The inhibitory vestibular efferent system and its relation to the cerebellum in the frog.

Abstract

1. In the frog, peripheral stimulation of the anterior and posterior branches of the VIIIth nerve evokes antidromic activation of Purkinje cells in the ipsilateral cerebellar auriculum. 2. The antidromic Purkinje cell invasion is followed by orthodromic action potentials due to the activation of direct and secondary vestibulo-cerebellar afferents. 3. In order to demonstrate the existence of vestibular efferent fibers at the level of the vestibular nerve, its anterior or posterior branches were electrically activated and a recording electrode placed proximally in the same nerve. Under these circumstances, efferent fiber action potentials with an average latency of 3 msec could be recorded, even after the peripheral vestibular organ had been removed. 4. Comparison of the latency for orthodromic activation of Purkinje cells at the auricular lobe with that of the action potentials recorded at the vestibular nerve level agrees very well, there being on the average a slight lead at the Purkinje cell level which can be explained by the conduction time (0.6 msec) from the cerebellum to the vestibular organ. 5. The vestibular efferent system could not be easily activated by moderate angular acceleration adequate for afferent fiber activation, or by tilting or vibratory stimulation. On some occasions, contralateral rotation evoked efferent discharge. 6. Activation of the vestibular efferent system following VIIIth nerve or auricular stimulation produces an inhibition on the spontaneous activity of saccular and utricular afferents for an average period of 15 msec. 7. It is therefore concluded that a direct cerebello-otolithic efferent system represented by axons of Purkinje cells is present in the frog. This cerebello-otolithic system, which is shown to be inhibitory, represents the first demonstration of cerebellar control of a sensory input.