Schematic localization of the elbow flexor and extensor centers and modeling of the neural networks required for walking. A, The localization of the flexor center in the C2 and part of C3 segments (F) and the extensor center in the C3 and C4 segment (E) are based on the integration of data from electrical stimulation and surgical isolation experiments. A small overlap region at C3 spinal root level is also shown. B, A model of neural networks required for walking is drawn based on the data presented in this work. This model contains independent flexor and extensor generators that drive the flexor motoneuron pool and extensor motoneuron pool, respectively. There are inhibitory interconnections between the two centers. Somatosensory input makes both excitatory and inhibitory connections with each of the two centers, modulates the activity of the neural network, and resets the ongoing walking-like rhythm in a phase-dependent manner.