Regeneration of sensory-motor synapses in the spinal cord of the bullfrog.

Sensory fibers innervating muscles in the arm of the bullfrog form specific patterns of monosynaptic connections with motoneurons in the spinal cord. We show here that these normal patterns are re-established after interruption of the second dorsal root (DR2) in tadpoles and postmetamorphic frogs. DR2 was either cut or crushed, and 2 to 8 months later the extent and specificity of regeneration were assessed anatomically and physiologically. Horseradish peroxidase labeling of DR2 showed that sensory afferents had regenerated back into the spinal cord to form local arborizations, as in the normal adult. However, few long-tract fibers in the dorsal columns regenerated. Intracellular recording from different classes of motoneurons at the level of DR2 revealed that triceps muscle sensory afferents had regenerated to form functionally appropriate synapses. As in the normal adult, stimulation of the triceps nerves elicited larger monosynaptic EPSPs in triceps motoneurons than in non-triceps motoneurons. Thus, in the central nervous system of the bullfrog, specific monosynaptic connections are re-formed within the region of local regeneration.