Reinnervation of glabrous skin in baboons: properties of cutaneous mechanoreceptors subsequent to nerve transection

1. A total of 758 fibers were isolated from previously transected and repaired ulnar nerves of five baboons. These fibers were compared to fibers from normal and previously crushed nerves studied in an earlier experiment. 2. The conduction velocities of the proximal portion of the injured axons dropped below normal, and this reduction persisted until reinnervation appeared nearly complete. 3. The receptive-field organization and response characteristics of 79 cutaneous afferent fibers serving the glabrous skin were studied in detail and compared to cutaneous afferent fibers of normal and previously crushed nerves studied earlier. 4. Initially, receptive fields were small and irregular, and often one fiber served several distinct skin regions. Ten months later, most of these abnormalities were no longer apparent. 5. Thresholds for single impulses elicited by von Frey hairs remained elevated for up to 4 mo after the receptive field reappeared, but then dropped abruptly to a near-normal range. 6. After reinnervation, rapidly adapting fibers displayed tuning curves characteristic of their submodality, but thresholds were elevated and only began to approach the normal range 6 mo after reinnervation. 7. After reinnervation, slowly adapting fibers displayed stimulus-response curves with elevated thresholds and they tended to saturate at lower stimulus intensities than normal fibers. 8. When compared to the return of function following a crushing injury, axons that had been transected displayed a slower time course for the return to normal values of conduction velocity and threshold. Receptive-field organization also remained abnormal for a longer time period. 9. These data support the hypotheses that a) breaking the continuity of the Schwann cells and extracellular matrix that occurs during transection but not during crush is a major factor leading to errors of axonal regeneration in the distal stump, b) submodality specificity is a property of the regenerating axon, and c) regenerating axons are influenced by an internal or external cue, causing them to form and maintain a single relatively homogeneous receptive field.

Reinnervation of glabrous skin in baboons: properties of cutaneous mechanoreceptors subsequent to nerve crush

1. A total of 791 fibers were isolated from the ulnar nerves of five baboons. Over half of these were obtained from the right ulnar nerves subsequent to reinnervation following a nerve crush; the other fibers were obtained from the undamaged left ulnar nerves. 2. The conduction velocities in the proximal portion of the injured axons dropped below normal, and this reduction persisted until reinnervation appeared nearly complete. 3. The response properties of 65 cutaneous afferent fibers serving reinnervated glabrous skin were compared to 80 fibers from normal skin. 4. Of the afferent fibers reinnervating skin, the proportion judged to have abnormal response properties was not significantly greater than the proportion in normal skin. 5. After reinnervation, cutaneous rapidly adpating fibers displayed tuning curves characteristic of their submodality, while some cutaneous slowly adapting fibers could still be differentiated into type I and type II fibers. However, both types of slowly adapting fibers displayed an increased rate of adaptation and a lowered sensitivity to sustained displacements even at 5 mo following reinnervation. 6. In conclusion, the cutaneous mechanoreceptors in reinnervated glabrous skin regained response properties that allowed them to be assigned to the same submodalities found in normal skin. Submodality was recognizable at an early stage when the receptive field was still immature and when the threshold was elevated. With time, the receptive-field sizes and shapes returned to normal, the thresholds approached normal, and the remaining differences from normal cutaneous afferent fibers became minor.