Developing networks of the chick activity-dependent depression of network excitability. spinal cord become spontaneously active early in de-
The interaction between high excitability and depresvelopment and remain so until hatching. Experiments sion results in an equilibrium in which episodes are using an isolated preparation of the spinal cord have expressed periodically by the network. The mechabegun to reveal the mechanisms responsible for this nism of the rhythmic bursting within an episode is not activity. Whole-cell and optical recordings have shown understood, but it may be due to a ''fast'' form of that spinal neurons receive a rhythmic, depolarizing network depression. Spontaneous embryonic activity synaptic drive and experience rhythmic elevations of has been shown to play a role in neuron and muscle intracellular calcium during spontaneous episodes.
development, but is probably not involved in the initial Activity is expressed throughout the neuraxis and can formation of connections between spinal neurons. It be produced by different parts of the cord and by the may be important in refining the initial connections, isolated brain stem, suggesting that it does not depend but this possibility remains to be explored. α§ 1998 John upon the details of network architecture. Two factors