Effects of caloric stimuli on frog ampullar receptors

The observation that caloric nystagmus can be evoked even in microgravity c onditions argues against Barany's convective theory. To justify this result , gravity-independent mechanisms (mainly endolymphatic volume changes and d irect action of the temperature on vestibular sensors) are believed to cont ribute to caloric-induced activation of vestibular receptors. To define the importance of both gravity-dependent and gravity-independent mechanisms, t he posterior semicircular canal of the frog was thermally stimulated by a m icrothermistor positioned close to the sensory organ. The stimulus produced a gravity-dependent transcupular pressure difference that, depending on th e position of the heater, could result in either excitation or inhibition o f ampullar receptor sensory discharge. When the heater was positioned on th e ampulla, or when the canal rested on the horizontal plane, no responses c ould be evoked by thermal stimuli. These results suggest that, in our exper imental conditions (Delta T up to 1.5 degrees C), neither a thermally induc ed expansion of the endolymph nor a direct action of the temperature on ves tibular sensors play any major role. (C) 1999 Elsevier Science B.V. All rig hts reserved.