Fig. 1.

Spontaneous episodes of activity recorded from ventral roots of chick embryo spinal cord in vitro at embryonic day (E) 7.5 and 10. Recordings were made in DC mode and digitized at 10 Hz, showing the synaptic depolarization recorded electrotonically from motoneurons. Periods of activity (episodes) are clearly distinct from silent phases. An interval is defined as the time between the beginning of two consecutives episodes. Episodes are composed of cycles, or “network spikes.” With development, both the interval and episode duration lengthen, and cycle frequency and the level of depolarization (tonic component) also increase. Also, on older embryos episodes have a leading tonic phase before cycling begins. For the examples shown here, interval and episode durations are, respectively, E 7.5: 440 ± 29 and 20 ± 1 sec; E 10: 1600 ± 140 and 68 ± 3.5 sec (mean ± SD).