Segment-specific branching patterns of single vestibulospinal tract axons arising from the lateral vestibular nucleus in the cat: A PHA-L tracing study.

The purpose of the present study was to detail the spinal cord (SC) trajectories and arborization patterns of vestibulospinal axons descending from the lateral vestibular nucleus (LVN). An anterograde neural tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L), was focally injected into the right-side LVN in 8 cats. Their subsequent survival times varied from 4 days to 12 weeks. The labeled axons were found mainly in the brainstem after 4-5 days and in successively more caudal spinal segments after longer survival times: i.e., in C1-T2 after 2-3 weeks, in C3-T11 after 6-7 weeks, and in T7-S1 after 10-12 weeks. The trajectories of 28 single, thick (diameter >/=2.4 microm) lateral vestibulospinal tract (LVST) axons were traced from serial transverse sections of the SC from C1-8 (n = 10), T1-9 (n = 11), and T11-L7 (n = 7). In the cervical segments, the LVST axons gave off collateral fibers, which terminated mainly in Rexed's laminae VII-VIII. The terminal-field patterns of these collaterals differed from one stem axon to another. In the thoracic segments, the terminal-field patterns from a given LVST axon were similar at each segmental level, i.e., a few main branches with or without short side branches. At the L3-5 midlumbar level, the collaterals usually arborized more extensively, such that their terminal fields occupied a much greater region of laminae VII-VIII. In contrast, at the L6-7 lower lumbar level, collaterals arising from thin axons (diameter <1.0 microm) tended to innervate, with even more extensive arborization, the medial part of the lamina VIII. These results revealed common and segment-specific collateral distribution patterns of LVST axons along the full extent of the spinal neuraxis.