DISPLACEMENT CONFIGURATION OF SEMICIRCULAR CANAL CUPULAE

Cupula displacement had traditionally been portrayed as a wiper motion of the cupula hinged at its base. In this study, semicircular canals of the bullfrog, which had been carefully dissected, were analyzed for cupula motion during compression of the canal wall. Cupulae marked with darkened oil droplets or alcian blue and displaced by endolymph exhibited a characteristic central bulging while the perimeter remained fixed to the ampullary wall. Small compressions resulted in displacements which were prominent near the receptor base while the center of the cupula mass had little movement. Large endolymph displacements produced a nearly symmetrical arching of the marker line that extended from the crista to the opposite wall. When endolymph was excessively displaced, the cupula became dislodged at its apical end and moved through the traditional wiper arc without the characteristic bulging. It is concluded that the cupula was dislodged in earlier studies by other investigators while, in the intact canal, the cupula has the properties of a modified elastic diaphragm. On the basis of these results, we suggest that the base of the cupula shears with the crista across the subcupular space. © 1979.