Neuronal networks in the spinal cord, termed central pattern generators (CPGs), have considerable motor capacity and can be activated by cutaneous stimulation in a spinal vertebrate with complete spinal cord transection. CPGs produce coordinated motor output in model systems of spinal vertebrates immobilized with neuromuscular blockade. The immobilized spinal turtle produces the motor patterns of mixed-synergy rostral scratch, conventional-synergy pocket scratch, and conventional-synergy flexion reflex. In rostral scratch, monoarticular knee extensors are active during hip-flexor activity; in pocket scratch, monoarticuiar knee extensors are active during hip-extensor activity; in flexion reflex, knee flexors are active during hip-flexor activity. We record single-unit descending propriospinal interneuron activity during these moter patterns. We test predictions of Grillner's unit-burst-generator hypothesis of CPG organization in which each direction of each degree of freedom is controlled by a specific module, e.g., hip flexor, hip extensor, knee flexor, knee extensor, etc. We study interneurons that are candidate members of each module during normal rostral scratch with rhythmic alternation between agonist and antagonist motor output at each degree of freedom; antagonist activity occurs during agonist quiescence. We test predictions of interneuronal activity or quiescence during 3 motor-pattern variations of rostral scratch, termed deletions. During an antagonist deletion, there is no quiescence between successive agonist bursts and no antagonist motor activity. We study hip-related and knee-related deletions. We test predictions of interneuronal phase constancy or shifting when pocket scratch is compared with rostral scratch and predictions of interneuroral activation or quiescence during flexion reflex. Our experiments reveal critical characteristics of turtle CPGs. There is considerable similarity in the organization of spinal cord in all vertebrates, including humans. Our findings of CPG organization will be of interest to those developing new therapies for spinal-injured humans.
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