Cochlear Functions Transduction- Converting acoustical- mechanical energy into electro-chemical energy. Frequency Analysis-Breaking sound up into its component frequencies

Transduction- 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-chemical

Hair Cell Activation Involves Ion Flow into cell Through channels in the stereocilia Bending stereocilia causes # of open channels to change. Toward Modiolus = Fewer channels open Away from Modiolus = More open

Ion Channels are opened by “TIP LINKS” Tip Links connect tip of shorter stereocilia to the side of a stereocilium in the next taller row Bending toward taller rows pulls tip links Bending toward shorter rows relaxes tip links

Tip Links

Resting (or Membrane) Potentials Inner Hair Cell = - 45 mV Outer Hair Cell = - 70 mV

Stereocilia bent toward tallest row Potassium flows into cell Calcium flows into cell Voltage shifts to a less negative value More neurotransmitter is released

Sensory Physiology The basic unit of the nervous system is the Neuron or nerve cell Neurons undergo voltage changes- –SPONTANEOUSLY –IN RESPONSE TO STIMULATION

The Neuron

Neural 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:

An 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 dB

Spike Rate Increases Thru a 30 dB Range

Cochlear Mechanics Passive: Bekesy’s Traveling Wave Theory Active: Outer Hair Cell Motion enhances stimulation of inner hair cells

The Traveling Wave BASE APEX

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 amplitude

The Active Component Adds to Bekesy’s Traveling Wave

The Active Component Improves Sensitivity for soft sounds Improves frequency resolution

Frequency Tuning Curves Show these Effects = plots of response threshold as a function of frequency They have a characteristic shape sharp tip (shows best sensitivity at one freq) steep high frequency tail shallow low frequency tail

Tuning Curves Passive Only Active + Passive

More on Tuning & Tuning Curves: Frequency of “tip” is called the CHARACTERISTIC FREQUENCY Can be seen for:  basilar membrane,  hair cells,  nerve cells

Tonotopic Organization Mapping of Characteristic Frequency by place in the auditory nervous system In the Auditory portion of cranial nerve VIII the map is spiral-shaped with highest CF neurons on the outside, lowest CF neurons at the center.

Head-Related Effects Head-Baffle--the mere presence of your head alters the sound field. Head Shadow - loss of energy at far ear for frequencies above approx 2000 Hz

Signs of Peripheral Activation Otoacoustic Emissions (OAEs) Cochlear Potentials

Otoacoustic Emissions Low-level sounds produced by the cochlea and recordable in the external ear canal. Spontaneous Click-evoked Distortion Product Stimulus Frequency

Recording OAEs

Spectrum of Sound in Ear Canal

OAEs: Measures of Cochlear Health

Cochlear Potentials: Resting Potentials: voltages which exist without external stimulation e.g., Endolymphatic Potential, Cell Membrane Potential Stimulus-Related Potentials: voltages occurring in response to sounds We’ll talk about 3 of these from the cochlea

Cochlear 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 cochlea

Summating 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 stimulus