Figure 4.

Cephalic descending signals are necessary and sufficient for appropriate intersegmental coordination during DA-biased fictive crawling. A, Schematic drawing of the whole nerve cord and placement of the bow-tie-shaped well containing saline (block OFF) or isotonic sucrose (block ON). Locations of extracellular recordings are marked by electrodes on 3 evenly spaced dorsal posterior (DP) nerves (M10, M12, and M14). Site of K⁺ excitation is indicated at the left of the cephalic ganglion. B, Initial record shows DP nerve activity during blockade of cephalic signals but in the presence of cephalic K⁺ excitation (vertical arrow). The largest unit in the DP recordings is from the dorsal longitudinal excitor motoneuron DE-3, which is active during the contraction phase of crawling (see below). When the block was removed (vertical dotted line), coordinated fictive crawling emerged. C, DP nerve activity from the same preparation after DA-biased crawling had subsided (initial traces). When the cephalic ganglion was briefly stimulated by K⁺ (vertical arrow), robust and coordinated fictive crawling ensued. A representative crawl period is highlighted with text, and the fictive elongation (E) and contraction (C) phases of crawling (2 cycles) are shown for the sake of clarity. D, Expanded trace of DP nerve activity when the sucrose block was in effect. Previous crawling exhibited before this block (C) was replaced with uncoordinated DE-3 bursting.