Static material properties of the tectorial membrane: a summary

The tectorial membrane (TM) is a polyelectrolyte gel. Hence, its chemical, electrical, mechanical, and osmotic properties are inextricably linked. We review, integrate, and interpret recent findings on these properties in isolated TM preparations. The dimensions of the TM in alligator lizard, chick, and mouse are sensitive to bath ion concentrations of constituents normally present in the cochlear fluids - an increase in calcium concentration shrinks the TM, and an increase in sodium concentration swells the TM in a manner that depends competitively on the calcium concentration. The sodium-induced swelling is specific; it does not occur with other alkali metal cations. We interpret these findings as due to competitive binding of sodium and calcium to TM macromolecules which causes a change in their conformation that leads to a change in mechanical properties. In mouse TM, decreasing the bath pH below 6 or increasing it above 7 results in swelling of the TM. Electric potential measurements are consistent with the notion that the swelling is caused by a pH-driven increase in positive fixed charge at low pH and an increase in the magnitude of the negative fixed charge at high pH which is consistent with the known protonation pattern of TM macromolecules. Increasing the osmotic pressure of the bathing solution with polyethylene glycol shrinks the TM and decreasing the ionic strength of the bathing solution swells the TM. Both results are qualitatively consistent with predictions of a polyelectrolyte gel model of the TM.