Figure 1: Conceptual frameworks for rhythm generation and hypothesized operation.

A. Half-center [1,2]. Flexor and extensor rhythm generating neurons are mutually inhibited by interposed inhibitory interneurons. Excitatory connections are shown with arrows and inhibitory with circles. In normal operation, flexor and extensor rhythm generating neurons alternate. In this model, rhythm generating neurons are not intrinsically rhythmic and do not oscillate when synaptically isolated. B. Flexor burst generator [3–8]. Flexor rhythm generating neurons are inherently rhythmic but extensor rhythm generating neurons are not. Activation of flexor rhythm generating neurons leads to the rhythmic inhibition of extensor rhythm generating neurons via interposed inhibitory interneurons. Coordinating connections from extensor rhythm generating neurons to flexor rhythm generating neurons are either less strong or non-existent. If rhythm generating populations were to be isolated, flexor rhythm generating neurons would be rhythmic and extensor rhythm generating neurons would be tonic or silent. C. Unit burst generator [10,11]. There are repeating modules of flexor and extensor rhythm generating populations for each joint. Coordinating connections in between flexor and extensor modules and populations at different joints are both excitatory and inhibitory to allow for flexibility in motor pattern. Note that only excitatory connections between modules of different joints are shown. When isolated, both populations oscillate but flexor populations may oscillate at higher frequency than extensor populations.