Figure 3.

Systems of interacting unit CPGs – intralimb coordination – forward locomotion. (A) The diagram to the left shows the four limb CPGs of a tetrapod and possible modes of coordination (in phase or alternation for fore- and hindlimbs, respectively). The CPGs are turned on by the descending drive from the MLR (mesencephalic locomotor region) or the DLR (diencephalic locomotor region). (B) Within each limb CPG there is most likely a further subdivision in unit CPGs controlling the synergists at one joint, such as hip (H), knee (K), ankle (A) and foot (F) extensors (E) or flexors (F). EDB (extensor digitorum brevis) has a particular pattern. The normal pattern of activity results from the interaction between the different unit CPGs at different joints. The advantage is that the unit CPGs may be recombined as in backward or forward walking, in the same way as with the different limb CPGs that can be recombined in the different gaits. Circles indicate inhibition, and forks/triangles excitation. (C) Exploratory simulation of locomotor activity, with a network arranged as in the diagram in which each unit CPG is designed in a similar way to the lamprey unit CPGs consisting of 100 excitatory interacting neurons–and the interaction between the 9-unit CPGs arranged as in the middle diagram. The output of the model network captures essential features of the locomotor output.