Supraspinal projections: origins and their terminals in the mouse spinal cord

The present study explored the origins of supraspinal projections in the mouse by injecting FG into the cervical cord. The majority of FG labelled cell groups were similar to those found in large mammals. The precuneiform and the epirubrospinal nuclei were found to project to the spinal cord for the first time. Next, we investigated the vestibulospinal projections arising from the lateral vestibular nucleus (LVe). FG labeled neurons at the caudal level of LVe formed two clusters. The boundary between LVe and MVe was tentatively defined. BDA injections into LVe led to the revelation of vestibulospinal fibers mainly in laminae 6-9 with an ipsilateral predominance. In the lumbar and sacral cord, collaterals extended from the ipsilateral to the contralateral side and mainly terminated in laminae 6-8. Next, termination pattern of raphespinal fibers arising from the hindbrain raphe nuclei was studied. These fibers terminate in both the dorsal and ventral horns in addition to lamina 10. There was a shift of fibers in the ventral horn towards the dorsal and lateral part of the gray matter. Fibers from the adjacent gigantocellular reticular nuclei show similar termination pattern as those from the raphe nuclei with slight difference. CLARITY experiments revealed that serotoninergic fibers are present in all laminae and form bundles regularly in a short distance along the rostrocaudal axis in the medial part of the ventral horn and extend towards the lateral motor neuron column.