Figure 1.

Model of two-joint planar arm actuated by antagonistic muscles under control of spinal interneurons. Shown are two spinal circuits, one for each pair of antagonistic muscles. Connections are drawn between interneurons regulating muscles acting on the same joint, as well as those coupling adjacent joints (only one direction is shown for simplicity; the structure of Ib connections between segments is symmetric in the model). Ia pathways are shown in red, Ib pathways in orange, and Renshaw cells in gray. Flexor related circuitry is drawn as solid and extensor as dashed lines. Excitatory synapses are displayed as triangles and inhibitory synapses as disks. Those that during optimization can be of either type are drawn as squares. Three types of signals descend from higher centers (blue). These are: the stretch reflex threshold λ^d (implying appropriate coordination of α and γ fusimotor drives, see section on threshold control); a coactivation signal λ^co to the α-MNs, and a GO-signal distributed to all spinal neurons (each receiving this signal via its own weighted connection, not shown here). The topology of the circuits is symmetric, but synaptic strengths can be assigned asymmetrically. The topology is also identical for the two joints, though for clarity some connections of the shoulder joint are omitted in the figure. Muscles wrap around joint capsules of radius r_1,2 and insert into arm segments of lengths l_1,2.