Morphology of lumbar-projecting lateral vestibulospinal neurons in the brainstem and cervical spinal cord in the squirrel monkey

The lateral vestibulospinal tract (LVST) is one of the major descending pathways controlling the extensor musculature of the body. To determine whether individual LVST neurons terminating in the lumbosacral spinal segments issue collaterals more rostrally to exert an influence of the cervical ventral horn intracellular recording and biocytin labeling techniques were used in the squirrel monkey. Only neurons monosynaptically related to the 8th nerve and antidromically identified to project below T12 were selected for study. The axon course through the brainstem and cervical spinal cord was examined in 37 LVST neurons. The average distance of recovered axon was 17.3 mm (4.5-31.7 mm). None could be antidromically activated from shocks applied to the rostral medial longitudinal fasciculus near the 3rd nuclei; and no collaterals were observed in the brainstem. Of the 37 neurons, only 1 axon issued a collateral to innervate the ventral horn, primarily in the region of the spinal accessory motoneurons; this single collateral provided a relatively minor input compared to that of LVST neurons terminating in the cervical cord. Thus, secondary, caudal-projecting LVST neurons represent a private, and mostly rapid, communication pathway between dorsal Deiters' nucleus and the motor circuits controlling the lower limbs and tail.