Patterns of projection and braching of reticulospinal neurons

Extracellular microelectrodes were used to record the activity of reticulospinal neurons within the medial ponto-medullary reticular formation in the cat. In one series of experiments reticulospinal neurons were activated from electrodes in the ventro-medial reticulospinal tract (RSTm) and in the ipsi- and contralateral lateral reticulospinal tracts (RSTi, RSTc) at spinal levels C1--2, C4, Th1 and L1. RSTm neurons were found primarily in n.r. pontis caudalis and the rostro-dorsal part of n.r. gigantocellularis. 71% of these neurons projected as far as the lumbar spinal cord. RSTi neurons projecting to C4 and beyond were clustered in the caudo-ventral part of n.r. gigantocellularis, but those RSTi neurons projecting to the first three cervical segments were located more rostro-dorsally. In all, 63% of the RSTi neurons projected to the lumbar spinal cord. RSTc neurons, which comprised only 5% of the reticulospinal population, were found throughout n.r. gigantocellularis. RSTm neurons had a median conduction velocity of 101 m/sec whereas RSTi and RSTc had median conduction velocities on the order of 70 m/sec. In a second series of experiments microstimulation was used to activate branches of reticulospinal neurons within the gray matter of the cervical enlargement. Twenty-two of thirty-three neurons found to project to the cerivcal ventral horn were branching neurons that also sent axons to the lumbar spinal cord. Thus much of the teticulospinal activity reaching the cervical enlargement also acts at one or more other spinal levels. Detailed investigation of the course of reticulospinal axons within the cervical gray matter indicated that a single axon may traverse wide areas of the ventral horn including regions on both sides of the spinal cord.