Mechanisms of pattern generation in co-cultures of embryonic spinal cord and skeletal muscle.

Spontaneous output patterns of embryonic spinal cord slices in vitro were investigated in order to study the formation of pattern-generating networks. Patterns of spontaneous contractions of skeletal muscle fibers were recorded in co-cultures of embryonic rat spinal cord, dorsal root ganglia and skeletal muscle. A part of these contractions was shown to be driven by spinal circuits. These neuron-driven activity patterns changed from random to rhythmic when the inhibitory synapses in the spinal cord were blocked by strychnine, bicuculline or both. Rhythmic patterns consisted of bursts of activity (tetanic contractions) followed by periods of relaxation. The transition from random to rhythmic patterns occurred during a period of heavily increased rate of activity. Presynaptic inhibition was not involved critically in the generation of rhythmic patterns. Such patterns were, however, modulated through muscarinic and alpha-adrenergic receptors. Neither NMDA nor glutamate nor its uptake blocker dihydrokainate induced rhythmic patterns of contraction, although NMDA in the presence of low magnesium increased moderately the rate of random activity. In order to study the size of pattern-generating networks, parts of the spinal cord slices were sectioned during rhythmic activity. Tangential cuts at the lateral or dorsal side of the slices reduced either the rate or the duration of the bursts or both. Sagittal cuts suppressed the activity almost totally. These findings suggest that the pattern generators in the slices consist of excitatory networks covering the entire slice, and that these networks reverberate following spontaneous activity of some distributed elements.