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AUDITORY / VESTIBULAR INTRO Similarities between auditory and vestibular systems (why consider together) Share: 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