Otolith-controlled response from the first-order neurons of the labyrinth of the bullfrog ( Rana catesbeiana ) to changes in linear acceleration

Abstract

The experiments reported here represent a study of otolith-controlled vestibular units showing an extreme degree of adaptation in stationary spatial positions. These units cannot, therefore, be characterized as static position sensors. They respond to tilting movements as such, showing an output related to the direction and velocity of the movements. Phase shifts in responses to oppositely directed interrupted and continuous sinu­soidal full-circle tilts about horizontally placed head axes indicate that, beside neuronal adaptation, mechanical factors may contribute to the observed response asymmetries. The possible structural basis for such asymmetries is discussed. A frequency analysis of the recorded data yields Bode plots of fre­quency-dependent phase and gain, and these relations are discussed against the background of various mathematical models suggested by a number of authors. The conclusion is reached that, in contrast to the case of the semicircular canals, it is difficult to fit the phase behaviour of otolith-controlled vestibular sense endings to model equations containing only one or two frequency constants. Evidently, the otolith organ is a mechanically complex system, and its receptor units are characterized by considerable nonlinearities in the transduction process apart from a wide range of adaptation in their responses to constant levels of acceleration. The results are discussed in comparison with recent vestibular re­search, especially on the mammalian labyrinth.