Middle Ear Functions Impedance Matching -- amplification of sounds to overcome difference in impedance between the air of EAC and the fluid of the inner ear. Filtering -- resonant frequency is approximately 1000 Hz, functions as bandpass filter. Acoustic Reflex -- Contraction of Stapedius muscle in response to loud sounds

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