Helmholtz Revisited: Direct Mechanical Data Suggest a Physical Model for Dynamic Control of Mapping Frequency to Place Along the Cochlear Partition

Abstract

There exists two and possibly three kinds of motility of the outer hair cells (OHC). The form of the motility first documented by in vitro measurement was evidence of length changes with electric stimulation and modification of the medium (Brownell et al., 1985; Zenner et al., 1985; Flock et al., 1986; Zenner, 1986; Slepecky et al., 1988; Can Ion et al., 1988; Brundin et al., 1989). The origin of this slow motility is not clear but may be due to some form of excitation-contraction coupling (Zenner et al., 1985; Flock et al., 1986; Zenner, 1986b). Isolated OHC have been demonstrated to exhibit fast motility under in vitro conditions with electric current stimulation (Brownell et al. 1985; Zenner et al., 1985; Ashmore, 1987). Electro-osmosis (Kachar et al., 1986) has been speculated to account for fast mechanical events. The OHCs are turgid, and osmotic modulation of their turgidity has been implicate: in the slow changes in cell shape (Brownell, 1984; 1989; Flock, 1986; Dulon et al. , 1988). Although the slow motility has now been demonstrated in several species, its utility has yet to be incorporated into any comprehensive model. Changes in efferent activity have been shown to result in changes in the mechanics by direct measurement (LePage, 1989) suggesting the involvement of tonic activity of the OHC. Changes in the activity of the efferent neurone would likely lead to concurrent changes in the tonus in the targeted OHC. It seems likely therefore, that the slow motility has at least one role associated with the medial efferent system (Liberman, 1988). The group of twenty to fifty OHC which are contacted by a single medial efferent neurone can be classed as a “motor unit” by analogy with the bundle of skeletal muscle fibers contacted by a single motoneurone from the spinal cord. It is possible that central control of the tonic fuing rate of individual efferent neurones may include electromechanical comb-mtering prior to IHC detection (LePage, 1989). OHC tonus may be the basis for central control of selective attention at the auditory periphery. Using otoacoustic emissions Puel el al., (1988) have provided some supportive evidence for the hypothesis.KeywordsHair CellOuter Hair CellBasilar MembraneOtoacoustic EmissionExcitation PatternThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.