Fig. 5. Interneuron C₁ activity during free walking. (Ai) Recordings from an animal that spontaneously initiated walking (vertical broken line) on the horizontal floor of the aquarium. The top trace shows the extracellular recording from the circumesophageal commissure (CC); the second trace shows the EMG recording from the mero-carpopodite flexor muscle of the right second leg; the third trace shows interneuron C₁ activity, which is also represented in the form of a frequency histogram with the time bin of 1 s underneath. (ii) Superimposition of interneuron spikes discriminated from the commissural recording. For clarity, the superimposed record is enlarged in both the time and voltage scale. The former scale is provided in the figure while the latter factor was 3.0 relative to the raw data. (B) Statistical comparison of interneuron C₁ spike activity between the resting and walking conditions (*P<0.05; Mann–Whitney U-test). (C) Interneuron activity when the animal walked on the tilted floor in different directions. The angular coordinate is shown in Fig. 1B. When the animal walked in 0° and 180° directions, the floor was bilaterally symmetrical for the animal body. In the direction of 90° and –90°, the animal body was tilted in the contralateral-side-down and ipsilateral-side-down directions, respectively. The gray bars depict the animal walking on the substratum tilted in the contralateral-side-down direction.