Hemisegmental localisation of rhythmic networks in the lumbosacral spinal cord of neonate mouse

In vitro isolated spinal cord preparations of newborn mice were used to examine the localisation of neuronal network(s) involved in the centrally-driven command of motor activities. Transections of reduced spinal cord preparations were performed under different extracellular bathing conditions, to obtain the smallest piece of cord capable of generating spinal motor rhythm. Under normal bathing medium, the whole lumbosacral cord from 0 to 2-day-old mice (P0-2 group) must be maintained to generate spontaneous motor bursts on lumbar ventral roots. In the P3-5 group, however, a three segment long section from the sacral part of the cord was still able to produce spontaneous episodes of rhythmic activity. Using a Mg2+-free medium to activate quiescent motor neuronal networks, transection procedures revealed that a double lumbar segment and a single segment (at both lumbar and sacral levels) of the cord continued to exhibit rhythmic locomotor-like discharges in P0-2 and P3-5 groups, respectively. In some experiments in which isolated reduced preparations did not generate any rhythmic activity in ventral roots, central inhibitory influences were blocked by addition of bicuculline (20-30 microM) or strychnine (20 microM) to the superperfusate. Under these conditions, a slow and synchronous rhythmic activity was typically recorded from lumbar and sacral outputs in both P0-2 and P3-5 groups. Finally, transection experiments showed that lumbar and sacral hemisegments of the cord retained the ability to generate a bicuculline- or strychnine-induced motor rhythm. These results suggest that (1) intersegmental connections appear to be stronger in P0-2 than in P3-5 group, since under both normal or Mg2+-free bathing medium, spinal rhythmic activity was more affected by transection procedures in preparations from the younger animals, and (2) neuronal networks producing rhythmic motor activities in mouse may be segmentally organised, each hemisegment being able to generate its own spinal motor rhythm.