DEVELOPMENT AND CHARACTERIZATION OF PATHWAYS DESCENDING TO THE SPINAL-CORD IN THE EMBRYONIC CHICK

1. We used an isolated preparation of the embryonic chick brain stem a nd spinal cord to examine the origin, trajectory, and effects of desce nding supraspinal pathways on lumbosacral motor activity. The in vitro preparation remained viable for less than or equal to 24 h and was su fficiently stable for electrophysiological pharmacological, and neuroa natomic examination. In this preparation as in the isolated spinal cor d, spontaneous episodes of both forelimb and hindlimb motor activity o ccur in the absence of phasic afferent input. Motor activity can also be evoked by brain stem electrical stimulation or modulated by the int roduction of neurochemicals to the independently perfused brain stem. 2. At embryonic day (E)6, lumbosacral motor activity could be evoked b y brain stem electrical stimulation. At E5, neither brain stem nor spi nal cord stimulation evoked activity in the lumbosacral spinal cord, a lthough motoneurons did express spontaneous activity. 3. Lesion and el ectrophysiological studies indicated that axons traveling in the ventr al cord mediated the activation of lumbosacral networks by brain stem stimulation. 4. Partition of the preparation into three separately per fused baths, using a zero-Ca2+ middle bath that encompassed the cervic al spinal cord, demonstrated that the brain stem activation of spinal networks could be mediated by long-axoned pathways connecting the brai n stem and lumbosacral spinal cord. 5. Using retrograde tracing from t he spinal cord combined with brain stem stimulation, we found that the brain stem regions from which spinal activity could be evoked lie in the embryonic reticular formation close to neurons that send long desc ending axons to the lumbosacral spinal cord. The cells giving rise to these descending pathways are found in the ventral pontine and medulla ry reticular formation, a region that is the source of reticulospinal neurons important for motor activity in adult vertebrates. 6. Electric al recordings from this region revealed that the activity of some brai n stem neurons was synchronized with the electrical activity of lumbos acral motoneurons during evoked or spontaneous episodes of rhythmic mo tor activity.7. Both brain stem and spinal cord activity could be modu lated by selective application of the glutamate agonist N-methyl-D-asp artate to the brain stem, supporting the existence of Functionally act ive descending projections from the brain stem to the spinal cord. It is not yet clear what role the brain stem activity carried by these pa thways has in the genesis and development of spinal cord motor activit y.