Ultrastructure of jaw muscle spindle afferents within the rat trigeminal mesencephalic nucleus

This study examined the ultrastructures of neuronal elements within trigeminal mesencephalic nucleus by labeling masseteric mesencephalic neurons and masseter motoneurons with injection of horseradish peroxidase into masseteric muscle. Of eight horseradish peroxidase-labeled muscle spindle afferents examined, four terminals showed synaptic contact with labeled dendrites of masseteric motoneurons, two with labeled somata, and the remaining two with unlabeled dendrites. A few of the labeled dendrites showed intimate contact with the somata of the trigeminal mesencephalic nucleus neurons. These results provide morphological evidence of synaptic contact of recurring masseteric muscle spindle afferents with the trigeminal mesencephalic nucleus somata and also suggest the presence of electrical synapses between the somata of the trigeminal mesencephalic nucleus neurons and dendrites of jaw-closing motoneurons.