Place principle

Travelling wave and the place principle

HHMI Cochlea animation (scroll to the very bottom of the list)

in response to a simple sinusoidal stimulus, the basilar membrane resonates in a "travelling wave" that gradually grows in amplitude as it moves along the cochlear duct: away from the stapes (base), toward the helicotrema (apex)
- same direction of travel is observed if the sound is introduced into the apex of the cochlea instead of the base

THE ENTIRE BASILAR MEMBRANE DOESN'T VIBRATE IN RESPONSE TO EVERY FREQUENCY OF A PARTICULAR SOUND!

for a pariticular frequency, basilar membrane vibration reaches a peak amplitude at a specific point that is determined by the resonant properties (acoustic impedance) of the basilar membrane at each point

both passive (structural) and active (energy requiring) processes establish the location of the peak vibration

PLACE PRINCIPLE: the peak of the travelling wave occurs at a different place along the basilar membrane for each frequency (the basilar membrane movement is said to be “tuned”)
- low frequencies peak near the apex; high frequencies near the base
as a result of this spectral analysis, complex stimuli produce vibration patterns where the component frequencies are represented in different locations along the basilar membrane
for "his discoveries of the physical mechanism of stimulation within the cochlea", Georg von Bekesy was awarded the 1961 Nobel prize in Physiology or Medicine