Kinociliary Mechanoreceptors in the Equilibrium Receptor Organs of Cephalopods

Abstract

The main cellular component of the linear and angular acceleration receptor systems in the equilibrium receptor organs of cephalopods is the mechanoreceptive hair cells. Although a few differences primarily in gross morphology exist, these cells show a striking number of similarities in structure and function with the vertebrate vestibular hair cells. The purpose of this paper is to summarize the main characteristics of the cephalopod hair cells and thus stimulate comparative research that, ultimately, may contribute to a better understanding of the primary features of both vertebrate and invertebrate hair cells. Most cephalopod hair cells are secondary sensory cells, but in some epithelia primary sensory cells do also occur. Unlike vertebrate vestibular hair cells, which have one kinocilium and 50-60 stereovilli (stereocilia), cephalopod statocyst hair cells have between 50 and 150 kinocilia and no stereovilli. All kinocilia of a single cell are of the same length and, consequently, have a different mechanical connection between their tips (membrane junctions instead of tip links), Despite these differences however, the cephalopod hair cells are morphologically and physiologically polarized in a way identical to that of the vertebrate hair cells. Furthermore, their activity is based on almost identical ionic currents, and they operate with the same afferent, as well as are modulated by the same set of efferent, transmitters.