NeurophysiologyThe NeuronAn Action Potential (or Spike)Two Descriptors for NeuronsAfferent & Efferent Neurons4 Types of Cochlear NeuronsIHC Innervation PatternOHC Innervation PatternInner hair cellsIHC activation alters firing rateAfferent neurons have their cell bodies in the Spiral Ganglion (4)Central Nervous System StructuresMajor Components of the Central Auditory Nervous System (CANS)Mid-Saggital View of BrainPowerPoint PresentationNeural Web-SiteSlide 17Slide 18Slide 19Section Thru Brainstem Shows Cochlear NucleusThe Superior Olivary ComplexConnections To the Superior Olivary ComplexSuperior Olivary Processing Supports LocalizationDorsal (back) Side of BrainstemInferior ColliculusThalamus in PurpleAuditory Radiations ConnectLateral-Superior view of brainPrimary Auditory Cortex (AI): superior surface of the temporal lobeBrain Photos Web-SitesNeurophysiological MeasuresAuditory Evoked PotentialsExamples of AEP EpochsAuditory Brainstem ResponseGENERATORS of ABR WAVESSingle-Unit MeasuresTuning CurvesTwo-Tone SuppressionTwo-tone SuppressionFrequency CodingIntensity CodingEfferent (Descending) ControlUncrossed OCB-- synapses on dendrites under inner hair cellsCrossed OCB-- synapses on outer hair cellsEfferent Control (cont’d)The Acoustic ReflexSlide 476 Cortical LayersCortical NeuronsCortical ProcessingCerebral Dominance/LateralityNeurophysiology•Neurons•Gross Anatomy•The Central Auditory Nervous System•Frequency and Intensity encoding•Central Auditory Processing•Binaural Processing•Central Control (Descending Systems)•Cortical ProcessingThe Neuron•Dendrites receive synaptic stimulation (neurotrans.)•Action Potential generated in soma near axon•AP conducted along axon from Node to Node (saltatory conduction)•AP produces release of neurotransmitter at terminal boutonsAn Action Potential (or Spike)Two Descriptors for Neurons•Afferent (sensory)-- carrying signals toward the brain• •Efferent (motor) -- carrying signals from brain to periphery•AfferentAfferent & Efferent Neurons4 Types of Cochlear Neurons•INNER HAIR CELLS>Multiple (10 to 20) Afferent synapses>(Efferents synapse on afferent dendrites)•OUTER HAIR CELLS:>Large Efferent synapses engulf base of cell>Small (& not very active) Afferent synapsesIHC Innervation PatternOHC Innervation PatternInner hair cells•Synapse at the base with up to 20 afferent neurons•“Divergence”•Efferents synapse on afferent dendrites under IHCsIHC activation alters firing rateAfferent neurons have their cell bodies in the Spiral Ganglion (4)Central Nervous System Structures•Nucleus = a group of nerve cell bodies•Fiber Tract = a group of axonsMajor Components of the Central Auditory Nervous System (CANS)•VIIIth cranial nerve•Cochlear Nucleus•Superior Olivary Complex•Lateral Lemniscus•Inferior Colliculus•Medial Geniculate Body•Primary Auditory CortexBrainstemThalamusMid-brainTemporal LobeMid-Saggital View of BrainPonsCerebellum4th VentricleThalamusCorpus Callosum•MedGen Body•Inf Coll •Lat Lemn•SOC•Coch Nuc•VIIIth CNNeural Web-Sitehttp://rprcsgi.rprc.washington.edu/neuronames/hierarchy.htmlSection Thru Brainstem Shows Cochlear NucleusThe Superior Olivary ComplexConnections To the Superior Olivary ComplexSuperior Olivary Processing Supports Localization•Lateral SO-- Interaural Intensity Differences•Medial SO-- Interaural Time Differences(These are the two primary acoustic cues for localizing sounds)Dorsal (back) Side of Brainstem•Thalamus(medial geniculate)•Inferior Colliculus•4th Ventricle•Area of PonsInferior ColliculusThalamus in PurpleAuditory Radiations Connect•Medial Geniculate Body (in purple)to•Primary Auditory Cortex (in blue)Lateral-Superior view of brainPrimary Auditory Cortex (AI):superior surface of the temporal lobeBrain Photos Web-Siteshttp://rpiwww.mdacc.tmc.edu:80/se/anatomy/brain/http://www.ets.uidaho.edu/med532/start.htmNeurophysiological Measures•Gross Evoked Potentials-- Voltage changes in response to auditory stimulation recorded from the scalp•Single-Unit Measures-- Voltage (or other) changes recorded within a neuronAuditory Evoked Potentials•Recorded in different time intervals (“epochs”) following a sound•Earlier epochs come from lower in the system•Later epochs come from higher in the systemExamples of AEP Epochs•Electrocochleography-- within 5 milliseconds•Auditory Brainstem Response-- thru 10 ms•Middle Latency Response-- thru 75 ms •Auditory Late Response-- thru 200 msAuditory Brainstem ResponseTime (ms)Amplitude (V) Wave V LatencyIIIIIIIVVAmp V 010GENERATORS of ABR WAVESIIIIIIIVV=Distal VIIIth nerve=Medial VIIIth nerve=Cochlear Nucleus=Superior Olivary Complex=Lateral Lemniscus & Inferior ColliculusSingle-Unit Measures•Post-Stimulus Time Histogram-- Shows firing rate changes over time•Period or Interval Histograms-- Show phase-locking of neural firingTuning Curves•Iso-Rate Function -- Shape similar to what we’ve already described (Fig 6.12 b)•Iso-level Function -- Shows spike rate as a function of frequency-- peak at a single frequency (Fig 6.12a)Two-Tone Suppression•The response to one tone can be reduced or eliminated by introducing a second tone near the neuron’s CF.•(Fig 6.16)•Second tone can be either one which normally would excite the neuron or notTwo-tone SuppressionRegular Tuning CurveFrequency Coding•The Place Code-- each neuron has a characteristic frequency•Periodicity Pitch-- neurons phase-lock to stimuliIntensity Coding•Firing rate increases in single neurons•Spread of activation to a wider range of neurons-- “Density of Discharges”•Latency of Firing (shorter delay at higher levels)Efferent (Descending) Control•Cochlear Efferents come from Superior Olivary Complex --The Olivo-Cochlear Bundle (OCB) •Uncrossed OCB-- synapses on dendrites under inner hair cells•Crossed OCB-- synapses on outer hair cells•Both use inhibitory neurotransmittersUncrossed OCB-- synapses on dendrites under inner hair cellsCrossed OCB-- synapses on outer hair cellsEfferent Control (cont’d)•The Acoustic Reflex •Auditory Cortex and Thalamus also send descending fibers to auditory brainstem locationsThe Acoustic ReflexAfferent:•VIIIth nerve•Cochlear Nucleus•Superior Olivary ComplexEfferent:•VIIth nerve nucleus•VIIth nerve•Stapedius musclePrimary Auditory Cortex (AI):superior surface of the temporal lobe6 Cortical Layers•Thalamic inputs >IV•project to