Figure 1.

Model schematics. The two-level spinal CPG consists of a half-center rhythm generator (RG, containing flexor, RG-F, and extensor, RG-E, half-centers), pattern formation (PF) circuits (PF-F and PF-E neurons) and inhibitory interneurons (In-F and In-E), which mediate reciprocal inhibition between flexor and extensor sides of the CPG. The CPG receives tonic “supraspinal” drive and generates a basic “locomotor” rhythm providing alternating activation of all flexor and extensor CPG neurons and corresponding flexor and extensor motoneurons (Mn-F and Mn-E). An additional circuit including In and Inab-E interneurons is incorporated to provide disynaptic excitation of Mn-E by extensor afferents during extension (see text for details). All interneurons are represented by spheres: light spheres are excitatory, and dark spheres are inhibitory interneurons. Motoneurons are represented by diamonds. Excitatory and inhibitory synaptic connections are shown by arrows and small circles, respectively. The biomechanical system represents a simplified model of a single-joint limb that is forced to oscillate by the forces of two antagonistic muscles (flexor, F, and extensor, E, activated by Mn-F and Mn-E, respectively). Muscle afferents provide length-dependent (Ia type from both muscles, and type II type from the flexor muscle) and force-dependent (Ib from the extensor) feedback to the CPG providing excitation to the homonymous (F or E) interneurons of the CPG (see text for details).