Figure 10.

Schematic diagram of the leech CNS depicting neural elements involved in the intersegmental coordination of crawling. Each of the 21 midbody ganglia contains a complete crawl oscillator (circle with sigmoid) (Puhl and Mesce, 2008). The descending long-distance coordination signal(s) from the cephalic ganglion is necessary and sufficient for the production of normal intersegmental phase delays between the oscillators (continuous gray line with lateral arrows). In addition, individual crawl oscillators provide weak drive (open arrows) to their anterior neighbor and much stronger excitatory drive (thick gray arrows) to two or more oscillators in posterior locations. Intersegmental fibers of passage delivering crawl-like drive can project uninterrupted at least two ganglia away from their source (thinner gray arrows).