Vestibulospinal projections in the toad. An experimental electron microscopic study

Abstract

An experimental electron microscopical study has been made on the mode of termination of the vestibulospinal projections in the toad. Degenerating fibers and boutons were observed in the ventromedial region of the spinal cord in which five types of terminals were identified in normal preparations: S-type, F-type, C-type, NfS-type and NfF-type. The latter two types represent the largest terminals in the spinal gray. In the medial region of the ventral horn they seem to belong to supraspinal fibers. Following electrolytic lesions of the dorsal and ventral nuclei nervi octavi, some of the NfF-type of boutons undergo filamentous degeneration which progresses into the dark type at longer survival periods (7–20 days). In two animals, a few NfS-type of boutons also display filamentous hypertrophy. Problems connected with the filamentous type of degeneration in amphibians and with the identification of bouton types belonging to the vestibulospinal pathway are discussed. Degenerating fibers and boutons are seen at all levels of the spinal cord, ipsilateral and also, in smaller number, contralateral to the lesions. The vestibulospinal fibers terminate predominantly on large and small dendrites, and only occasionally on the somata of large neurons on the ipsilateral side.