Topographic Projections from Cortex to Trigeminal Motoneurons through Premotor Neurons

Objective: The aim of this study was to verify the neuronal characteristics of cortical disynaptic projections to the jaw-opening and -closing motoneurons controlling masticatory jaw-movements, since it is well known that the cortical direct projections to them are none or very weak.

Methods: Injections of a retrograde tracer, Fluorogold (FG) and/or an anterograde tracer, biotinylated dextranamine (BDA) were made in anaesthetized rats in order to reveal (1) the distribution of premotor neurons for jaw-opening or -closing motoneurons, (2) the distribution of cortical neurons projecting to the two kinds of premotor neurons, (3) the distribution of efferent fibers and terminals of the cortical neurons in the brainstem, and (4) the connections between the premotor neurons and the cortical neurons.

Results: Three kinds of brainstem areas which include a large number of premotor neurons for only jaw-opening motoneurons (e.g., reticular formation medial to the jaw-opening motor nucleus), only jaw-closing motoneurons (e.g., intertrigeminal region) or both (e.g., trigeminal oral nucleus and reticular formation medial to the trigeminal interpolar nucleus) were found in the lower brainstem. These three areas received topographic projections from the cerebral cortex; areas including only jaw-opening motoneurons, only jaw-closing motoneurons and both received projection mainly from the secondary motor cortex (M2), the primary motor cortex (M1) and the primary sensory cortex (S1), respectively. Contacts between the cortical neurons and the premotor neurons were also observed.

Conclusion: The present study suggests that the jaw-opening and jaw-closing motoneurons receive strong disynaptic projections from large sensorimotor cortical areas through a large number of premotor neurons in the lower brainstem, and are regulated in a distinct fashion by the distinct motor cortex (M2/M1) and/or in a similar fashion from the sensory cortex (S1).