Dense distributed processing in a hindlimb scratch motor network.

In reduced preparations, hindlimb movements can be generated by a minimal network of neurons in the limb innervating spinal segments. The network of neurons that generates real movements is less well delineated. In an ex vivo carapace-spinal cord preparation from adult turtles (Trachemys scripta elegans), we show that ventral horn interneurons in mid-thoracic spinal segments are functionally integrated in the hindlimb scratch network. First, mid-thoracic interneurons receive intense synaptic input during scratching and behave like neurons in the hindlimb enlargement. Second, some mid-thoracic interneurons activated during scratching project descending axons toward the hindlimb enlargement. Third, elimination of mid-thoracic segments leads to a weakening of scratch rhythmicity. We conclude that densely innervated interneurons in mid-thoracic segments contribute to hindlimb scratching and may be part of a distributed motor network that secures motor coherence.