Figure 4.

Onset of Hb9 interneuron calcium peak lags the onset of ipsilateral motor output. A, Fluorescence trace shows the calcium transient of an Hb9 IN (GFP⁺/LacZ⁺) at the beginning of a fictive locomotor episode elicited by tonic electrical stimulation of the caudal tip of the spinal cord. Fictive locomotion followed seconds after electrical stimulation began. The onset of the calcium peak clearly lagged the beginning of ipsilateral ventral root activity. B, The lag time between onset of calcium peak and ipsilateral ventral root activity was averaged over multiple fictive locomotor episodes for each Hb9 IN. The distribution of the mean lag times are summarized in a histogram. C, Circular plot shows the onsets of calcium peaks (triangles; black for electrical stimulation, white for chemical) from one Hb9 IN relative to the locomotor cycle (ipsilateral half-cycle shaded gray). Most calcium peak onsets lie in the first quadrant of the circular plot, indicating that they lagged behind the beginning of ipsilateral motor output (labeled 0.0). Vector shows the mean phase and r value, which is statistically significant and indicates that this Hb9 IN was rhythmically active. The inner circle indicates r = 0.5, the threshold for rhythmicity. The locomotor cycle periods were 2.4 s for electrical stimulation and 4.7 s for chemical stimulation. D, Population circular plot summarizes the phase of spiking activity from all Hb9 INs that had calcium peaks during fictive locomotion. Each dot represents the end of a mean phase vector from a single Hb9 IN, i.e., from a single-cell plot such as C (black for electrical stimulation, white for chemical). contra., Contralateral; ipsi., ipsilateral.