Ultrastructural analysis of inputs around the soma of an intra-cellularly labeled masseter muscle spindle afferent in cat mesencephalic trigeminal nucleus.

Intra-axonal recording and labeling with HRP was performed to examine synaptic inputs around soma of mesencephalic trigeminal neuron (Vme) innervating the masseter muscle-spindles on electron microscopy. Intra-axonal injection of HRP was made in the mesencephalic tract. About twelve boutons contacting the labeled soma were observed from serial ultrathin sections. Among them eight boutons synapsed on the labeled soma, one bouton contacted on the soma but synapsed on an unlabeled dendrite nearby. Serially observing the other three boutons, no synaptic junctions were identified. The synapses between boutons and soma were mainly asymmetrical. Six boutons contained clear spherical vesicles (S-ending) and the other six contained spherical, oviod and flattened, namely pleomorphic clear vesicles (P-ending). Both S-endings and P-endings frequently contained large or small dense-core vesicles. Typical F-endings with predominantly flattened vesicles was not seen, though some P-endings contained more flattened vesicles. The significance of these observations was discussed.