Schematic representation of the mechanical coupling between OHCs and surrounding structures

Stages 1 to 3 represent the coupling to their surrounding structures of OHCs in the bat (very tight), in the basal portion of a guinea pig cochlea, and in the apex of the guinea pig cochlea (very loose). In an individual cochlea, the strength of this mechanical coupling follows a decreasing gradient from base to apex - the higher the frequency, the stronger the coupling. This gradient accounts for the efficacy of the reverse transduction process (active mechanism).

NOTE. In the scheme above as well as in diagrams below: OHC in yellow, basilar membrane in dark blue, Deiters' cell in pink with microtubules in brown, tectorial membrane in light blue.

In this cochlea, specialised for coding very high frequencies, a very short OHC (yellow), about 10 µm in length, is half-ensheathed in the Deiters' cell cup; a dense fascicle of microtubules (brown) in the Deiters' cell links the cup with a thick basilar membrane (dark blue).

a) The tallest stereocilia are deeply embedded in a massive tectorial membrane (light blue) parallel to the BM.

b) In the space between the membranes of both the OHC and the Deiters' cell dense material and septa are seen.

Together, these features account for a very firm coupling and the active mechanism can feedback energy even above 100 kHz.

At a place coding for frequencies around 10 to 20 kHz, the Deiters' cup is more like a close-fitting seat: one free side of a longer OHC (around 20 to 30 µm) being occupied by the characteristic huge medial efferent endings.

Compared to the bat, the fascicle of microtubules in the Deiters' cell remains dense whilst the phalangeal process is longer; both TM and BM are thinner than in the bat

a) The tallest OHC stereocilia are not so deeply embedded in the TM.
b) The extracellular material (and septa) in the Deiters' cell cup is less obvious.

In an area coding below 1 kHz, an elongated OHC (>70 µm) is now very loosely sited on a Deiters' cell, which is almost devoid of microtubules and has a very long and thin phalangeal process; both TM and BM are much thinner and they form a much wider angle (no longr parallel).

a) Even the tallest OHC stereocilia are not embedded with the TM (no imprints in the TM are seen).
b) The Deiters' cup is now devoid of dense material.

Within the Deiters' cell (d), a fascicle of microtubules running through the cell (red arrows) links its close junction with the OHC (o) superiorly to its junction with the basilar membrane inferiorly

(see scheme above).

scale bar: 500 nm

A higher magnification of this junction shows material and septa (yellow arrows) linking both membranes.

scale bar: 150 nm

Inferior face of the tectorial membrane as see by scanning electron microscopy.

At the base of a rat cochlea, imprints of the tallest stereocilia are clearly seen for all three rows of OHCs.

scale bar: 10 µm

See also the electronic zoom.

In this transmission electron micrograph, the tallest stereocilium of an OHC is just detached (by the fixation procedure) from the tectorial membrane.

scale bar: 1 µm