CHAPTER 8PowerPoint PresentationIHC SynapsesOHC SynapsesTransduction-Sensory PhysiologyThe NeuronTwo Descriptors for NeuronsAfferent & Efferent Neurons4 Types of Cochlear NeuronsIHC Innervation PatternOHC Innervation PatternInner hair cellsIHC activation alters firing rateNeural ActivityAn Action Potential (or Spike)Action PotentialsSpike Rate Increases Thru a 30 dB RangeSigns of Peripheral ActivationOtoacoustic EmissionsRecording OAEsSpectrum of Sound in Ear CanalOAEs: Measures of Cochlear HealthCochlear Potentials:Cochlear MicrophonicSummating Potential (SP)Compound Action Potential (CAP)Slide 28Afferent neurons have their cell bodies in the Spiral Ganglion (4)CHAPTER 8Peripheral Auditory Nervous System and HaircellsIHC SynapsesOHC Synapses•Efferents have greater surface area than afferentsTransduction-•Inner Hair Cells are the true sensory transducers, converting motion of stereocilia into neurotransmitter release. Mechanical Electro-chemical•Outer Hair Cells have both forward and reverse transduction-- Mechanical  Electro-chemical Mechanical Electro-chemicalSensory Physiology•The basic unit of the nervous system is the Neuron or nerve cell•Neurons undergo voltage changes-–SPONTANEOUSLY–IN RESPONSE TO STIMULATIONThe 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 boutonsTwo 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 rateNeural Activity•Post-synaptic Potentials-- Local, Variable changes in voltage near synapse•Action Potentials-- Conducted through axon, “all or none,” “spike” •For image of AP’s traveling down an axon:http://bio.winona.msus.edu/berg/ANIMTNS/actpot.htmAn Action Potential (or Spike)Action Potentials•Are generated spontaneously–At a slow rate by some neurons–At a faster rate by some neurons•And occur more frequently with STIMULATION•Spike rate increases through a range of about 30 dBSpike Rate Increases Thru a 30 dB Range01020304050607080900 5 10 15 20 25 30 35 40 45 50 55 60Stimulus Level (dB SPL)Spike Rate (APs/sec)Spike RateSigns of Peripheral Activation•Otoacoustic Emissions (OAEs)•Cochlear PotentialsOtoacoustic Emissions•Low-level sounds produced by the cochlea and recordable in the external ear canal.•Spontaneous•Click-evoked•Distortion Product•Stimulus FrequencyRecording OAEsSpectrum of Sound in Ear CanalOAEs: Measures of Cochlear HealthCochlear Potentials:•Resting Potentials: voltages which exist without external stimulatione.g., Endolymphatic Potential, Cell Membrane Potential•Stimulus-Related Potentials: voltages occurring in response to soundsWe’ll talk about 3 of these from the cochleaCochlear Microphonic•Least valuable from a clinical standpoint. •Is an alternating current (AC) response that mirrors the waveform of low to moderately intense sound stimuli•Appears to arise from outer hair cells in the basal-most turn of the cochleaSummating Potential (SP)•Is a direct current or DC potential•Lasts for duration of stimulus.Compound Action Potential (CAP)•Summation of APs in large number of VIIIth nerve neurons•following onset (and offset) of stimulusIHC activation alters firing rateAfferent neurons have their cell bodies in the Spiral Ganglion