Download auditory / vestibular

AUDITORY / VESTIBULAR
INTRO
Similarities between auditory and vestibular systems (why consider together)
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Labyrinth in temporal bone
Eighth nerve
Hair cells
Differences
Auditory
Massive cortical representation
Focuses on external world
Pattern recognition ("what is it")
Spatial info ("where is it")
High-order cognitive (language)
Vestibular
Weak cortical representation (little consciousness, except in
disturbance)
Focuses on internal world, orientation/movement of body through space
Tight coupling to motor system
AUDITORY SYSTEM
Peripheral apparatus:
External ear and tympanic membrane
Middle ear
Air filled
Ossicular chain
Impedance match to fluid in inner ear
Large amplitude, low power oscillations of eardrum  low
amplitude, high power oscillations of inner-ear fluid
Cochlea
Spiral, complex chamber
Fluid filled
Unrolled view: basilar nearly divides chamber in half
Oval window  Scala vestibuli
Scala tympani  round window
Helicotrema is point of confluence of the two scalae
Mechanics
Pressure waves make floppy basilar wiggle
Frequency determines site of maximal wiggle
High frequency = basal
Low frequency = apical
Transduction
Apparatus (Organ of Corti) rests on basilar membrane
Hair cells
Non-neuronal
Cilia on apical surface
Bending of cilia = adequate stimulus for hair cell
 Depolarization of hair cell
 transmitter release from hair cell
Eighth nerve terminals
Receive synaptic input from hair cells
 Depolarization  action potentials
Spiral ganglion cells within cochlea are
first order neurons of auditory pathway
cells of origin of eighth nerve fibers
Inner vs. outer hair cells
Inner play fine sensory role
~10 eighth nerve fibers per hair cell (divergence)
Outer play cruder sensory and possible “motor” role
Convergence of several outer hair cells on single
afferent fibers
Eighth nerve efferents cause…
outer hair cell contraction
changes mechanical properties of basilar
membrane and hence auditory transduction
Shearing force upon basilar deflection
Cilia embedded in tectorial membrane
Shearing between Organ of Corti and tectorial
Maximal where deflection biggest
Tonotopy in hair-cell array (preserved in central projections)
Cochlear nuclei
Location
Upper medulla (near nerve entrance)
Dorsolateral to sulcus limitans (sensory zone)
Ascending path - overview
Many more waystations than in visual or lemniscal somatic sensory
One major nucleus per division (show)
Pons - superior olive
Midbrain - inferior colliculus
Thalamus - medial geniculate body
Laterality
Predominantly crossed (above cochlear nuclei)
But heavily cross-connected
Clinical correlate: no unilat. deafness with lesions above cochlear
nuclei
Functional logic. Why mix inputs from two ears?
Contra-space representation (as in visual)
Requires computation
Superior olive and spatial localization
Cues to spatial location in binaural input
Interaural time differences
Useful only at low stimulus frequencies
Interaural intensity differences
Useful only at high stimulus frequencies (shadowing effect of
head lost at lower frequencies)
Superior olive does computation
Medial superior olive
Computes time differences
Gets low frequency inputs from both ears via cochlear nuclei
(as expected)
Mechanism
Delay line in inputs from two ears
Facilitation with coincident excitation
Medial superior olive cells are "coincidence detectors"
(maximally excited by specific interaural time
differences and hence specific sound source
locations)
Lateral superior olive
Computes intensity differences
High frequency inputs from both ears
Mechanism
Excitatory input from one ear
Inhibitory input via interneuron from other ear
So response is
maximal for sounds nearer the ear providing
excitatory input
minimal for sounds nearer the ear providing the
inhibitory input
Olivocochlear bundle:
Efferent axons (bilateral) from some cells in superior olive to inner
and outer hair cells in cochlea.
Role: not fully understood but…
Stimulation of efferent bundle suppress acoustic responses
in auditory nerve,
Perhaps because causes outer hair cells to contract, in turn
affecting mechanical properties of basilar membrane (to
which they are linked).
Inferior colliculus [midbrain]:
Obligatory synapse on route to cortex.
Contrast with superior colliculus which is
Visual
Off the lemniscal path
Lateral lemniscus Main afferent input to inferior colliculus
Carries fibers from all lower auditory nuclei destined for the inferior
colliculus
Medial geniculate [thalamus]
Subdivisions (as in LGN)
Lemniscal = ventral division
Input from part of inferior colliculus getting "express" input
(direct from cochlear nuclei)
Output direct to primary auditory cortex
"Discriminative" audition
Extralemniscal
Inputs from other subnuclei of inferior colliculus
Outputs to "association areas" of auditory cortex
Analogous to extralemniscal visual pathway from retina to
superior colliculus to pulvinar to extrastriate cortex
Cortex
Primary auditory area = Brodmann 41 (dorsal surface temporal lobe)
Tonotopy (topographic map of stimulus frequency)
Orthogonal "ear dominance" columns
Summation (binaural better than either monaural)
Suppression (binaural worse than best monaural)
Association areas
Surround AI, merge with Wernicke's area (language
comprehension)
Lateralization Planum temporale (gross feature on dorsal surface of
temporal lobe, buried in lateral sulcus)
Bigger on left in most people (language dominance)
Especially big in people with perfect pitch