Peripheral organization of the vestibular efferent system in the frog: an electrophysiological study

The distribution and the properties of efferent fibers in vestibular nerve were studied in the isolated frog labyrinth. Electrical stimulation of the central stump of any vestibular nerve branchlet elicited compound action potentials in all the other eighth nerve branchlets, indicating the existence of neural links between the various vestibular organs. The same experimental paradigm, when repeated in frogs with chronic section of the eighth nerve roots, demonstrated that these pathways are efferent collaterals extending to all vestibular organs. There are more collaterals linking the 3 semicircular canals than the otolith organs and the otoliths with the canal organs. Efferent connections in the eighth nerve were preserved in full after ablation of the ipsilateral hemi-cerebellum, suggesting that the efferent pathways probably originate in the brainstem. Intracellular recordings from single afferent fibers of both canal and otolith organs revealed that efferent fiber activation could elicit either inhibition or facilitation of the receptor discharge. It was concluded that the frog efferent vestibular system is endowed with non-selective control channels which allow single neurons to influence the receptor activity of different labyrinthine organs.