Specific Regeneration of Cutaneous Sensory Afferent Fibers in the Amphibian Spinal Cord

Regeneration of cutaneous sensory fibers within the spinal cord was studied in frogs after lesions of sensory axons in dorsal roots. Sensory fibers were disrupted by repeatedly freezing the exposed brachial dorsal root in postmetamorphic frogs. Previous experiments indicated that this procedure destroyed virtually all sensory axons within the dorsal root. The pattern of regrowth of these fibers within the spinal cord was then assessed at different times by labeling them with horseradish peroxidase applied to the superior cutaneous ulnar nerve in the front leg. Arborizations of cutaneous axons within the spinal gray matter were remarkably similar to those in normal frogs. The arborizations were restricted to dorsal levels of the cord, never projecting ventrally to the region where muscle spindle sensory axons terminate. These experiments complement earlier studies demonstrating that muscle spindle afferents also regenerate specifically. Together these studies indicate that sufficient cues exist in the mature amphibian spinal cord to allow different classes of sensory fibers to reinnervate their appropriate target areas.