Anatomical specificity of regenerated muscle sensory afferents in the spinal cord of the bullfrog.

The specificity of central projections made by regenerated muscle sensory fibers in the brachial spinal cord was studied with anatomical tracing methods. Sensory fibers were interrupted by freezing dorsal roots in postmetamorphic bullfrogs. After several months, regenerated sensory fibers were labeled with horseradish peroxidase applied to the triceps brachii muscle nerve, and their arborizations within the spinal cord were reconstructed from serial cross sections. Most of the regenerated projections from triceps muscle sensory afferents ended in or near their normal terminal field. A few branched and appeared to terminate more dorsally than normal, however, sometimes within the region where cutaneous afferents normally terminate. In contrast to the normal pathway followed by muscle afferents within the spinal cord, many regenerated afferents grew along the circumference of the spinal cord, just under the pial surface, and then turned abruptly toward the midline and into their appropriate terminal region. This suggests that regenerating afferents may actively seek out their appropriate targets and are not simply passively guided to them.