The stereocilia of hair cells have a molecular structure similar to those described on page 110. Tightly packed actin filaments cross-linked by fimbrin and espin (actin-bundling proteins) form their internal core structure. The high density of actin filaments and the extensive cross-linking pattern  imparts rigidity and stiffness to the shaft of the stereocilium. The shaft tapers at its proximal end near the apical surface of the cell, where the core filaments of each stereocilium are  anchored within the terminal web (cuticular plate). When stereocilia are deflected, they pivot at their proximal ends like stiff rods (see Fig. 25.10). Transmission electron microscope  examination of the distal free end of the stereocilium reveals an electron-dense plaque at the cytoplasmic site of the plasma membrane. This plaque represents mechanoelectric transducer (MET) channel protein. A fibrillar cross-link called the tip link connects the tip of the stereocilium with the shaft of an adjacent longer stereocilium (see Fig. 25.10). The tip link plays an important role in activating the MET channels at the tip of the stereocilia and opening additional transduction K channels at the site of its attachment to the shaft of neighboring stereocilium (see Fig. 25.10). The molecular structures of the transduction K channels and tip links are as yet unknown. Individual stereocilia are also connected by a variety of fibrillar extracellular cross-links.
A mutation that disrupts the gene that produces espin causes cochlear and vestibular symptoms in experimental mice. They lose their hearing early in life; these animals also spend most of their time walking or spinning in circles. The stereocilia of these animals do not maintain the rigidity necessary for proper functioning of the MET channels.
All hair cells use mechanically gated ion channels to generate action potentials.
All hair cells of the internal ear appear to function by moving (pivoting) their rigid stereocilia. Mechanoelectric transduction