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. Anterograde tracer injections into the Su5 revealed that the Su5 sends contralateral projections (or bilateral projections with a contralateral predominance) to the Su5, basilar pontine nuclei, pontine reticular nucleus, deep mesencephalic nucleus, superior colliculus, caudo-ventromedial edge of the ventral posteromedial thalamic nucleus, parafascicular thalamic nucleus, zona incerta, and lateral hypothalamus, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) to the intertrigeminal region, trigeminal oral subnucleus, dorsal medullary reticular formation, and hypoglossal nucleus as well as the Mo5. Retrograde tracer injections into the Su5 demonstrated that the Su5 receives bilateral projections with a contralateral predominance (or contralateral projections) from the primary and secondary somatosensory cortices, granular insular cortex, and Su5, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) from the dorsal peduncular cortex, bed nuclei of stria terminalis, central amygdaloid nucleus, lateral hypothalamus, parasubthalamic nucleus, trigeminal mesencephalic nucleus, parabrachial nucleus, juxtatrigeminal region, trigeminal oral and caudal subnuclei, and dorsal medullary reticular formation. These findings suggest that the Su5, which receives JCMS proprioception, has efferent and afferent connections with multiple brain regions that are involved in emotional and autonomic functions as well as orofacial motor functions.