Figure 2.

Spinal circuitry between true antagonists. The diagonal muscles are called true antagonists because they oppose each other in all tasks. There are two such circuits in the integrated model. Five interneuronal pathways were modeled: (1) PN—propriospinal, (2) monosynaptic Ia-excitatory, (3) reciprocal Ia-inhibitory, (4) R—Renshaw inhibitory, and (5) Ib-inhibitory pathways. The inputs are differentiated as SET and GO. The SET inputs regulate the background activity in the spinal cord and consist of the gains in the neural pathways plus Ia presynaptic inhibition (small white circles marked S in the figure), and biases to the interneurons and motoneurons and fusimotor drive (γ static and γ dynamic) to the muscle spindles (the large white circles marked SET).