The ultrastructural morphology and distribution of trigemino-hypoglossal connections labeled with horseradish peroxidase.
Axon terminals projecting to the hypoglossal nucleus have been identified and characterized by electron microscopy following injections of horseradish peroxidase (HRP) into pars interpolaris of the spinal trigeminal nucleus (SPVN) in adult rats. Over 70% of the anterogradely labeled terminals contained spherical vesicles (S-terminals) and their synaptic densities were chiefly asymmetrical (Gray Type I). The rest (28%) of the labeled terminals had flattened vesicles (F-terminals) and predominantly established symmetrical (Gray Type II) synaptic contacts. The diameters of labeled terminals were 0.5-2.5 micron. Two-thirds of the S-terminals had diameters less than 1.25 micron, whereas, F-terminals were distributed equally in the higher (greater than 1.25) and lower (less than 1.25) diameter ranges. Most axon terminals ended on dendrites of hypoglossal neurons; some, chiefly F-terminals, formed axosomatic endings. Dendrites had diameters of 0.5-5 micron. The majority of S- and F-terminals ended on dendrites with diameters of less than 2.5 micron. However, more F-terminals (17%) than S-terminals (11%) were presynaptic to dendrites greater than 2.5 micron in diameter. Experiments in which anterograde HRP labeling of trigemino-hypoglossal projections was combined with retrograde WGA-HRP labeling of motoneurons projecting to the tongue, demonstrated that SPVN axons end on dendrites of these motoneurons. Whether some of the trigeminal fibers also terminate on intrinsic hypoglossal interneurons remains to be determined.
D000344:Afferent Pathways / Q000033:anatomy & histology / Q000648:ultrastructure; D000818:Animals; D001370:Axonal Transport; D006735:Horseradish Peroxidase; D007002:Hypoglossal Nerve / Q000033:anatomy & histology / Q000648:ultrastructure*; D008297:Male; D008854:Microscopy, Electron; D051381:Rats; D011919:Rats, Inbred Strains; D014276:Trigeminal Nerve / Q000033:anatomy & histology / Q000648:ultrastructure*; D014279:Trigeminal Nucleus, Spinal / Q000033:anatomy & histology / Q000648:ultrastructure*
D006735:Horseradish Peroxidase