Sound Transduction 1 Or, if the a tree falls in a forest and no one is around does it still reflect light?

What is Resonance?  Resonance Characteristic frequency response  Density & Tuning Sharpness Hi dense = Lo Sharpness  Size/Cavity Breaking Glass Demo

The Outer Ear: All about resonance  Pinna  Immobile cartilage side of head Flange  ~ Hz resonance Concha  ~1 – 7000 Hz resonance  Why such high frequencies? Thoughts?  Directionality Finger in folds demo Spectral Filter (e.g., Rayker et al., 2004)  Resonance Frequencies ~1000 – 7000 Hz  Notch Filter ~ 700, 3500, 7000, Hz

Outer Ear: con’t  Meatus Cartilaginous – bone  Density & Resonance  3000 Hz resonator Wax + Hair  Dirt Filter  Tympanic Membrane Elastic Skin stretched across a bony ring Stiff cone (2 mm height)  High Fidelity Transfer

Middle Ear: The Saga Continues  The Impedance Problem Getting sound to the sensors  Tympanic Membrane to Oval Window (stapedial footplate) Orders of magnitude size difference  Ossicular chain Malleus – Incus – Stapes – Stapedial Footplate  High density benefits!

More middle ear goodness  Air filled pressure equal to outside Eustacian Tube regulation  High intensity sound response Multiple muscles Sound attenuators

Quick Interruption!  TLA 1: Hearing Under Water (HUW) Why is this important? Ingredients:  Sound source (Clicker?)  Still water (bath, sink, pool) Stick your ear, or a friend’s ear under water Make sound in air and under water and listen with:  Out of water ear  Under water ear Questions: Which produces the loudest sound? Is it difficult to determine directionality?

Into the Inner Ear  Major subdivisions of the Bony Labyrinth Vestibular & Auditory  Auditory-side = Cochlea  Cochlea Cavity within bone, Fluid-filled caverns Curls like a snail

Life in the Cochlea  Three major subdivisions Scala Vestibuli: Largest cavity, filled with perilymph (e.g., Ricci & Fettiplace, 1998)  Positively charged Sodium Ions (Na+) Scala Media: Smallest cavity, filled with endolymph (e.g., Ricci & Fettiplace, 1998)  More positively charged Potassium & Calcium Ions (K+, Ca++)  Where the action is!!! Scala Tympani: Mid size cavity, filled with perilymph  Connected to Scala Vestibuli

Scala Media, come get some!  ‘Organ of Corti’  Organ o’ Corti contains Basilar Membrane (base) Tectorial Membrane Inner Hair Cells Outer Hair Cells  Hair cells embedded in Bas. Membrane  Outer Hair Cells contact Tect. Membrane

Basilar Membrane  Properties of the Basilar Membrane Apex thin and stiff, Base broad and flexible  Standing Waves Upward spread of masking Why do higher Frequencies get masked by lower frequencies?

Why does it matter that the Basilar Membrane moves?  Hair cell magic  Outer Hair Cells ~ 12,000 in three rows Afferent and Efferent connections Attached to muscle fiber  Inner Hair Cells ~ 3,000 in single row Afferent connection Passive Motion

Actual Transduction!  Wave along Basilar Membrane Causes inner hair cell shearing Shearing opens channel  Endolymph in Scala Media attracts perilymph in Scala Tympani  Charges up Hair cell to cause neural firing

What are the outer hair cells doing?  Outer Hair cells motile & embedded in Tectorial Membrane Theory 1. Stiffen to attenuate sound along the basilar membrane, shear to add energy to the basilar membrane Theory 2. Stiffen to raise the Tectorial membrane away from the inner hair cells, shear to lower the Tectorial membrane and obstruct the inner hair cells

The big picture  Outer/Middle ear filter and intensify sound  Inner ear detects sound Inner Hair Cell movement along the basilar membrane  Converts Mechanical energy to Electrical energy (nerve impulse) Outer hair cells help modulate movement along the basilar membrane

Questions?