Two Halves: §Vestibular--transduces motion and pull of gravity §Cochlear--transduces sound energy (Both use Hair Cells) INNER EAR

Subdivision into spaces containing endolymph (blue), and spaces containing perilymph (red)

The Endolymphatic Sac §Termination of vestibular aquaduct §Outside of temporal bone; next to dura mater lining of the brain §Thought to maintain endolymphatic volume/pressure

Cochlea is Divided into 3 “Scala” §Scala Vestibuli l Reissner’s Membrane §Scala Media l Basilar Membrane §Scala Tympani §Helicotrema - the opening between 2 outer Scala

Fluids filling the Inner Ear §Perilymph- in S. Vestibuli and S. Tympani l High Sodium / Low Potassium concentrations l Low Voltage (0 to +5 mV) §Endolymph- in S. Media l High Potassium / Low Sodium concentrations l High Positive Voltage (80 mV)

Cross-Section of the Cochlea Third Turn Second Turn First Turn

A Cross Section Shows the 3 Scala

Within S. Media is the Organ of Corti

I = Inner Hair Cells P = Pillar Cells O = Outer Hair Cells D = Deiter’s Cells

IHCs, OHCs And Their Stereocilia §OHCs (at top) l 3, 4 or 5 rows l Approx 12,000 cells l 10 to 90 microns l V- or W-shaped ranks of stereocilia l 50 to 150 stereocilia per cell §IHC (at bottom) l 1 or 2 rows l Approx 3,500 cells l 35 microns l straight line ranks of stereocilia l 50 to 70 stereocilia per cell

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

Frequency Analysis - the Traveling Wave §Bekesy studied cochleae from cadavers, developed the Traveling Wave theory §1. Response always begins at the base §2. Amplitude grows as it travels apically §3. Reaches a peak at a point determined by frequency of the sound §4. Vibration then dies out rapidly

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: §Seen for basilar membrane, hair cells, nerve cells §Frequency of “tip” is called the CHARACTERISTIC FREQUENCY

OHC Length and CF High Freqs Low Freqs

Tectorial Membrane

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

Synapse Basics §Pre-Synaptic cell contains vesicles §Gap between cells is Synaptic Cleft §Post synaptic cell may show darkened area adjacent to membrane

Afferent Afferent & Efferent Neurons

4 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 synapses

IHC Innervation Pattern

OHC Innervation Pattern

Inner hair cells §Synapse at the base with up to 20 afferent neurons §“Divergence” §Efferents synapse on afferent dendrites under IHCs

IHC activation alters firing rate

Afferent neurons have their cell bodies in the Spiral Ganglion (4)

An Action Potential (or Spike)

IHC activation alters firing rate

Spike Rate Increases Thru a 30 dB Range

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

Electrocochleography

The SP/AP Ratio