Distribution of Central Pattern Generators for Rhythmic Motor Outputs in the Spinal Cord of Limbed Vertebratesa

Neuronal networks in the spinal cord are capable of producing rhythmic movements, such as walking and swimming, when the spinal cord itself is isolated from the brain and sensory inputs. These spinal networks, also called central pattern generators or CPGs, serve as relatively simple model systems for our understanding of brain functions. In this paper we concentrate on spinal CPGs in limbed vertebrates and in particular address the question: Where in the spinal cord, in the longitudinal and transverse planes, are they located? We will review the use of lesions to isolate the rhythm and pattern-generating parts of the CPG network, indirect methods like activity-dependent labeling with 14C-2-deoxyglucose, c-fos, sulforhodamine 101, and WGA-HRP, which label presumed rhythmically active neurons en bloc, and direct methods such as calcium-imaging, extracellular and intracellular recordings, which identify rhythmically active cells directly. With this review we hope to highlight the scientific disagreements and the consensus, which have emerged from these studies with regard to the distribution of the CPG networks in the spinal cord.