Critical Review of the Concept of Cupula Functon

Abstract

The cupula has long been regarded as the link between the inertial fluid movements in the canal duct and the bending of the sensory cilia. This concept has been challenged by recent experiments, indicating that hair cell stimulation can occur unaffected by position and/or mobility of the cupula. There are, further, indications that contrary to expectation, a direct mechanical deflection of the cupula would have less effect on the nerve response than a fluid displacement in the duct.

Further investigations have pursued this problem in regard to the following points

1) The cross-section area of the duct is generally small in comparison to the larger volume of the ampulla, resulting in a reduction in cupula deflection without any evident functional advantage.

2) The ratio between these cross-section areas shows variations of between 1:4 and 1: 100 in different animal species without any rational relation to physiological function.

3) The inertial endolymph movement from the canal is not directed towards the surface of the cupula, but towards the entrance to the subcupular space. This seems to indicate that there might exist a direct fluid pathway to the sensory hairs in this area.

4) The position of the ampulla inside the ring of the semicircular canal initiates inertial forces counteracting the deflection of the cupula.

Conclusions from these facts indicate that the assumption of the cupula as an inescapable physiological link between the endolymph in the canal and the sensory hairs, is erroneous and that the function of the cupula instead is that of a damper and a safety device for the protection of the fragile sensory hairs, and that the hairs are stimulated by direct exposure to the inertial endolymph movements in the subcupular space.