Chapters 9,10 Auditory and Vestibular Systems

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Chapters 9,10 Auditory and Vestibular Systems

• Chris Rorden
• University of South Carolina
• Norman J. Arnold School of Public Health
• Department of Communication Sciences and
  Disorders
• University of South Carolina

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Audition

• The ear converts sound energy into patterns of
  neural firing transduction
• Outer ear collect and amplify sound, aid in
  localization
• Middle ear impedance matching
• Cochlea frequency and intensity analysis
• Auditory pathway complex signal processing

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Ear structures

• Peripheral
• Outer ear
• Middle ear
• Inner ear
• Auditory nerve
• Central
• Brainstem
• Midbrain
• Cerebral

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Anatomy and function

• Pinna the projecting part of the ear lying
  outside of the head (also called auricle, or just
  ear lobe)
• Reflection of sound in pinna provides spectral
  cues about elevation of a sound source

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Outer Ear Auditory Canal

• External auditory meatus
• Provides communication between middle and inner
  ears by conducting sound to the ear drum
• S-shaped tube _at_ 2.5cm long and _at_ 7mm wide
• Lining of the lateral 1/3rd of canal has cilia
  and glands
• Cerumen (ear wax) protects ear canal from drying
  out and prevents intrusion of insects

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Outer Ear - Ear drum

• Tympanic membrane
• Separates outer and middle ear
• Compliant
• Thin, three-layered sheet
• Epithelium of EAM outer layer
• Middle layer fibrous (strong) tissue
• Inner layer of middle ear mucous membrane

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Outer Ear - Ear drum

• Slightly concave to EAM, cone-shaped
• Most depressed and thinnest point is called the
  umbo
• End of the attachment of malleus
• Cone of light from umbo to periphery reflects
  light when viewed with otoscope
• Slightly oval, taller than wide
• Otoscope if you pull the pinna up and back the
  tympanic membrane is visible

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Middle Ear Tympanic Membrane
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Role of outer ear

• To augment the sound shadow
• Ear canal protects delicate parts of middle and
  inner ear from impact.
• To heighten our sensitivity to sounds
• Ear canal boosts sounds 15 to 16 dB between 1.5
  and 8 kHz (in the area of speech)
• This is due to resonance of ear canal
• Just like vocal tract this tube amplifies and
  dampens certain frequencies based on its length
  and composition

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Localization and shadowing

• Intensity differences louder if nearer, less
  shaded
• Inter-aural timing differences
• Frequencies influenced by location relative to
  pinna.

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Middle Ear Eustachian Tube

• Establishes communication between middle ear and
  nasopharynx
• 35 to 38 mm long, typically closed
• Biological functions
• To permit middle ear pressure to equalize with
  external air pressure
• On the air plane, change in atmospheric pressure
  but not pressure in middle ear
• Yawning or swallowing opens pharyngeal orifice of
  tube to equalize pressures
• To permit drainage of normal and diseased middle
  ear secretions into the nasopharynx

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Middle Ear - Ossicles

• 3 of the smallest bones
• Malleus (hammer)
• Incus (anvil)
• Stapes (stirrup)
• Ossicular chain Transmits acoustic energy from
  tympanic membrane to inner ear
• Acts as lever large weak motion of TM causes
  small forceful movement of stapes.
• Takes force from gas (air) and matches impedance
  to liquid (inner ear).
• Muscles allow movement to be attenuated Prevents
  the inner ear from being overwhelmed by
  excessively strong vibrations

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Middle Ear Ossicles
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Middle Ear Ossicles - Malleus
Malleus (hammer) 9 mm long Manubrium (handle)
attaches to tympanic membrane pulls the drum
medially Caput (head) jointed (quite inflexibly)
to Incus
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Middle Ear Ossicles - Incus

• The ossicles give the eardrum mechanical
  advantage via lever action and a reduction in the
  area of force distribution
• Pressure Force/Area so less area more
  pressure
• the resulting vibrations would be much smaller if
  the sound waves were transmitted directly from
  the outer ear to the oval window.
• The movements of the ossicles is controlled
  muscles attached to them (the tensor tympani and
  the stapedius).These muscles can dampen the
  vibration of the ossicles, in order to protect
  the inner ear from excessively loud noise and
  that they give better frequency resolution at
  higher frequencies by reducing the transmission
  of low frequencies

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Middle Ear Ossicles - Stapes
Head (caput) jointed to incus Anterior and
posterior crura (legs) Footplate joins oval
window of inner ear (opening in temporal bone)
via annular ligament
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Cochlea and neighbors
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Inner Ear - Cochlear
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Osseous cochlea

• Oval window
• Connects scala vestibuli and middle ear
• Round window
• Connects scala tympani and middle ear

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Cochlear Structures

• Cochleus from 5mo fetus
• Oval window (blue arrow)
• Round window (yellow arrow)

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Tonotopic

• Base
• High Freq

• Apex
• Low Freq.

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Travelling wave

• Always starts at the base of the cochlea and
  moves toward the apex
• Its amplitude changes as it traverses the length
  of the cochlea
• The position along the basilar membrane at which
  its amplitude is highest depends on the frequency
  of the stimulus

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Traveling wave

• High frequencies have peak influence near base
  and stapes
• Low frequencies travel further, have peak near
  apex
• A short movie
• www.neurophys.wisc.edu/ychen/auditory/animation/a
  nimationmain.html

• Green line shows 'envelope' of travelling wave
  at this frequency most oscillation occurs 28mm
  from stapes.

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Cochlear structure

• Cross-section shows the coiling of the cochlear
  duct The red arrow is from the oval window, the
  blue arrow points to the round window.
• Scala media filled w Endolymph
• scala vestibuli filled w Perilymph
• scala tympani filled w Perilymph
• spiral ganglion
• nerve fibres
• www.iurc.montp.inserm.fr/cric/audition/english/coc
  hlea/fcochlea.htm

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Inner Ear - Labyrinth
Endolymph K 100 mV Perilymph Na 20mV
Reissners Membrane Basilar Membrane
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Inner Ear Organ of Corti
Both types of hair cells protrude into endolymph
of scala media,
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Inner Ear OHC IHC

• Inner Hair Cells
• Non-motile
• Outer Hair Cells
• Vibrates when triggered acts as preamplifier.

Hair cells are mechanically gated ion channels
deflection of hairs depolarizes the cell,
resulting in a receptor potential causing
calcium ions to enter, which in turn stimulates
the release of neuroreceptors.
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Neural connections

• Inner hair cells many nerve fibers for each cell
  (many-to-one innervation) 3500
• Outer hair cells each nerve fiber connected to
  many hair cells (one-to-many innervation). 12000

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Function of the cochlea

• First stage of auditory processing
• 1. Spectral analysis
• Extracts frequency and amplitude information from
  sound waves
• 2. Temporal analysis
• Basic temporal characteristics of sounds

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• The ear codes frequency in two ways
• Position of neural responses along basilar
  membrane changes with frequency - tonotopic
  organization or the place coding
• Timing of neural responses follows the time
  waveform of sound phase-locking

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Place coding

• Place coding Auditory frequency coded by
  location of stimulation.

• Base
• High Freq

• Apex
• Low Freq.

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Phase locking

• The rate of neural firing matches the sound's
  frequency.
• Problem some auditory frequencies much faster
  than neurons can fire
• Each neuron can only fire around 200 times per
  sec.
• Solution volley principle large numbers of
  neurons that are phased locked can code high
  frequencies.

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Afferent and efferent innervation

• Afferent signals from sense organ to brain
• Auditory signals
• Efferent signals from brain to sense organ
• Inhibits auditory signals
• Both cochlear and ossicles
• Improves signal-to-noise ratio by suppressing
  noise

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Primary auditory cortex
Medial geniculate Body (thalamus)
Inferior colliculus (in midbrain)
Auditory radiation
Cochlear and Superior olivary Complex in the
Medulla
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Central Auditory Mechanism

• Auditory input projects to the cortex
  bilaterally, with stronger contralateral
  connections.
• The superior olive and the inferior colliculus
  send efferent fibers back to attenuate motion of
  the middle ear bones (dampen loud sounds)

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• Cochlear Nucleus
• Evidence of signal processing (monaural)
• Superior Olivary Complex (SOC)
• Binaural processing
• Localization of sound source
• Low frequency sounds arrival time compared
• High frequency sounds intensity level compared

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Auditory Pathway

• Lateral Lemniscus
• Fiber tract within CNS
• From SOC to IC
• Inferior Colliculus (IC)
• Bilateral innervation
• Frequency, intensity and temporal processing
• Medial Geniculate Body (MGB)
• Tonotopic mapping
• Complex responses to contralateral signals

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Cerebral cortex

• Signal comes primarily from contralateral ear via
  ipsilateral MGB
• Heschls gyrus
• Tonotopic mapping in columns
• Each column has one characteristic frequency
• Neurons in column responsive to different
  stimulus parameters, like frequency and intensity

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Anatomy and function

• Many sound features are encoded before the signal
  reaches the cortex

- Cochlear nucleus segregates sound
information - Signals from each ear converge on
the superior olivary complex - important for
sound localization - Inferior colliculus is
sensitive to location, absolute intensity, rates
of intensity change, frequency - important for
pattern categorization - Descending cortical
influences modify the input from the medial
geniculate nucleus - important as an adaptive
filter
cortex
medial geniculate body
inferior colliculus
cochlear nucleus complex
cochlea
superior olivary complex
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Clinical Notes

• Conductive
• Cerumen in canal, Otitis Media of Middle Ear (ear
  infection)
• Sensorineural
• Menieres disease (abnormality in the fluids of
  the inner ear vertigo), Presbycusis (age
  related hearing loss)
• Central
• Pathology in cortex
• Bilateral auditory cortex lesions result in
• Profound loss of auditory discriminative skills
• Impaired speech perception
• Hearing loss