ANATOMICAL STUDY OF SPINOBULBAR NEURONS IN LAMPREYS

The present study was aimed at identifying spinal neurons ascending to the brainstem outside the dorsal columns in the lamprey. Two retrogra de tracers (cobalt-lysine and horseradish peroxidase [HRP]) were injec ted in the brainstem or rostral spinal cord in vivo or in vitro. Label ed cells were distributed bilaterally with a contralateral dominance, along the whole rostrocaudal extent of the spinal cord. The density of cells markedly decreased rostrocaudally. Several classes of brainstem -projecting neurons were identified. Most cells with a short axon were small and formed columns, in the dorsolateral and ventrolateral gray matter, at the transition between the rhombencephalon and the spinal c ord. Dorsal elongated cells were spindle shaped, located medially, in the first two spinal segments. Lateral elongated cells were medium to large size neurons, located in the intermediate and lateral gray matte r, mainly contralateral to the injection site. Their axon emerging fro m the lateral part of the soma crossed the midline, ventral to the cen tral canal. These cells were present throughout the rostral spinal cor d. Cells were also labeled in the lateral white matter. Some of them h ad the typical dendritic arborizations of edge cells (intraspinal stre tch receptor neurons) and were located in the most rostral segments, b ilaterally. Other medium to large size neurons were identified dorsal and medial to most of the edge cells. We suggest that at least the gro up of lateral elongated cells exhibits rhythmic membrane potential osc illations during fictive locomotion. These cells may, together with th e rostral edge cells, be responsible for the locomotor-related modulat ion of activity in reticulospinal and vestibulospinal neurons. J. Comp . Neurol. 397:475-492, 1998. (C) 1998 Wiley-Liss, Inc.