Gravity Reception

Abstract

The orientation of animals with respect to the direction of the Earth’s gravitational field is effected, no matter how high their organizational level, with the aid of the otolith apparatus. In invertebrates, this apparatus is generally represented by statocysts, while in vertebrates its functions are performed by the vestibular portion of the labyrinth. A gravitational stimulus is associated with the function of mechanoreceptor cells, which at the lower levels of evolution consist of cells containing in their cytoplasm a concretion; this cell is known as a lithocyte. As the organizational level rises, the mechanoreceptor cells can perceive movements of the otolith by means of the flagella or stereocilia, with which the primary sensory cells in the labyrinth of vertebrates are equipped. The flagella of primary sensory cells in the statocysts of invertebrates contain 9×2+2 fibrils in which ATPase is located, but the movement of these flagella is apparently inhibited under normal conditions. In vertebrates, the secondary sensory cells of the labyrinth retain only a single mobile kinocilium, which is “opposed” to a whole bundle of immobile stereocilia; the latter can move, but only under the effect of the shearing forces generated by the otolith when the animal changes its position in the gravitational field of the Earth.