliculus
– This constitutes the indirect binaural pathway from the
superior olive to the inferior colliculus
Lateral Lemniscus
• The lateral lemniscus contains axons from
– Second-order neurons in the cochlear nuclei,
– Third-order neurons in the superior olive,
– Fourth-order neurons in the adjacent nuclei of the lateral
lemniscus.
• This heterogeneous collection prevents a simple
correlation of nuclei or tracts with specific wave
components of the auditory evoked responses used to
assess clinically the level of brainstem function.
• The interposition of synaptic delays in each of these
components contribute to at least the second, third, and
fourth wave components of the evoked responses.
Anterior Lateral
Lemniscus Nuclei
• The anterior (ventral, larger) nucleus of the
lateral lemniscus consists of cells scattered
among the ascending fibers of the lateral
lemniscus.
• It extends from the rostral limit of the superior
olive to just below the inferior colliculus.
• These cells project to the inferior colliculus,
completing an indirect monaural pathway
Posterior Lateral
Lemniscus Nuclei
• The posterior (dorsal, smaller) nucleus of the lateral
lemniscus is intercalated in the ascending fiber bundles
of the lateral lemniscus
• It is caudal to the inferior colliculus.
• Its ascending projections decussate in the posterior
tegmental commissure and terminate in the contralateral
inferior colliculus and in the contralateral posterior
nucleus of the lateral lemniscus
• This pathway is largely inhibitory, using GABA as the
neurotransmitter.
• It conveys binaural information and inhibits activity from
the opposite hemifield.
Inferior Colliculus
• All ascending auditory pathways terminate in the inferior
colliculus
• The egg-shaped central nucleus, is formed by fibers
(major source of input) of the lateral lemniscus.
• Paracentral nuclei are in a shell around the central
nucleus
– the pericentral nucleus, which lies posterior and is traversed by
fibers from the commissure of the inferior colliculus
– the external (lateral) nucleus, which lies lateral and is intersected
by fibers that form the brachium of the inferior colliculus.
Inferior Colliculus
• Many cells in the inferior colliculus respond to
input from either ear
• Binaural responses of inferior collicular neurons
resemble those of the superior olivary neurons,
from which they receive a dominant binaural
input
• Other cells in the fibrodendritic laminae of the
central nucleus are monaural and are mainly
excited only by the contralateral ear
Inf Coll. Central Nucleus
• The central nucleus integrates info from
hindbrain auditory sources and projects it to the
anterior division of the medial geniculate
nucleus.
• The central nucleus consists of parallel layers of
cells with disc-shaped dendritic fields. Afferents
from the lateral lemniscus (fibrodendritic
laminae) course parallel to these dendritic fields.
Inf Coll. Central Nucleus
• Ascending projections diverge and converge in a
point-to-plane order in the central nucleus
each point along the cochlear spiral is
represented in an isofrequency lamina.
• Functionally, cells in the central nucleus are
narrowly tuned, with the lowest frequencies
represented posterolaterally and higher
frequencies anteromedially.
Inf Coll. Paracentral Nucleus
• Cells in the paracentral nuclei are broadly tuned to
frequency, and they habituate rapidly to repetitive stimuli.
• They receive input from the central nucleus and the
cerebral cortex and
• They receive nonauditory input from the spinal cord,
posterior column nuclei, and superior colliculus.
• These nuclei project to the medial geniculate nucleus
superior colliculus, reticular formation, and precerebellar
nuclei.
• Thus, the paracentral nuclei are probably involved in
functions related to attention, multisensory integration,
and auditory-motor reflexes.
Medial Geniculate Nucleus
•
It forms a small protuberance on the lower
caudal surface of the thalamus between
lateral geniculate body and the pulvinar
Medial Geniculate Nucleus
• The anterior division receives afferents from
the central nucleus of the inferior colliculus and
projects to the primary auditory cortex
• Isofrequency contours in the anterior division are
arranged so that low frequencies are
represented laterally and higher frequencies are
represented medially
• As a result of collicular and thalamic integration
most cells are not reliably excited by simple
tones and are probably involved in complex
Medial Geniculate Nucleus
• The posterior division receives input from the
pericentral nucleus of the inferior colliculus and
projects to secondary auditory cortex
• Also tonotopically arranged
• This pathway is more broadly tuned and
sensitive to habituation,it convey information
about moving or novel stimuli that direct auditory
attention.
Medial Geniculate Nucleus
• The medial (magnocellular) division receives afferents
from the external nucleus of the inferior colliculus and
projects to association areas of auditory cortex
• It contains cells that are broadly tuned to auditory and
other sensory stimuli (vestibular & somesthetic inputs).
• It projects to temporal and parietal association areas and
to the amygdala, putamen, and pallidum.
• In view of the multisensory convergence that occurs in
this pathway, it may be a part of the reticular activating
system.
Clinical / Central Deafness
• Central deafness results from damage to the
cochlear nuclei and/or the central pathways that
relay auditory information to the auditory cortex.
• Damage to the cochlear nuclei may cause
deafness in the ear on the affected side.
• Central lesions within the brainstem,
diencephalon, or auditory cortices may alter the
perception of sound but infrequently result in
deafness in one ear.
Clinical / Central Deafness
• Pontine lesions may result in pontine auditory
hallucinosis, such as an orchestra out of tune, buzzing
insects, or strands of music.
• These perceived auditory events are accompanied by
more typical symptoms of pontine lesions, such as
cranial nerve deficits and/or long tract signs.
• A perception of noise or sounds may also be
experienced by patients with temporal lobe seizures or a
temporal lobe lesion that damages auditory cortices.
Auditory Cortex
• The primary auditory cortex (AI, Brodmann area 41) is
located in the transverse gyri of Heschl
• It is located in the first (anterior) transverse temporal
gyrus but may extend into the second (posterior) gyrus
• Cytoarchitecturally, area 41 encompasses the granular
cortex, with its well-developed layer IV containing small
granule cells and densely packed small pyramidal cells
in layer VI
• The secondary auditory cortex (AII, area 42) is adjacent
to the granular cortex in the second transverse gyrus
and planum temporale
Auditory Cortex
• Caudal to the
transverse temporal
gyri is a smooth area,
the planum
temporale, which is
usually larger on the
left side than on the
right
Auditory Cortex
• Area 41 is connected with the anterior division
of the medial geniculate body
• Area 42 is connected with the posterior division,
of the medial geniculate body.
• Through the corpus callosum, each auditory
cortical area is connected with the reciprocal
areas in the other cerebral hemisphere.
Auditory Cortex
• The tonotopic organization of constituent cells of the
cortical layers and incoming afferent fibers form a series
of orderly isofrequency columns that extend through the
primary auditory cortex as long stripes.
• High frequencies are represented medially and low
frequencies laterally.
• The series of stripes so formed have one subcomponent
composed of cells excited by stimulation of both ears
(EE) alternating with a subcomponent composed of cells
excited by the contralateral ear and inhibited by the
ipsilateral ear (EI).
Auditory Cortex
• The auditory association
cortex surrounds the
primary auditory area and
is located mainly in the
posterior portion of the
superior temporal gyrus.
• It is connected to the
primary auditory cortex by
the arcuate fasciculus.
Auditory Cortex
• Area 22 includes a part of the planum temporale and the
posterior portion of the superior temporal gyrus. It
receives connections from the primary auditory cortex,
as well as visual and somesthetic information.
• This speech receptive area, known as the Wernicke
area, may be as much as seven times larger on the left
side than on the right.
• When this area is damaged by occlusion of branches of
the MCA, an auditory aphasia (Wernicke aphasia)
results where comprehension of speech sounds is
impaired but discrimination of nonverbal sounds is
largely unaffected.
Auditory Cortex
• Brodmann areas 44 and 45 are known as the Broca area
for expressive speech and language.
• They are located in the pars opercularis and pars
triangularis of the inferior frontal gyrus.
• The major pathway connecting these areas with the
primary and association auditory cortex is the arcuate
fasciculus.
• If areas 44 and 45 are damaged along with other motor
cortices on the left side by a stroke involving branches of
the middle cerebral artery, the result is Broca aphasia
where the speech is nonfluent, but comprehension of
verbal and nonverbal sounds is largely unimpaired.
Descending Auditory Pathways
• Descending projections make reciprocal
connections throughout the auditory pathway.
• They form feedback loops that provide circuits to
modulate information processing from the
peripheral level to the cortex.
• For example, the auditory cortex projects to the
medial geniculate nucleus and nuclei of the
inferior colliculus. The inferior colliculus projects
to the periolivary nuclei, which, in turn, send
olivocochlear efferents to the cochlea.
The Olivocochlear Bundle
• The olivocochlear efferent system arises from groups of
cells in the periolivary nuclei of the superior olivary
complex.
• They travel as the olivocochlear bundle in the vestibular
part of the vestibulocochlear nerve.
• Lateral olivocochlear efferent cells project to the
ipsilateral inner hair cells, where they make axoaxonic
synapses with type I spiral ganglion afferent fibers.
• Medial olivocochlear efferent cells have bilateral
projections that terminate directly on outer hair cells
Cochlear Mechanics
• Direct efferent feedback to outer hair cells may
influence cochlear mechanics and the sensitivity
and frequency selectivity of the cochlea.
• Efferent-induced changes in outer hair cell
membrane potentials result in changes in the
height of the cells and the stiffness of their
stereocilia.
• These changes modulate basilar membrane
motion and thereby influence cochlear function.
Cochlear Mechanics
• The tight coupling of the basilar membrane
to the tectorial membrane by the outer hair
cells enables this efferent mechanism to
feed energy back to the cochlea to amplify
responses to specific tones.
• The cochlear amplifier effect is important
in selectively tuning the cochlea to
important sounds.
Middle Ear Reflex
• The middle ear reflex activate the small
striated muscles:
– The stapedius muscle is innervated by facial
motor neurons (associated with the caudal
end of the superior olivary complex)
– The tensor tympani muscle is innervated by
trigeminal motor neurons (associated with the
rostral end of the superior olivary complex).
Middle Ear Reflex
• Auditory input via axons of neurons in the
cochlear nuclei or the superior olivary complex
provides the sensory limb of the reflex.
• The sensory pathways are bilateral, so that
stimuli may be presented by earphones to one
ear while the device to measure impedance is
placed in the ear canal on the other side.
Thanks