MECHANISMS OF SPONTANEOUS ACTIVITY IN DEVELOPING SPINAL NETWORKS

Developing networks of the chick spinal cord become spontaneously acti ve early in development and remain so until hatching. Experiments usin g an isolated preparation of the spinal cord have begun to reveal the mechanisms responsible for this activity. Whole-cell and optical recor dings have shown that spinal neurons receive a rhythmic, depolarizing synaptic drive and experience rhythmic elevations of intracellular cal cium during spontaneous episodes. Activity is expressed throughout the neuraxis and can be produced by different parts of the cord and by th e isolated brain stem, suggesting that it does not depend upon the det ails of network architecture. Two factors appear to be particularly im portant for the production of endogenous activity. The first is the pr edominantly excitatory nature of developing synaptic connections, and the second is the presence of prolonged activity-dependent depression of network excitability. The interaction between high excitability and depression results in an equilibrium in which episodes are expressed periodically by the network. The mechanism of the rhythmic bursting wi thin an episode is not understood, but it may be due to a ''fast'' for m of network depression. Spontaneous embryonic activity has been shown to play a role in neuron and muscle development, but is probably not involved in the initial formation of connections between spinal neuron s. It may be important in refining the initial connections, but this p ossibility remains to be explored. (C) 1998 John Wiley & Sons, Inc..