Abstract
This study presents direct evidence that in Xenopus laevis embryos ipsi- and contralaterally descending reticulospinal fibers from the caudal brain stem project to the spinal cord, where they directly contact primary motoneurons. At stage 30, occasional contacts between primary motoneurons and descending axons are present. These contacts are possibly already functional since presynaptic vesicles were sometimes observed. Furthermore, the physiological data obtained in this study suggest that reticulospinal neurons in the caudal brain stem are involved in the central generation of early swimming. The first ingrowth of reticulospinal axons was observed in the rostral spinal cord after application of HRP to the caudal brain stem of stage 27/28 embryos. By stage 32, many supraspinal axons could be found in the spinal cord at the level of the 12/13th myotome, near the time of the first rhythmic swimming. Both lamellipodial and varicose growth cones were found. Intracellular recordings from the brain stem and extracellular recordings from the myotomal muscles in curarized embryos around stage 30 revealed neurons in the caudal brain stem which were active during early fictive swimming. After intracellular staining with Lucifer yellow neurons with descending axons were found in the brain- stem reticular formation. These reticulospinal neurons showed “motoneuron-like” phasic activity, producing one spike each swimming cycle. Rhythmically occurring spikes with swimming periodicity were superimposed on a sustained depolarization level of some 5–30 mV. Reticulospinal neurons in the brain stem resemble descending interneurons in the spinal cord by their morphology, projection pattern, and activity during early swimming. Reticulospinal neurons and descending interneurons might therefore form one continuous population of projecting interneurons with a different location but a similar function. In support of this we propose that the embryonic brain-stem reticular formation forms part of the swimming pattern generator.