The motoneuron level (MNL) network is the “network of networks” that directly converges on the spinal cord system’s motoneurons (MNs). The propriospinal level (PPSL) network has the MNL for its immediate target, and given the level of complexity we have already seen at the MNL, it should be no surprise that the PPSL network is considerably more difficult for neurobiologists to probe. When a network’s target is a motoneuron, there is comparatively little doubt about what the direct function of the MNL network involves. In one way or another, its role is to activate or inhibit the proper groups of muscles at the proper time in the execution of a reflex or a voluntary movement. For the PPSL network, which targets interneurons (INs) at both the MNL and within the PPSL, the significance of signal processing with which any particular IN is involved is at least one level removed from direct MN activity and consequently interpretation becomes a major issue for neurobiologists. In this sense, exploration of the PPSL networks is handicapped by the same sort of “what does this do?” issues that confront neuroscience research of the brain.
