I was interested by the experiment you described with the nervous system of the cat-- After cutting the dorsal roots and severing the connection between the spinal cord and the brain, the cat can still be made to walk, given the proper stimulation. That leads to the conclusion that the "motor symphony" for walking is stored in the spinal cord. What is it, then, that stimulates this "symphony" in an intact nervous system? And doesn't this hold implications (albeit at a long, long stretch) for human paraplegics, in terms of artificially stimulating certain limited types of movements in the spinal cord below the point of damage (and thus regaining some function)?
Presumably descending signals normally activate the CPG's (although this can happen as well from sensory input into the spinal cord). And yes, indeed, it provides a possible route, which people are exploring, for human paraplegics to regain some control over "paralyzed" body regions. There are real problems with this approach, however, since many inputs relevant to walking (sight, balance) would also need to somehow be conveyed to the spinal cord to get generally effective movement. PG