The coactivation command for antagonist muscles involving Ib interneurons in mammalian motor control systems: an electrophysiologically testable model.

The hypothesis of a weighted combination of independent reciprocal (R) and co-activation (C) commands to agonist and antagonist motoneurons (MNs) underlying movement is considered. In contrast to the R command, C command does not influence the equilibrium position of the joint. This constraint together with experimental data on descending and segmental afferent pathways to MNs forms the basis of the neuronal model for the C command. In the model, descending systems issue identical signals to agonist and antagonist MNs. To prevent shifts in the equilibrium position, these signals are adjusted by interneurons, in proprioceptive pathways compensating the asymmetry of muscle action.