The Cerebellar Cortex Receives Orofacial Proprioceptive Signals from the Supratrigeminal Nucleus via the Mossy Fiber Pathway in Rats

Proprioceptive sensory information from muscle spindles is essential for the regulation of motor functions. However, little is known about the motor control regions in the cerebellar cortex that receive proprioceptive signals from muscle spindles distributed throughout the body, including the orofacial muscles. Therefore, in this study, we investigated the pattern of projections in the rat cerebellar cortex derived from the supratrigeminal nucleus (Su5), which conveys orofacial proprioceptive information from jaw-closing muscle spindles (JCMSs). Injections of an anterograde tracer into the Su5 revealed that many bilateral axon terminals (rosettes) were distributed in the granular layer of the cerebellar cortex (including the simple lobule B, crus II and flocculus) in a various sized, multiple patchy pattern. We could also detect JCMS proprioceptive signals in these cerebellar cortical regions, revealing for the first time that they receive muscle proprioceptive inputs in rats. Retrograde tracer injections confirmed that the Su5 directly sends outputs to the cerebellar cortical areas. Furthermore, we injected an anterograde tracer into the external cuneate nucleus (ECu), which receives proprioceptive signals from the forelimb and neck muscle spindles, to distinguish between the Su5- and ECu-derived projections in the cerebellar cortex. The labeled terminals from the ECu were distributed predominantly in the vermis of the cerebellar cortex. Almost no overlap was seen in the terminal distributions of the Su5 and ECu projections. Our findings demonstrate that the rat cerebellar cortex receives orofacial proprioceptive input that is processed differently from the proprioceptive signals from the other regions of the body.