Abstract
Bulbospinal systems (BS) originate from various regions of the brainstem and influence spinal neurons by classical synaptic and modulatory mechanisms. Our aim was to determine the brainstem locations of cells of origin of BS pathways passing through the medial longitudinal fasciculus (MLF) and the caudal ventrolateral medulla (CVLM). We also examined the transmitter content of spinal terminations of the CVLM pathway. Six adult rats received Fluorogold (FG) injections to the right intermediate gray matter of the lumbar cord (L1–L2) and the b-subunit of cholera toxin (CTb) was injected either into the MLF or the right CVLM (3 animals each). Double-labeled cells were identified within brainstem structures with confocal microscopy and mapped onto brainstem diagrams. An additional 3 rats were injected with CTb in the CVLM to label axon terminals in the lumbar spinal cord. Double-labeled cells projecting via the MLF or CVLM were found principally in reticular regions of the medulla and pons but small numbers of cells were also located within the midbrain. CVLM projections to the lumbar cord were almost exclusively ipsilateral and concentrated within the intermediate gray matter. Most (62%) of terminals were immunoreactive for the vesicular glutamate transporter 2 while 23% contained the vesicular GABA transporter. The inhibitory subpopulation was glycinergic, GABAergic or contained both transmitters. The proportions of excitatory and inhibitory axons projecting via the CVLM to the lumbar cord are similar to those projecting via the MLF. Unlike the MLF pathway, CVLM projections are predominantly ipsilateral and concentrated within intermediate gray but do not extend into motor nuclei or laminia VIII. Terminations of the CVLM pathway are located in a region of the gray matter that is rich in premotor interneurons; thus its primary function may be to coordinate activity of premotor networks.
Highlights
Bulbospinal (BS) systems are composed of heterogeneous pathways that originate from the brainstem
caudal ventrolateral medulla (CVLM) injections were centered upon the lateral reticular nucleus but spread into surrounding structures including the parvicellular reticular nucleus, the internal reticular nucleus and the nucleus ambiguous; they did not encroach on the rubrospinal tract
In this study we have shown that the cells that form pathways from the brainstem to the lumbar spinal cord passing through the medial longitudinal fasciculus (MLF) and CVLM, for the most part, have overlapping spatial distributions
Summary
Bulbospinal (BS) systems are composed of heterogeneous pathways that originate from the brainstem They influence a variety of spinal networks including those concerned with motor control, sensory input (including pain) and autonomic function. The neurons that give rise to BS pathways project to the spinal cord via fiber tracts in the medulla (Mitani et al, 1988) These tracts include the MLF, which contains axons principally involved in motor control (Jankowska et al, 2007; Jankowska, 2008) and axonal systems within the caudal ventrolateral medulla (CVLM) which have a role in sensory and autonomic control in addition to an influence on motor activity (Tavares and Lima, 2002). Some classes of bulbospinal neuron which arise principally from the medullary reticular formation are responsible for conveying signals from motor centers in the brain which include the mesencephalic locomotor center and the primary
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