Activity patterns and synaptic organization of ventrally located interneurons in the embryonic chick spinal cord

To investigate the origin of spontaneous activity in developing spinal netw orks, we examined the activity patterns and synaptic organization of ventra lly located lumbosacral interneurons, including those whose axons project i nto the ventrolateral funiculus (VLF), in embryonic day 9 (E9)-E12 chick em bryos. During spontaneous episodes, rhythmic synaptic potentials were recor ded from the VLF and from spinal interneurons that were synchronized, cycle by cycle, with rhythmic ventral root potentials. At the beginning of an ep isode, ventral root potentials started before the VLF discharge and the fir ing of individual interneurons. However, pharmacological blockade of recurr ent motoneuron collaterals did not prevent or substantially delay interneur on recruitment during spontaneous episodes. The synaptic connections of int erneurons were examined by stimulating the VLF and recording the potentials evoked in the ventral roots, in the VLF, or in individual interneurons. Lo w-intensity stimulation of the VLF evoked a short-latency depolarizing pote ntial in the ventral roots, or in interneurons, that was probably mediated mono- or disynaptically. At higher intensities, long-latency responses were recruited in a highly nonlinear manner, eventually culminating in the acti vation of an episode. VLF-evoked potentials were reversibly blocked by extr acellular Co2+, indicating that they were mediated by chemical synaptic tra nsmission. Collectively, these findings indicate that ventral interneurons are rhythmically active, project to motoneurons, and are likely to be inter connected by recurrent excitatory synaptic connections. This pattern of org anization may explain the synchronous activation of spinal neurons and the regenerative activation of spinal networks when provided with a suprathresh old stimulus.