The Ear - Tulane University

The Ear

Round
Window

Outer, Middle and Inner Ear
Middle Ear – Tympanic Membrane (ear drum)
Inner Ear – Vestibular Apparatus and Cochlea

Ossicles – small bones that connect tympanic membrane to inner ear
- Malleus, Incus, Stapes
- Connected to Oval Window

The Cochlea

- Coiled tube

- 3 chambers -

- 2 large chambers: Scala Vestibuli
Scala Tympani

- 1 smaller channel: Scala Media
- within cochlear partition
- Contains Organ of Corti

- Helicotrema -
- Apex of cochlea
- Connects Scala Vestibuli and Tympani

- Fluids -
- Scala Vestibuli/Tympani – Perilymph
- Scala Media - Endolymph

Transduction of Sound

Organ of Corti -
- Located in Scala Media
- Hair Cells
- Basilar Membrane
- Tectorial Membrane
- Afferent axons

Transduction
⇒ Fluid waves - Scala Tympani
⇒ Oscillation in Basilar Membrane
⇒ Movement of Hair Cells
⇒ Movement of stereocilia
- within Tectorial Membrane, which remains stationary

Mechano-Electric Transduction

- Basilar Membrane oscillations
⇒ Forward/back movements of

stereocilia

⇒ Opening/closing of mechanically
gated K+ channels
- Toward long cilia ⇒ Opening
- Toward short cilia ⇒ Closing

- Opening causes influx of K+
- Depolarization
- Opening of voltage-gated Ca2+ channels
- Transmitter release (glutamate)

Hair Cell Receptor Potential
Generation

Endolymph – Hi [K+], low [Na+]
- +80 mV potential

Perilymph – Hi [Na+], low [K+]
- 0 mV potential

Hair Cell – -45 – -60 mV resting potential

- Opening of K+ channels ⇒ depolarization

- Closing of K+ channels ⇒ hyperpolarization

- Repolarization
- opening K channels
in base of hair cell

- 2 different EK’s
- 1 at base, 1 at apex

Auditory Signals

Sound waves -
- Compression and rarefication
- Sinusoidal wave
- Sinusoidal wave in perilymph
- mechanical pressure on oval
window

Opening/closing of mechanically gated K channels
- Depolarization/hyperpolarization of hair cell
- Reproduces sinusoidal wave

Tonotopy

Basilar membrane
- Narrow at base, wide at apex
- Base more rigid than apex

Different frequency sensitivities
- Base sensitive to high frequencies
- due to high rigidity
- Apex sensitive to low frequencies
- due to lower rigidity

⇒ Tonotopic organization of cochlea
⇒ Tonotopic transmission to brain