The Vestibular System

Hair cells of the guinea pig cochlea were preserved for electron microscopic examination by fixing in glutaraldehyde without the use of osmium. An extensive array of cross-links was seen between the stereocilia, by both scanning and transmission electron microscopy. The stereocilia were linked together laterally, particularly near their apical ends, by links running approximately at right angles to the long axis of the stereocilia. One set joined stereocilia of the same row, and another set joined stereocilia of the different rows, holding the tips of the shorter stereocilia in towards the longer stereocilia of the next row. In addition, the tip of each shorter stereocilium on the hair cell gave rise to a single, upwards-pointing link, which ran up to join the taller stereocilium of the next row. We suggest that distortion of this link would give rise to sensory transduction. On this basis, we are able to explain the V shape of the rows of stereocilia on outer hair cells. Within the rows, the three-dimensional arrangement of the stereocilia was different from that seen conventionally. Rather than standing parallel, the stereocilia of the different rows tapered in together at the tips, presumably held by the laterally-running cross-links. In addition, a membrane roughness, particularly pronounced in the region of the stereocilium which gives rise to the cross-links, was seen. However, the lateral and basal surface membranes of the hair cell, and the membranes of the internal organelles, had a more conventional appearance.

The receptor current of hair cells from the bullfrog's sacculus was measured by voltage clamp recording across the isolated sensory epithelium. Several hundred hair cells were stimulated en masse by moving the overlying otolithic membrane with a piezoelectrically activated probe. As measured by optical recording of otolithic membrane motion, the step displacement stimuli reached their final amplitudes of up to 1 micrometer within 100 microseconds. The relationship between displacement and steady-state receptor current is an asymmetric, sigmoidal curve about 0.5 micrometer in extent. The time constant of the approach to steady state depends upon the magnitude of the hair bundle displacement and ranges from 100 to 500 microseconds at 4 degrees C; the time course is faster with larger displacements or at higher temperatures. Both the displacement-response curve and the kinetics of the response are changed by alterations in the Ca2+ concentration at the apical surface of the cells. The characteristics of the response are not consistent with simple models for the transduction process that involve enzymatic regulation of channel proteins or diffusible second messengers. Mechanical stimulation is instead posited to act directly by altering the free energy difference between the open and closed forms of the transduction channel, thereby inducing a redistribution between these states. The dependences of the response kinetics on displacement and on temperature suggest that the thermal interconversion between open and closed transduction channels is limited by an enthalpy of activation of about 12 kcal/mol.