Fatigue and adaptation of the cupular mechanism of the human horizontal semicircular canal: an experimental investigation

Abstract

The nature of the physiological mechanism underlying the response decline (r. d.) of the cupula as the result of prolonged deflexion has been studied in the human horizontal semicircular canal by means of a revolving chair of new design. The normal duration of the after-sensation of turning which follows the application of a standard test stimulus (4° s -2 for 6 s) was determined, and this was followed by a prolonged iso-directional angular acceleration designed to bring about a response decline. The occurrence of this decline was demonstrated by the considerable shortening of the after-sensation duration found to occur when the test stimulus was then reapplied. Further experimental findings are described which lead to the following conclusions: (1) That the response decline of the cupula, as described, is due to end-organ adaptation (Matthews) and not to true physiological fatigue (Bronk). (2) That increase of stimulus strength, with its accompanying increase of the cupular deflexion, involves an increase in the number of activated receptors. (3) That the adapting elements of the cupula show a directional specificity, that is to say, prolonged cupular deflexion in one direction diminishes its subsequent response to deflexions in the same but not in the opposite direction. The theoretical implications of these findings are discussed, and a new theory propounded of the nervous mechanism of the cupula based in part upon the histological studies of Cajal and Lorente de Nó.