The Auditory and Vestibular Systems

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The Auditory and Vestibular Systems
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• I. Functional Anatomy of the 2 Systems - Overview
• Parallel ascending auditory pathways.
• Ascending vestibular system.
• II. Regional Anatomy
• Sensory organs
• 1. Auditory receptor cells and apparatus.
• 2. Vestibular sensory organs.
• B. Brainstem nuclei
• 1. Vestibular and its projections.
• 2. Auditory and its projections.
• a. Lateral lemniscus
• b. Inferior colliculus
• c. Medial geniculate nucleus
• d. Primary auditory cortex.
• e. Wernikes Area a higher-order auditory
  cortex.

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Auditory System
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I. Functional Anatomy of the 2 Systems - Overview

• Auditory system.
• Tonotopic basis for frequency coding along the
  length of the cochlea.
• Tympanic membrane ? transduced info into neural
  signals.
• (next slides)

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II. Regional Anatomy Hearing - Equilibrium
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Hearing

• Sounds are waves of compressed air traveling
  through space
• - sound intensity ?wave height
• - pitch ? wave frequency

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Organ of hearing (and equilibrium) inner ear

• Cochlea
• Vestibular apparatus

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Sensory Organs Hearing

• 1- The sound waves enter the external auditory
  canal and trigger vibrations of the tympanic
  membrane
• 2- The tympanic membrane induces a vibration of
  the ossicles
• 3- the last ossicle, the stapes, transmits
  amplified vibrations to the oval window
• 4- The vibrations induce waves in the perilymph
  of the various inner ear chambers
• 5- the round window absorbs excess energy and
  prevent wave reverberation
• 6- the fluid wave is transduced into an
  electrical signal by the auditory receptors, the
  organs of Corti located on the basilar membrane

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Receptors for sound the organ of Corti

• The hair cells of the organ of Corti transduce
  fluid wave into an electrical signal
• The energy of the wave causes the basilar and
  vestibular membrane to move, thus displacing the
  cilia from the organ of Corti

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Organ of Corti
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Signal transduction

• Movements of the cilia open or close potassium
  channels ? changes in the state of polarization
  of the hair cell
• Changes in potassium leakage due to cilia bending
  trigger changes in neurotransmitters exocytosis
• The neurotransmitters send an electrical signal
  to an afferent neuron of the cochlear nerve
• The louder the sound, the more the cilia bend,
  the more numerous are the APs produced

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Coding for pitch

• The location of the organs of Corti on the
  basilar membrane codes for pitch
• - Organs of Corti located near the oval window
  are more sensitive to high pitch sounds while the
  ones located toward the tip of the cochlea
  respond more readily to low pitch sound

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Coding for sound intensity
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Neural pathway for sounds

• Cochlear nerve ? nucleus in medulla oblongata ?
  thalamus ? auditory cortex in the temporal lobe
• So, how do we perceive the direction from which a
  sound is coming from?

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B. Brainstem nuclei and their Projections

• 2. Auditory nuclei
• 3 major auditory relay nuclei of the brainstem
• A. cochlear nuclei (same side from cn VIII)
  (medulla).
• B. superior olivary nuclear complex (integration
  from both sides) (pons).
• C. inferior colliculus (midbrain).
• 2 major divisions we noted earlier.
• -superior olivary complex is important in sound
  localization (major input from AV cochlear
  nucleus).

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• Auditory System
• Overview Cochlear division of
• Nerve VIII ? cochlear n. (same side) in rostral
  medulla.
• 1. Anteroventral cochlear n.
• ? sup olivary n. (both
• sides) ? lateral lemniscus
• inferior colliculus.
• Important for horizontal location of sounds, as
  well as for other aspects of sound patterns,
  other than location.
• 2. Dorsal posteriorventral
• cochlear n. ? lat lemniscus
• Inferior colliculus (opp.
• side).
• Note the decussations Important for integration.
• Clin one can experience loss in only 1 ear only
  if lesion is peripheral.

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B. Brainstem nuclei and their Projections

• 2. Auditory nuclei (contd)
• Low-freq sounds are distinguished in space by
  interaural time difference.
• High-freq sounds are distinguished by difference
  in intensity between the ears.
• Different parts of the superior olivary n.
  (medial and lateral) are sensitive to these 2
  types of differences.
• Decussation is visible in trapezoid body.
• Feedback pathway some superior olivary neurons
  project back to the cochlea (both sides).

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B. Brainstem nuclei and their Projections

• 2. Auditory nuclei (contd)
• Olivocochlear bundle regulates flow of auditory
  info to the brain (much like inhibitory dorsal
  horn n. inhibit somatic sensory info.).
• Lateral Lemniscus
• Most auditory path neurons course in lateral
  lemniscus ? inferior colliculus.
• Some synapse on nucleus of lateral lemniscus ?
  contralateral inferior colliculus.
• Another important site of decussation (Probsts
  Commisure)

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Probsts commisure
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B. Brainstem nuclei and their Projections

• 2. Auditory nuclei (contd)
• Inferior Colliculus
• Within the midbrain tectum.
• The central nucleus within the inf colliculus
  receives the auditory info, which will proceed to
  the medial geniculate nucleus of the thalamus and
  the 1 auditory cortex.
• Laminated neurons in a single lamina are
  maximally sensitive to similar tonal frequencies.
• Receives input from superior olivary n., n. of
  lateral lemniscus (both sides), and dorsal n.
  pv cochlear (direct).
• Projects to thalamus through the brachium of the
  inferior colliculus.

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B. Brainstem nuclei and their Projections

• 2. Auditory nuclei (contd)
• Medial geniculate nucleus
• Thalamic auditory relay nucleus.
• The major part (ventral division) is
  tonotopically organized (receiving its input from
  the central n. of the inf coll, which is also
  tonotopically organized.
• Therefore, the MGN is also laminated layers
  maximally sensitive to similar frequencies.
• Thalamocortical auditory projections are called
  auditory radiations ? 1 auditory cortex with 2
  gyri within sulcus of temporal lobe Heschles
  Gyri

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B. Brainstem nuclei and their Projections

• 2. Auditory nuclei (contd)
• Medial geniculate nucleus
• Columnar organization of neurons sensitive to
  tones of similar frequencies (isofrequency
  columns).
• Also binaural columns (similar interaural
  intensity differences for localization of
  high-frequency sounds).
• Like other 1 sensory cortices, this has a
  prominent layer 4.

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Thalamic nuclei
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B. Brainstem nuclei and their Projections

• 2. Auditory nuclei (contd)
• Wernickes Area
• A higher-order auditory cortex for the
  interpretation of language. (language on L side
  of brain interpreting emotional content of
  language on R side of brain).
• One projection of Wernickes area is to Brocas
  motor speech area in the frontal lobe.

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Primary Auditory cortex
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Higher order auditory cortices
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• Auditory System
• Overview Cochlear division of
• Nerve VIII ? cochlear n. (same side) in rostral
  medulla.
• 1. Anteroventral cochlear n.
• ? sup olivary n. (both
• sides) ? lateral lemniscus
• inferior colliculus.
• Important for horizontal location of sounds, as
  well as for other aspects of sound patterns,
  other than location.
• 2. Dorsal posteriorventral
• cochlear n. ? lat lemniscus
• Inferior colliculus (opp.
• side).
• Note the decussations Important for integration.
• Clin one can experience loss in only 1 ear only
  if lesion is peripheral.

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Vestibular System
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Sensory Organs Vestibular Ampulae of
semicircular canals Maculae of utricle (linear
acceleration) saccule Endolymph gel-like
fluid flows over the hair cells with
movement and deflects them Ca carbonate
crystals (otoliths)
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Equilibrium

• Ability to detect head position and movement (or
  acceleration)
• Change of speed linear acceleration (utricle
  and saccule)
• Turning rotational acceleration (semi-circular
  canals)

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Utricle and saccule

• Sensory cells have cilia extending into a
  gelatinous material topped by otoliths
• Saccule detects backward-frontward movement
• Utricle detects changes relative to gravity

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Semi-circular canals

• The receptors in the ampulla are hair cells with
  cilia extruding into a gelatinous mass (cupula)
• When the head rotates, the cupula moves ? cilia
  pulled ?APs (vestibular nerve ? cerebellum )

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• So why does a person become dizzy after he/she
  stops spinning?

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B. Brainstem nuclei and their Projections

• Vestibular nuclei on floor of 4th ventricle.
• 4 nuclei inferior, medial, lateral, superior ?
  ascending projections to VPN of thalamus ? 1
  vestibular cortex in parietal lobes (just behind
  the 1 somatic sensory cortex).
• Can project to nearby parietal areas for
  integration of info regarding head motion with
  info from somatic sensory receptors in trunk and
  limbs.

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Vestibular Nuclei
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Vestibular System Overview Head motion ?
vestibular hair cell receptors ? 4 vestibular n.
in rostral medulla and caudal pons 2 Descending
projections to sc and extraocular muscles
(control movements) ? cerebellum. 2 Ascending
projections ? VPN of thalamus ? 1 vestibular ctx
in parietal lobe (for conscious awareness of
orientation and motion).