Search

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.

1. Micro‐electrode recordings were made from normal and regenerating sural nerve fibres in cats. Increases in the excitability of the central terminals of these fibres after conditioning stimulation of other sural nerve fibres were taken as evidence for primary afferent depolarization. 2. At all recovery times studied the excitability changes seen were significantly less than those seen in control animals. Two factors contributed to the changes in primary afferent depolarization. First, the proportion of fibres that showed no evidence of primary afferent depolarization increased significantly. This proportion became smaller as recovery progressed. Secondly, where primary afferent depolarization was present, the magnitudes of the effects were slightly but significantly decreased compared with control values. 3. Excitability changes of the central terminals of sural nerve fibres were also measured after conditioning stimulation of the ipsilateral, unlesioned accessory sural nerve. One month after sural nerve transection there was a significant increase in the proportion of fibres showing no evidence of excitability changes following accessory sural nerve conditioning stimulation compared with control animals. Thus, the loss of primary afferent depolarization of regenerating sural nerve fibres was neither simply a consequence of desynchronization of the volley of impulses entering the spinal cord after conditioning stimulation of other regenerating sural fibres, nor due to fewer fibres being activated during conditioning stimulation of the lesioned nerves. 4. A possible explanation of these results is that after peripheral nerve crush or transection the central terminals of the damaged fibres retract or atrophy. Then as regeneration of the nerve proceeds, the central terminals of the fibres re‐form.

1. Individual type I sensory neurons in cutaneous nerves typically innervate two to four type I cutaneous mechanoreceptors (Haarscheiben). The extent to which these neurons replicate the original innervation patterns of the type I receptors after peripheral nerve regeneration and the means by which these neurons are guided back to their old receptor sites during regeneration were studied in cats using neurophysiological techniques. 2. By recording activity of type I neurons in small cutaneous nerves and isolated dorsal rootlets, it was possible to map the distribution of these neurons in the skin. Maps made before nerve lesion were compared to maps made after recovery from nerve crush and transection. 3. Fibers regenerating after nerve crush return to their old receptor sites, probably by following their old Schwann tubes in the distal stump of the nerve, and replicate the original receptor innervation pattern. Essentially all the type I fibers successfully regenerate in this case. 4. In contrast, after nerve transection the regenerating fibers do not restore the original innervation pattern, although they do preferentially return to other old type I receptor sites. About 60% of the type I fibers reinnervate the skin after transection. 5. These observations provide a basis for the difference in functional recovery seen after crush and transection lesions of peripheral nerves.

1. Type I sensory fibres in cat hairy skin innervate structures characterized by twenty to fifty specialized epithelial (Merkel) cells aggregated in a small dome‐shaped elevation. Only one fibre enters each dome and it branches repeatedly to supply at least one terminal to each Merkel cell. After the nerve is cut, the Merkel cells and the dome ultimately disappear.