MESENCEPHALIC TRIGEMINAL NUCLEUS NEURONS SUPPLYING THE JAW-CLOSING MUSCLES HAVE NO SPINAL PROJECTION - A FLUORESCENT DOUBLE-LABELING STUDY IN BIRDS AND MAMMALS

Background: The present study deals with the possibility that the mese ncephalic trigeminal nucleus (MeV) neurons that innervate the muscle s pindles of the jaw closing muscles may also have collaterals projectin g to the cervical spinal cord, At the same time, we reexamine the morp hology of these cells and their location within the MeV, Methods: The fluorescent retrograde tracers Fast Blue (FB) and Diamidino Yellow dih ydrochloride (DY) were injected into the jaw closing muscles and C2-C3 spinal cord segments, respectively, of duck, rat, and rabbit in one s eries of experiments. In a second series of animals, the targets of th e tracers were reversed, Results: Retrogradely double-labeled cells (F B+DY) were not found in the MeV. On the contrary, the tracer injected into the muscles retrogradely labeled only large unipolar MeV cells, w hereas the tracer injected into C2-C3 spinal cord segments labeled onl y small multipolar cells which were intermingled with the MeV somata o f muscle spindle afferents, Conclusions: These findings exclude the po ssibility of spinal projections via collaterals of MeV cells supplying muscle spindles of jaw closing muscles in duck, rat, and rabbit. More over, the retrograde double-labeling technique evidences two cellular populations within the MeV of the duck, rat, and rabbit: large unipola r neurons which are the cell bodies of primary afferents from jaw clos ing muscles and small multipolar cells projecting to the upper cervica l spinal cord. (C) 1997 Wiley-Liss, Inc.