On the number and nature of regenerating myelinated axons after lesions of cutaneous nerves in the cat.

1. Electrophysiological and anatomical techniques were used to investigate normal and regenerating sural and posterior femoral cutaneous nerve fibres in the cat. 2. One and a half years after transection of these nerves it was found that the regenerating neurones supported multiple sprouts in the distal stump of the nerve. The branching occurred at or beyond the level of the neuroma and some of the branched fibres innervated split receptive fields on the skin. 3. Counts of the number of axons in the proximal stumps of transected nerves showed that the whole original population of myelinated fibres persisted for at least 18 months. About 75% of these fibres successfully crossed the unrepaired transection site and regenerated into the distal stump of the nerve to re-form functional connexions in the skin. 4. After nerve crush all the myelinated axons regenerated. None showed signs of abnormal branching. 5. After crush the conduction velocities of the regenerated axons in the distal stump of the nerve reached nearly normal values by 6 months. After nerve transection the distal conduction velocities were reduced to 50% of normal even 18 months after the injury. 6. The implications of these findings for the recovery of function after nerve injury in man are discussed.