Projections from Regions of the Cerebellar Nuclei Receiving Jaw Muscle Proprioceptive Signals to Trigeminal Motoneurons and Their Premotoneurons in the Rat Pons and Medulla.

The cerebellum plays a crucial role in sensorimotor control through cerebellofugal projections from the cerebellar nuclei. However, little is known about the cerebellofugal projection features involved in jaw sensorimotor control, although the dorsolateral parts of the interposed cerebellar nucleus (IntDL) and medial cerebellar nucleus (MedDL) do receive proprioceptive signals bilaterally from rat jaw-closing muscle spindles (JCMSs). This study aimed to detail the cerebellofugal projection features involved in jaw sensorimotor control. Anterograde tracer was injected into regions of the rat IntDL and MedDL receiving JCMS proprioceptive inputs (i.e., jcms-IntDL and jcms-MedDL). Axon terminals arising from the jcms-IntDL were labeled bilaterally with an ipsilateral predominance in several pontomedullary regions, although very few terminals were labeled in the dorsolateral and ventromedial divisions (5dl and 5vm) of the trigeminal motor nucleus. In contrast, terminals from the jcms-MedDL were labeled bilaterally with a contralateral predominance in several pontomedullary regions and a few terminals were labeled in the contralateral 5dl and 5vm. Thus, the projections from the jcms-IntDL and jcms-MedDL were well segregated. Subsequent retrograde tracer injections into the pontomedullary regions demonstrated that amongst the entire cerebellar nuclei the nucleofugal projections principally arose from the IntDL and MedDL. Additionally, many premotoneurons for the 5dl or 5vm were widely labeled in the pontomedullary regions where many axons from the jcms-IntDL or jcms-MedDL terminated. The various connections involving the jcms-IntDL and jcms-MedDL may play a crucial role in jaw sensorimotor control, mainly through indirect cerebellofugal pathways to the 5dl and 5vm via their premotoneurons.