Vestibulospinal, Reticulospinal and Interstitiospinal Pathways in the Cat

Experiments on the properties and motor actions of three descending systems, the vestibulospinal, reticulospinal and interstitiospinal tracts, have been reviewed. The vestibulospinal tracts are the most direct pathways between the labyrinth and spinal motoneurons. The MVST is the predominant direct pathway to axial motoneurons, the LVST the only direct pathway to limb motoneurons; not much is known about the recently discovered caudal vestibulospinal tract. The role of these direct pathways in functionally meaningful vestibulospinal reflexes remains to be determined. The reticulospinal tracts consist of three groups of descending fibers: one descending in the ventromedial funiculus (RST,), one in the ipsilateral ventrolateral funiculus (RST,), and one in the contralateral ventrolateral funiculus (RST,). Excitatory RST, neurons scattered throughout nucleus reticularis (n.r.) pontis candalis and the dorsal part of n.r. gigantocellularis establish direct synaptic connections with motoneurons supplying a wide variety of muscles throughout the body. RST, and RST, neurons located in n.r. gigantocellularis and ventralis establish direct inhibitory and excitatory connections with neck and direct excitatory connections with back motoneurons, but do not establish direct connections with limb motoneurons. The reticulospinal systems receive major direct inputs from many different regions including vestibular nuclei, suggesting that they participate in vestibulospinal reflexes. The interstitiospinal tract, which has not been studied extensively, includes neurons that establish direct excitatory connections with neck motoneurons, but do not establish direct connections with limb and back motoneurons.