The primary objective of this work is to elucidate the cellular and synaptic mechanisms responsible for the genesis of spontaneous rhythmic activity in the developing spinal cord. Experiments are performed on isolated preparations of the chick spinal cord maintained in vitro. We utilize electrophysiological, optical and anatomical methods to analyze the function and properties of the developing networks. We have found that rhythmic network activity persists in the presence of excitatory or inhibitory neurotransmitter receptor antagonists but not in the presence of both. This result suggests that the production of rhythmic activity can be supported either by predominantly inhibitory networks or by predominantly excitatory networks. Since the synaptic connections and properties of the cells in such networks are likely to differ, we propose that the genesis of rhythmic activity in developing neuronal networks does not depend on the detailed connections within the network nor on the particular neurons involved. Rather, the emergence of rhythmic activity, requires only a certain strength and level of connectivity between a critical number of neurons. Such a mechanism differs from the traditional central pattern generating circuits that have been proposed to account for the genesis of rhythmic activity in several vertebrate species. We have begun to examine the synaptic connections of individual premotor interneurons in the spinal cord. Whole cell recordings from antidromically identified interneurons are used to characterize the firing behavior and projections of single cells. One class of interneuron receives synaptic inputs from motoneurons and in turn projects onto motoneurons. We believe this to be the avian equivalent of the mammalian Renshaw cell. We have also identified interneurons that receive synaptic inputs from specific muscle nerves but not from others. Such a cell may be similar to the 1a-inhibitory interneurons identified in mammalian spinal cord. The majority of these interneuronal classes are rhythmically active suggesting that they play a role in rhythmogenesis in the developing spinal cord. In another set of experiments, we established that motoneurons are not required for the generation of the rhythm because spontaneous rhythmic activity persists after the surgical removal of most of the motoneurons. The result indicates that a network of interneurons is sufficient to generate rhythmic activity in the developing spinal cord.
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