Brownian Motion in the Cochlear Partition

Abstract

Estimates have been made of the Brownian-pressure fluctuations at the eardrum. These are 15 dB below threshold (MAP) for 3000 cps. It is thus necessary to look into the ear itself to see whether thermal fluctuations limit the sensitivity of the ear. It is assumed that the first stage of amplification of acoustical energy takes place at the hair cell, and that it is the relative shear displacement between the top of the hair cell and the stereocilia that is the relevant parameter. Two estimates of Brownian fluctuation are made. The first assumes a rigid connection between the stereocilia and the tectorial membrane, and thus the source of noise is the fluctuation in amplitude of the basilar-membrane displacement. The second estimate assumes a connection rigid enough to provide the greatest flow of acoustical energy to a hair cell, i.e., proper impedance match, but which is loose enough so that each hair cell has independent thermal displacements. The impedance used in making these estimates were based on the in vivo measured impedances at the eardrum and the anatomy of the middle ear. The first estimate gives a signal to noise ratio of 15 dB at threshold for 3000 cps. The second estimate gives a signal to noise ratio of −37 dB. This calculation shows that it is necessary to assume that the stereocilia are attached to the tectorial membrane and that the hair cell bodies are embedded in the organ of Corti with a high degree of rigidity. There is no direct experimental evidence for the assumption of rigid attachment, although there is some indirect evidence from ultrastructural studies.