Site of Cortical Utricular Representation with Special Reference to the Somatosensory Barrel Field in the Gerbil

Abstract

In mammals, the osseous semicircular canals of the vestibular labyrinths are usually embedded in the pyramis of the temporal bone. Thus, the osseous semicircular canals are a cavity system that can only be visualized by injection molding. Exceptionally, the walls of the osseous semicircular canals of the Mongolian gerbil are not embedded, but exposed in the hollow space in the temporal bone. Under urethane anesthesia, a window was made in the periotic capsule of the gerbil to reach this hollow space (semicircular canal triangle), and a pair of wire electrodes were inserted through an opening made in the horizontal semicircular canal toward the utricular nerve endings. Repetitive electric stimuli at 80 Hz were applied, and the direction of eye movements was noted. Subsequently, the stimulation frequency was reduced to 0.3 Hz, and evoked potentials were recorded from the neocortex. Positive surface potentials and negative deep potentials were recorded in the somatosensory area and, more specifically, in the vibrissa “barrel field,” as judged by later histochemical staining of the cortical specimens. This unique anatomic feature of the gerbil labyrinth offers an opportunity whereby the vestibular organ can be reached without any heavy surgical insult, and the presence of fine-grain vibrissa barrels in this species (seven rows instead of five rows in most rodents) will help elucidate functional interactions between vestibular and somatosensory sensations.