I found that these bilaterally projecting neurons also innervate multiple other orofacial motor nuclei, premotor regions, and midbrain regions implicated in motor control.
Moreover, these studies uncovered two distinct roles of supratrigeminal bilaterally projecting neurons in bilaterally synchronized activation of masseter muscles, and active balancing of bilateral masseter muscle tone against an excitatory input.
These studies identified a population of bilaterally projecting neurons in the supratrigeminal nucleus as essential for maintenance of an appropriate level of masseter activation during natural chewing behavior in the freely moving mouse.
Finally, labeling of bilaterally projecting MesV primary afferent neurons, although very few, was another unexpected finding.
Thus, such bilaterally projecting MesV neurons could be another case that only transiently exists in neonatal animals.
Furthermore, we observed axon terminals of premotor neurons transsynaptically labeled from left masseter motoneurons extensively innervating the contralateral (right) MoV and forming boutons opposing motoneurons, supporting the existence of large numbers of bilaterally projecting premotor neurons.
Previous studies have implicated commissural interneurons located inside MoV and bilaterally projecting neurons extending axons to both left and right MoV in some brainstem regions as a potential source of bilaterally coordinated inputs (Appenteng and Girdlestone, 1987; Ter Horst et al., 1990; Juch et al., 1993; Kamogawa et al., 1994; Yoshida et al., 2005; McDavid et al., 2006).
However, because MoV contains both motoneurons and interneurons (Appenteng and Girdlestone, 1987; Ter Horst et al., 1990; Juch et al., 1993; McDavid et al., 2006), it is possible that these bilaterally projecting premotor neurons synapse with motoneurons on the ipsilateral side, and with interneurons on the contralateral side.
We have shown that when injections are made into both sides of the LPb or CVLM, most lamina I cells in L4 that are labelled from the ipsilateral side are also labelled from the corresponding site on the contralateral side, which suggests that the majority of lamina I cells have purely contralateral projections, while a smaller number project bilaterally (Spike et al., 2003).
L5 MOs neurons also project bilaterally to amygdala nuclei, specifically the anterior basolateral amygdala (BLAa) and capsular central amygdala (CEAc) (Table 1 and Supplementary Movie S1).
