Effects of caloric stimuli on frog ampullar receptors

Abstract

The observation that caloric nystagmus can be evoked even in microgravity conditions argues against Barany's convective theory. To justify this result, gravity-independent mechanisms (mainly endolymphatic volume changes and direct action of the temperature on vestibular sensors) are believed to contribute to caloric-induced activation of vestibular receptors. To define the importance of both gravity-dependent and gravity-independent mechanisms, the posterior semicircular canal of the frog was thermally stimulated by a microthermistor positioned close to the sensory organ. The stimulus produced a gravity-dependent transcupular pressure difference that, depending on the position of the heater, could result in either excitation or inhibition of ampullar receptor sensory discharge. When the heater was positioned on the ampulla, or when the canal rested on the horizontal plane, no responses could be evoked by thermal stimuli. These results suggest that, in our experimental conditions (vT up to 1.5³C), neither a thermally induced expansion of the endolymph nor a direct action of the temperature on vestibular sensors play any major role. ß 1999 Elsevier Science B.V. All rights reserved.

A Biomechanical Model of the Inner Ear: Numerical Simulation of the Caloric Test

“…Based on experimental and theoretical analyses, Gentine et al [10–13] and later both Zucca et al [14] and Valli et al [15] concluded that the gravity-dependent buoyancy force is the largest contributor to the caloric response. However, Kassemi et al [16] argued that the previous researchers did not incorporate the effects of fluid dynamics, and they further discovered through a finite element (FE) study that the dominant mechanism is the natural convection driven by the temperature-dependent variation in the bulk fluid.…”