CHAPTER 9The 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)Tuning CurvesBekesy’s Theory describes Passive MechanicsThe Active Component Adds to Bekesy’s Traveling WaveThe Active ComponentFrequency Tuning Curves Show these EffectsSlide 17More on Tuning & Tuning Curves:Single-Unit MeasuresSlide 20Two-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 ReflexCHAPTER 9The Neural Response and the Auditory CodeThe 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 peripheryAfferentAfferent & 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)Tuning 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)Bekesy’s Theory describes Passive Mechanics•Based on work in “dead” cochleae•Highly damped -- not sharply tuned•Active Undamping occurs in live and healthy cochleae•Like pumping on a swing--adds amplitudeThe Active Component Adds to Bekesy’s Traveling WaveThe Active Component•Improves Sensitivity for soft sounds•Improves frequency resolutionFrequency Tuning Curves Show these Effects= plots of response threshold as a function of frequencyThey have a characteristic shape•sharp tip (shows best sensitivity at one freq)•steep high frequency tail•shallow low frequency tailTuning CurvesPassive OnlyActive + PassiveMore on Tuning & Tuning Curves:•Frequency of “tip” is called the CHARACTERISTIC FREQUENCY •Can be seen for: basilar membrane,hair cells, nerve cellsSingle-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