So far as the signaling pathways to -motoneurons (-MNs) are concerned, the motoneuron level (MNL) of the spinal cord neural network system is composed of five types of neurons: The -MNs 1, the Ia inhibitory interneurons (Ia-INs), the Ia/Ib non-reciprocal interneurons (Ib-INs), the group II ventral horn interneurons (IIVH-INs), and the Renshaw cells (RCs). Perhaps the easiest way to explain the -MNL is to look at it in terms of the individual -MNs. You will recall from Part I that each -MN drives one muscle unit (MU). Thus, the network for each -MN constitutes a sub-network within the MNL. We will call these subnetworks “alpha motoneuron groups” (-MNGs).
