ELECTRON-MICROSCOPIC OBSERVATION OF SYNAPTIC CONNECTIONS OF JAW-MUSCLE SPINDLE AND PERIODONTAL AFFERENT TERMINALS IN THE TRIGEMINAL MOTOR AND SUPRATRIGEMINAL NUCLEI IN THE CAT

Previous studies indicate that the trigeminal motor nucleus (Vmo) and supratrigeminal nucleus (Vsup) receive direct projections from muscle spindle (MS) and periodontal ligament (PL) afferents. The aim of the p resent study is to examine the ultrastructural characteristics of the two kinds of afferent in both nuclei using the intracellular horseradi sh peroxidase (HRP) injection technique in the cat. Our observations a re based on complete or near-complete reconstructions of 288 MS (six f ibers) and 69 PL (eight fibers) afferent boutons in Vmo, and of 93 MS (four fibers) and 188 PL (four fibers) afferent boutons in Vsup. All t he labeled boutons contained spherical synaptic vesicles and were pres ynaptic to neuronal elements, and some were postsynaptic to axon termi nals containing pleomorphic, synaptic vesicles (P-endings). In Vmo neu ropil, MS afferent boutons were distributed widely from soma to distal dendrites, but PL afferent boutons predominated on distal dendrites. Most MS afferent boutons (87%) formed synaptic specialization(s) with one postsynaptic target while some (13%) contacting two or three dendr itic profiles; PL afferents had a higher number of boutons (43%) conta cting two or more dendritic profiles. A small but significant number o f MS afferent boutons (12%) received contacts from P-endings, but PL a fferent boutons (36%) received three times as many contacts from P-end ings as MS afferents. In Vsup neuropil, most MS (72%) and PL (87%) aff erent boutons formed two contacts presynaptic to one dendrite and post synaptic to one P-ending, and their participation in synaptic triads w as much more frequent than in Vmo neuropil. The present study indicate s that MS and PL afferent terminals have a distinct characteristic in synaptic arrangements in Vmo and Vsup and provides evidence that the s ynaptic organization of primary afferents differs between the neuropil s containing motoneurons and their interneurons. (C) 1996 Wiley-Liss, Inc.