IDENTIFICATION, LOCALIZATION, AND MODULATION OF NEURAL NETWORKS FOR WALKING IN THE MUDPUPPY (NECTURUS-MACULATUS) SPINAL-CORD

We tested the hypothesis that the neural networks for walking in the m udpuppy can be divided into a flexor and an extensor center, each of w hich contains collections of interneurons localized in the vicinity of their motoneuron pools. Combining a battery of techniques, we identif ied and localized the elbow flexor center and its motoneuron pool in t he C2 segment and the elbow extensor center and its motoneuron pool in the C3 segment. Rhythmic flexion or extension of the limb in isolatio n could be induced by continuous trains of current pulses of the C2 or C3 segments, respectively. Independent activation could also occur af ter application of glutamate receptor agonist NMDA. Part of segment C2 in isolation generated rhythmic elbow flexor bursts. whereas part of segment C3 in isolation generated rhythmic elbow extensor bursts. An i solated region spanning the C3 roots generated both flexor and extenso r bursts. The step cycle was modulated in a phase-dependent manner by stimulation of the dorsal roots, the ventral roots, or either of the t wo centers. The effects of ventral root stimulation were removed by de afferentation to block reafferent input attributable to muscle contrac tion induced by the stimulation. We conclude that the neural networks for walking contain at least a flexor and an extensor generator that a re localized in close apposition to the motoneuron pools, that the two centers can work independently despite the fact that there are recipr ocal inhibitory interconnections between them, and that sensory input interacts with the spinal neural networks to reset the ongoing walking rhythm in a phase-dependent manner.