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).

Efferent and afferent connections of supratrigeminal neurons conveying orofacial muscle proprioception in rats.

The supratrigeminal nucleus (Su5) is a key structure for controlling jaw movements; it receives proprioceptive sensation from jaw-closing muscle spindles (JCMSs) and sends projections to the trigeminal motor nucleus (Mo5). However, the central projections and regulation of JCMS proprioceptive sensation are not yet fully understood. Therefore, we aimed to reveal the efferent and afferent connections of the Su5 using neuronal tract tracings.

Ascending projection of jaw-closing muscle-proprioception to the intralaminar thalamic nuclei in rats.

An invasive intralaminar thalamic stimulation and a non-invasive application of oral splint are both effective in treating tic symptoms of patients with Tourette syndrome (TS). Therefore, these two treatments may exert some influence on the same brain region in TS patients. We thus hypothesized that the proprioceptive input arising from the muscle spindles of jaw-closing muscles (JCMSs), known to be increased by the application of oral splint, is transmitted to the intralaminar thalamic nuclei.

Transcortical descending pathways through granular insular cortex conveying orofacial proprioception.

Our motor behavior can be affected by proprioceptive information. However, little is known about which brain circuits contribute to this process. We have recently revealed that the proprioceptive information arising from jaw-closing muscle spindles (JCMSs) is conveyed to the supratrigeminal nucleus (Su5) by neurons in the trigeminal mesencephalic nucleus (Me5), then to the caudo-ventromedial edge of ventral posteromedial thalamic nucleus (VPMcvm), and finally to the dorsal part of granular insular cortex rostroventrally adjacent to the rostralmost part of secondary somatosensory cortex (dGIrvs2).

Thalamo-insular pathway conveying orofacial muscle proprioception in the rat.

Little is known about how proprioceptive signals arising from muscles reach to higher brain regions such as the cerebral cortex. We have recently shown that a particular thalamic region, the caudo-ventromedial edge (VPMcvm) of ventral posteromedial thalamic nucleus (VPM), receives the proprioceptive signals from jaw-closing muscle spindles (JCMSs) in rats. In this study, we further addressed how the orofacial thalamic inputs from the JCMSs were transmitted from the thalamus (VPMcvm) to the cerebral cortex in rats.

Thalamic afferent and efferent connectivity to cerebral cortical areas with direct projections to identified subgroups of trigeminal premotoneurons in the rat.

The roles of supramedullary brain mechanisms involved in the control of jaw movements are not fully understood. To address this issue, a series of retrograde (Fluorogold, FG) and anterograde (biotinylated dextran amine, BDA) tract-tracing studies were done in rats. At first, we identified projection patterns from defined sensorimotor cortical areas to subgroups of trigeminal premotoneurons that are located in defined brainstem areas.

Distribution of premotoneurons for jaw-closing and jaw-opening motor nucleus receiving contacts from axon terminals of primary somatosensory cortical neurons in rats.

To clarify features of direct projections from the primary somatosensory cortex (S1) to premotoneurons for the jaw-closing (JC) and jaw-opening (JO) components of the trigeminal motor nucleus, biotinylated dextranamine (BDA) and Fluorogold (FG) were used as the anterograde and retrograde tracers. The BDA and FG injections were made in the S1 and the JC or JO component, respectively, in rats. The distribution of FG-labeled JC and JO premotoneurons receiving contact(s) from BDA-labeled axon terminals of S1 neurons was quantitatively examined; the contacts were identified microscopically by using a X100 oil immersion objective.