Abstract
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.
Highlights
Proprioceptive signals arising from muscles in the entire body play a crucial role in sensorimotor reflexes through the reflex arcs located at the levels of the lower brainstem or the spinal cord
The proprioceptive signals arising from jaw-closing muscle spindles (JCMSs) are conveyed by primary afferents, whose neuronal cell bodies are located in the trigeminal mesencephalic nucleus (Me5), to the jaw-closing motoneurons in the trigeminal motor nucleus (Mo5) in the rostral pons (Luo et al 1995, 2001; Fujio et al 2016; for review, see Dubner et al 1978; Taylor 1990); this monosynaptic reflex arc induces jaw-jerk reflex
The present study has demonstrated the detailed features of efferent and afferent projections of the Su5 in the entire brain
Summary
Proprioceptive signals arising from muscles in the entire body play a crucial role in sensorimotor reflexes through the reflex arcs located at the levels of the lower brainstem or the spinal cord. We have revealed that the Me5 afferents transmit the sensory signals almost exclusively from the JCMSs, but rarely from the periodontal ligaments around upper and lower teeth in the rat (Fujio et al 2016) This finding indicates that the efferent projections from the rat Su5 signals the JCMS proprioception. The JCMS proprioceptive signals via the VPMcvm are subsequently conveyed to the dorsal part (dGIrvs2) of the granular insular cortex (GI) rostroventrally adjacent to the rostralmost part of the secondary somatosensory cortex (S2), whereas the signals via the OPC to the rostral part of the primary somatosensory cortex (S1), the rostral S2, and the rostral GI (Sato et al 2017; Tsutsumi et al 2021) These cortical projection pathways strongly suggest that the JCMS proprioceptive signals are more involved in emotional function than sensory discriminative and motor functions. In the first experiment, we sought to reveal efferent projections of the Su5 in the entire brain by means of anterograde tracer injections into the rat Su5
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