Auditory Brainstem ReflexesHST 722 Descending SystemsChrisBrownFrom Slama, unpublishedAscending System: SolidDescending System: DashedHuge Cortico-Thalamic ProjectionInjection of HRPIn MGB (thalamus)CorticalLabeling:Terminals of thalamo-cortical axons in layer IV(Ascending System)Cell bodies of cortico-thalamic neurons in layers VI and Vb(Descending System)From Peterson and Winer, 1993Injection Site TracerCortex FluoroRuby (red)(to label axons & terminals of descending system)Inferior Colliculus(to label cell bodies of the ascending system)contra Fast Blueipsi fluorescein dextran(green)Cortical Descending Projections to cochlear nucleus neurons that project to inferior colliculus - studied with multiple tracersFrom Schofield and Coomes (2005)From Winer, 2006DescendingProjections ofAuditory CortexMethods for studying descending systems:1. Anatomical tracings2. Activate - electrical stimulation, sound3. Inactivation - lesion, temporary block (cooling), gene deletion4. Record responsesFrom Yan & Suga, 1998Electrical Stimulation of Cortex changes frequency responses of neurons in the Inferior ColliculusLateral (L) OC Neurons Medial (M)OC neuronsOrigin lateral regions near LSO medial regions near MSOAxon caliber thin thickPeripheral Target auditory-nerve dendrites outer hair cellsOlivocochlear (OC) Neurons are a Descending System that Projects from the Olive (superior olivary complex) to the CochleaMOC Efferent Fibers Terminate on Outer Hair CellsResponse Action of MOC EfferentsOAEs usually decreasedCochlear Microphonic (CM) increasedAfferent Fiber Responses usually decreased(including compound action potential, CAP)Alpha 9/10 cholinergic receptoron OHCsSPLOther MOC Functions:1) Reduce Masking2) Protection from overstimulation3) Selective AttentionFiring Rate ofAuditory Nerve FiberActivation of MOC Neurons Shifts Rate-Level FunctionsThis shift adjusts the dynamic range of nerve fibers,enabling better signaling at higher levelsOC Activation Reduces Masking in Auditory-Nerve ResponseBehavioral experiments suggest tones in noisy backgroundsbecome harder to detect after lesion of the OC systemFrom May and McQuone, 1995Recordings from single MOC neurons: Ipsi vs. Contra Response and locations of NeuronsResponse of Single MOC NeuronFrom Brown, 2001Recordings from single MOC Neurons: Sharp Frequency Tuning Tonotopic Cochlear Frequency MapBrown, unpublishedWhat is the pathway of the MOC Reflex?PVCN Lesions Interrupt the MOC ReflexFrom Brown et al., 2003Interneurons are probably PVCN ChoppersPlasticity of OC System?MOC Postsynaptic Target in Cochlear NucleusFrom Benson & Brown, 1990Auditory Brainstem Reflexes1. MOC Reflex2. Middle Ear Muscle Reflex3. Acoustic Startle Reflex(all are under descending control of higher
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