Guidance of regrowing sensory axons after cutaneous nerve lesions in the cat

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.