Structure of the Inner Ear and Its Mechanical Response CHAPTER 7 Structure of the Inner Ear and Its Mechanical Response

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

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

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

Fluids filling the Inner Ear Perilymph- in S. Vestibuli and S. Tympani High Sodium / Low Potassium concentrations Low Voltage (0 to +5 mV) Endolymph- in S. Media High Potassium / Low Sodium concentrations High Positive Voltage (85 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

The Stereocilia on IHCs and OHCs OHCs (at top) V or W shaped ranks IHC (at bottom) straight line ranks

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

The Traveling Wave BASE APEX

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

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