Fine structure of the primary afferent vestibulocochlear terminals in the frog

The synaptology of the primary afferent vestibulocochlear fibers of the frog's nucleus vestibularis lateralis (NVL), nucleus cochlearis dorsalis (NCD) and nucleus saccularis (NS) was studied with the aid of cobalt labeling technique. On the basis of their sizes and morphological characters, two types of boutons could be distinguished. The type one large boutons making only axosomatic contacts were found in the NVL and NS. The form of contact in these boutons was mainly attachment plaque without vesicle accumulation in the contact zone. The second type of bouton having a smaller diameter and spheric vesicles, were engaged both in axosomatic and axodendritic synapses in all three nuclei studied. There was about the same number of axosomatic and axodendritic boutons in the NCD and NS; while the numerical distribution of type one and type two axosomatic boutons was similar in the NVL and NS. On the basis of our results it is suggested that the NS may be a transitory form between the NVL and NCD.