Fig. 6. Interneuron C₁ spike activity dependent on the abdominal posture movements during free walking. (A) Tracing of two bouts of walking on the tilted floor. The body position and head orientation are plotted at 1 s intervals. The filled circles indicate the head while the straight lines indicate the longitudinal body axis of the animal. In the first bout of walking, the animal walked forwards with the abdomen extended (Ab. Ex. FW). The animal then turned left and started the second walking in the backward direction with the abdomen flexed (Ab. Fl. BW). Smooth-line arrows in the figure indicate approximate displacement of the animal. (B) Spike activity of interneuron C₁ during the walking shown in A. Represented in the form of a frequency histogram with the time bin of 1 s, the record shows that the interneuron activity was enhanced when the animal started abdominal flexion (indicated by a vertical line). The crayfish behavior is shown in the top bar: P, pause; Ab. Ex. FW, forward walk with abdomen extended; Ab. Fl. BW, backward walk with abdomen flexed. Head orientation is monitored in the top trace. (C) Spike activities of interneuron C₁ in different orientation angles. Filled and open bars indicate the spike activity during abdominal flexion and extension, respectively.