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Structural Organization

In the cochlea, sound creates a traveling wave that moves from base to apex, increasing in amplitude as it moves along a tonotopic axis in the basilar membrane (BM). This pressure wave travels along the BM of the cochlea until it reaches an area that corresponds to its maximum vibration frequency; this is then coded as pitch. High frequency sounds stimulate neurons at the base of the structure and lower frequency sounds stimulate neurons at the apex. This represents cochlear tonotopic organization. This occurs because the mechanical properties of the BM are graded along a tonotopic axis; this conveys distinct frequencies to hair cells (mechanosensory cells that amplify cochlear vibrations and send auditory information to the brain), establishing receptor potentials and, consequently frequency tuning. For example, the BM increases in stiffness towards its base.

Mechanisms of Cochlear Tonotopy

Hair bundles, or the “mechanical antenna” of hair cells, are thought to be particularly important in cochlear tonotopy. The morphology of hair bundles likely contributes to the BM gradient. Tonotopic position determines the structure of hair bundles in the cochlea. The height of hair bundles increases from base to apex and the number of stereocilia decreases (i.e. hair cells located at the base of the cochlea contain more stereo cilia than those located at the apex).

Furthermore, in the tip-link complex of cochlear hair cells, tonotopy is associated with gradients of intrinsic mechanical properties. In the hair bundle, gating springs determine the open probability of mechanoelectrical ion transduction channels: at higher frequencies, these elastic springs are subject to higher stiffness and higher mechanical tension in tip-links of hair cells. This is emphasized by the division of labor between outer and inner hair cells, in which mechanical gradients for outer hair cells (responsible for amplification of lower frequency sounds) have higher stiffness and tension.

Tonotopy also manifests in the electrophysical properties of transduction. Sound energy is translated into neural signals through mechanoelectrical transduction. The magnitude of peak transduction current varies with tonotopic position. For example, currents are largest at high frequency positions such as the base of cochlea. As noted above, basal cochlear hair cells have more stereocilia, thus providing more channels and larger currents. Tonotopic position also determines the conductance of individual transduction channels. Individual channels at basal hair cells conduct more current than those at apical hair cells.

Finally, sound amplification is greater in the basal than in the apical cochlear regions because outer hair cells express the motor protein prestin, which amplifies vibrations and increases sensitivity of outer hair cells to lower sounds.