Synaptic relations of the trigeminal motoneurons in a frog (Rana esculenta).

The cobalt tracing technique was used to study synaptic relations of trigeminal motoneurons and the axon terminals of the mesencephalic trigeminal nucleus (Vmes) in a frog (Rana esculenta). Most of the small calibre Vmes terminals containing spherical synaptic vesicles occupied presynaptic position in axodendritic contacts. Axoaxonic contacts of Vmes boutons were exclusively found at the postsynaptic site. The synaptic coverage of the trigeminal motoneuron dendritic tree increased somatofugally along the dendrites. Monosynaptic contacts between the trigeminal motor and mesencephalic nuclei occur very rarely indicating that this type of activation does not play a significant role in the frog.