The vestibular type I hair cells: a self-regulated system?

In this study the functional role of afferent nerve calyx surrounding the type I vestibular hair cells was investigated. Synaptic microvesicles were present at the apex of the calyx in the vestibular epithelium of human foetuses at 9 weeks from gestation. Whole cell clamped type I hair cells isolated from guinea pig epithelium presented active movements as shortening of the neck and tilting of the cuticular plate at the cessation of the depolarising step. These movements were calcium dependent. With the aim of establishing the kinetics of calcium influx during the cell depolarisation, intracellular free calcium rate variations were investigated by coupling cytofluorimetry technique with whole cell patch clamp. An increase of intracellular calcium was only observed at the repolarisation of type I hair cells. Thus, a regulatory short-loop is thought to exist to control adaptation phenomena at the upper part of the type I hair cell. It is suggested that this occurs through the release of a neurotransmitter from the apex of the afferent calyx.