Neuronal Mechanisms of Cerebral Cortex Controlling Trigeminal Mesencephalic Spindle Atfetents for Smooth Mastication

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In subprimates, projections from the cerebral cortex to the trigeminal motor nucleus (Vmo) are considered to be not direct, but indirect through premotoneurons in the brainstem. However, the projection patterns of the corticofugal direct projections to the premotoneurons have not been examined well. Whereas, the premotoneurons for the Vmo include the mesencephalic nucleus (Vmes) neurons that are primary afferents whose cell bodies are located in the brain. Therefore the present study was conducted to examine the projection patterns morphologically in the rat. At first, we examined the distribution of the premotoneurons for the Vmo by making an injection of a retrograde tracer into the Vmo. The premotoneurons were found in the Vmes ipsilateral to the injection site, and in the trigeminal oral and principal nucleus, parabrachial nucleus, supratrigeminal nucleus, parvocellular reticular formation (pRF) surrounding the Vmo, and pRF medial to the trigeminal sensory nuclei bilaterally with an ipsilateral predominance to the injection site. Then, we examined the distribution of cortical neurons projecting directly to the premotoneurons for the Vmo, by making an injection of the retrograde tracer into the brainstem where the premotoneurons were labeled in the first experiment. The cortical neurons projecting to the reticular formation around the Vmo, the trigeminal oral nucleus, and the Vmes nucleus were found in the primary and secondary motor cortex, the primary somatosensory cortex, and the insular and limbic cortex, respectively.The present study demonstrated several, strong projections from the distinct cortical areas to the Vmo through premotoneurons including the Vmes neurons, which are topographically organized. The present study contributed to reveal complex neuronal mechanisms in the cortical control of jaw movements.