Mesencephalic trigeminal nucleus neurons supplying the jaw closing muscles have no spinal projection: a fluorescent double-labeling study in birds and mammals.

Abstract

The present study deals with the possibility that the mesencephalic trigeminal nucleus (MeV) neurons that innervate the muscle spindles of the jaw closing muscles may also have collaterals projecting to the cervical spinal cord. At the same time, we reexamine the morphology of these cells and their location within the MeV. The fluorescent retrograde tracers Fast Blue (FB) and Diamidino Yellow dihydrochloride (DY) were injected into the jaw closing muscles and C2-C3 spinal cord segments, respectively, of duck, rat, and rabbit in one series of experiments. In a second series of animals, the targets of the tracers were reversed. Retrogradely double-labeled cells (FB+DY) were not found in the MeV. On the contrary, the tracer injected into the muscles retrogradely labeled only large unipolar MeV cells, whereas the tracer injected into C2-C3 spinal cord segments labeled only small multipolar cells which were intermingled with the MeV somata of muscle spindle afferents. These findings exclude the possibility of spinal projections via collaterals of MeV cells supplying muscle spindles of jaw closing muscles in duck, rat, and rabbit. Moreover, the retrograde double-labeling technique evidences two cellular populations within the MeV of the duck, rat, and rabbit: large unipolar neurons which are the cell bodies of primary afferents from jaw closing muscles and small multipolar cells projecting to the upper cervical spinal cord.