Locomotor behavior is believed to be produced by interneuronal networks that are intrinsically organized to generate the underlying complex spatiotemporal patterns. In order to study the temporal correlation between the firing of individual interneurons and the pattern of locomotion, we utilized the spinal cord-forelimb preparation from the mudpuppy, in which electrophysiological recordings of neuronal activity were achieved during walking-like movement of the forelimb induced by bath application of N-methyl-D-aspartate (NMDA). Intra- and extracellular recordings were made in the C2 and C3 segments of the spinal cord. These segments contain independent flexor and extensor centers for the forelimb movement about the elbow joint during walking. Among the 289 cells recorded in the intermediate gray matter (an area between the ventral and dorsal horns) of the C2 and C3 segments, approximately 40% of the cells fired rhythmically during "walking". The firing rates were 6.4±0.4 impulses/s (mean ± SE). These rhythmically active cells were classified into four types based on their phase of activity during a normalized step cycle. About half the rhythmic cells fired in phase with either the flexor (F) or extensor (E) motoneurons. The rest fired in the transitions between the two phases (F→E and E→F). Longitudinal distributions of the four types of interneurons along the spinal cord were in agreement with observations that revealed distinct but overlapping flexor and extensor centers for walking. Some cells triggered short-latency responses in the elbow flexor or extensor muscles and may be last-order interneurons. These observations suggest that there is a differential distribution of phase-specific interneurons in the central pattern generator of the mudpuppy spinal cord for walking.
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