RESPONSE OF MAMMALIAN VESTIBULAR NEURONS TO HORIZONTAL ROTATION AND CALORIC STIMULATION

Abstract

By inserting a needle electrode in the vestibular nerve of the cat, single functional units have been isolated and those belonging to the horizontal semicircular canal selected. In most cases the units showed spontaneous activity. In other instances it was possible to induce a formerly inactive neuron to signal a gradual increase in the velocity of rotation. Upon rotation of the cat with its head placed in the center of the rotating slab (to eliminate any "linear" component in the stimulation) and with the horizontal semicircular canal in the horizontal plane, one can distinguish with certainty 2, probably 3, types of reaction to rotation in both directions I. The discharge is accelerated by rotating to the side of recording and inhibited by rotation in the opposite direction (of 100 units studied 83 gave this type of response); II. an excitatory response to rotation in both directions was noted in 12% of the units; III. an inhibitory response to rotation in both directions was noted in the remaining cases. In the expts. with caloric stimulation, a soft rubber tube attached to a syringe was placed in the external auditory canal. To caloric stimulation with cold (20[degree]C) or hot (43[degree]C) water, the discharge in the units activated by horizontal semicircular canal varied in different ways, depending upon the angle formed by the canal and the horizontal plane. For the majority of units it followed the rule to be expected from indirect stimulation by movement of the endolymph according to the theory of Barany. In addition, a direct effect on some peripheral nervous structure was noted, consisting in inhibition by cold and excitation by heat. Only a limited number of units were susceptible to this direct effect which was of very long duration. The indirect effects were much more transitory. The thresholds were detd. for Types I and II and found to vary over as large a range of acceleration as from 50 to 30[degree] per sec.