Direct connections of primary trigeminal afferent axons with trigeminocerebellar projection neurons in the border zone of rat trigeminal nucleus oralis.

This study presents electron microscopical evidence for direct synaptic contact between primary trigeminal axonal (PR) endings and trigeminocerebellar projection neurons (TCPNs), in the border zone (BZ) of rat trigeminal nucleus oralis. The combined techniques of anterograde degeneration following trigeminal sensory root rhizotomy and retrograde transport of horseradish peroxidase subsequent to injections into the orofacial tactile portions of crura I and II of the cerebellar hemispheres were utilized. Degenerating PR endings lie centrally in glomeruli where they form axodendritic synapses on higher order dendrites of identified BZ TCPNs. It is at these synapses that the PR endings are probably transferring tactile input to BZ TCPNs for direct relay to the cerebellar cortex. Transmitter release at the axodendritic synapses may be modified within the glomeruli by axoaxonic synapses between terminals containing flattened synaptic vesicles and the PR ending.