Vestibular thermodynamics and motion detection: A complementary hypothesis

OBJECTIVE: An inner-ear thermodynamic mechanism that contributes to the detection of motion is proposed. Endolymph in the semicircular ducts loses thermal energy at a constant rate via direct conduction and radiation to the adjacent perilymph and to the middle and external ear canals. Consequently, endolymph displaced by inertial movement during head rotation reaches the cupular hair cell coupling at a lower temperature than the vascular crista. This thermal gradient may have a complementary role in the stimulation and instantaneous response of sensory cilia, initiating (in the central nervous system) the identification of movement. Models for the mechanism by which humans detect motion do not satisfactorily explain the precise mode of ciliary cell stimulation. Current thinking supports a diaphragm- cupula model. METHODS: Literature review. CONCLUSIONS: Endolymph in the human semicircular ducts loses heat at a constant rate by conduction and radiation via the adjacent perilymph to the bone of the otic capsule and to the middle and external ear. During head rotation, relatively cooler endolymph displaced by inertial movement from the duct affects the constant arterial temperature environment around the crista. These instantaneous thermal changes act as cofactors to the Ca++ and K+ channels that respond to the bending of the stereocilia, create changes in the mechanism that modulates the discharge rate of the primary vestibular afferent fibers, allowing detection of motion. The speed of thermal transfer by radiation matches that of the proprioreceptive and visual systems; this thermal transfer plays a complementary role to current models of balance and equilibrium in humans.