Cochlea Conduction & Reception of Auditory Stimuli

Cochlea: Organization Fluid-filled Divided Longitudinally into 3 compartments Scala tympani: perilymph ↑ Na+, ↓ K+ (extracellular) Scala vestibuli: perilymph Scala media: Endolymph ↓Na+, ↑K+ (intracellular)

Cochlea: Sound Conduction Snail-shaped; 2.25 turns in humans Basal end of basilar membrane set in motion by movement of cochlear fluid Frequency sorting along basilar membrane Wave amplitude decreases and becomes extinct at a particular distance Hair cells along basilar membrane within organ of Corti

Sensory Reception: Hair Cells Mechanoreptors: Type II –Afferent fibers (part of CN VIII) synapse with receptor cell –Efferents synapse with receptor cell or with afferents Two types –Inner: (IHC) one row; convert movement of basilar membrane into a neural signal; multiple afferents –Outer: (OHC) 3-5 rows; participate in movement & amplification of basilar membrane Efferent fibers innervate both IHCs and OHCs Efferents may control mechanical properties of OHCs and thus the mechanical properties of the medium that stimulates IHCs

Sensory Reception: Hair Cells Stereocilia –Large microvilli arranged by increasing height on each hair cell –Emerge from cuticular plate –Composed of cross-linked actin filaments –Tips of stereocilia of OHCs embedded in tectorial membrane

Sensory Reception: Hair Cells Movement of cochlear fluid sends a traveling wave along the basilar membrane beginning at the basal end Stiffness decreases toward apex causing wave amplitude to increase Wave travels a particular distance dependent on frequency Different populations of hair cells are activated by sounds of different frequencies Positive deflection of stereocilia Each stereocilium has a very fine filament that connects it to its taller adjacent neighbor Bundle of stereocilia tilts which puts tension on filaments Tension pulls on mechanically gated ion channels Influx of K+ & Ca++ from endolymph (scala media) → depolarization Faintest sounds: stretch filaments 0.04 nm (1/2 the diameter of a hydrogen atom)

Sensory Reception: Hair Cells Receptor potential leads to opening of voltage gated Ca++ channels Ca++ enters & triggers release of glutamate at basal end of hair cell Glutamate diffuses across space between hair cell and dendrites of CN VIII Action potential triggered in nerve Repolarization of the hair cell: low concentration of positive ions in perilymph of scala tympani

Nonlinearity of the Cochlea William S. Rhode & Alberto Recio (2000) Chinchilla cochlea Region of sensitivity to a particular frequency shifts toward the basal end of the basilar membrane with increasing intensity of sound Tuning is sharpest with lower intensity sounds