NEUROSCIENCE: AUDITORY SYSTEM -- COCHLEAR PROCESSING: Sequence in the Cochlea

COCHLEAR PROCESSING

A. Sequence in Cochlea

vibration of basilar membrane bending of hair cells depol. of hair cells transmitter release from hair cell somas APs on auditory nerve axons

1. Pressure is transmitted through the oval window to cochlear fluid

2. Pressure wave moves through the scala vestibuli and scala media, beginning at the base and moving toward the apex, then through the basilar membrane, then through the scala tympani, and finally causing vibration at the round window

3. This sets up a traveling wave in the basilar membrane, beginning at the base of the cochlea. This wave travels down the basilar membrane, first increasing and then decreasing in amplitude (depends on mechanical characteristics of cochlea). The site of maximum displacement depends upon frequency. The high frequencies are rapidly damped, so their peak amplitude is near the base; the low frequencies travel further toward the apex. Lowest frequency waves cause vibration along whole length of basilar membrane (==>)

Note: In the illustration, the cilia of the inner hair cells should contact the tectorial membrane

4. Vibration of the basilar membrane causes shear (bending) of the hair cells. This causes inner hair cell depolarization due to cation (mainly K⁺) influx (generator potential), followed by Ca²⁺ influx (due to opening of voltage-dependent Ca channels), leading to release of excitatory transmitter, causing action potentials to be developed on auditory nerve axons. (==>)

a. inner hair cells and primary auditory afferent neurons are separate cells. Hair cells depolarize and release transmitter but do not generate action potentials. Primary auditory afferent neurons generate action potentials when excited by neurotransmitter released from hair cells.

b. influx of K⁺ into inner hair cell is due to K⁺ channel opening, which occurs when the cilia flex toward the stria vascularis. K⁺ influx is made possible by the high concentration of K⁺ in the scala media fluid, due to K⁺secretion by the stria vascularis.

c. hair cell flexing is promoted by the tectorial membrane, which contacts the hair cell cilia

d. influx of Ca²⁺ into the soma also causes opening of soma K⁺ channels, terminating the depolarization