The Non-centrifugal Degeneration of Severed Peripheral Nerve

NEUMANN in 1868 proposed the idea that severed peripheral nerve degenerates centrifugally; i.e., the degenerative processes start at the cut end and progress slowly towards the periphery. This view, opposing the conception of simultaneous degeneration advanced by Lent in 1856, started a controversy as yet unsettled. Most of the arguments on both sides have been based on histological evidence-only relatively recently has there been any physiological experimentation on the subject. The early physiological studies were concerned chiefly with the time of disappearance of indirect excitability of the muscles innervated by cut nerves (Bethe, 1903; Courrier, 1926). Apostolaki and Deriaud (1925) found no change in chronaxie following section until the “complete degeneration” of the nerve (i.e., loss of indirect excitability of the muscle). This occurred by 9-10 days in frogs kept at 12-15OC. The nerve’s rheobase increased by the fourth day after the cut, but there was no appreciable change in gastrocnemius chronaxie for 20-30 days. Titeca (1932, 1935) confirmed this for frogs kept at 22OC. (no change in chronaxie for 8 days). He also described an early “fatiguability,” manifested as an increasing threshold with repeated stimulation. This was found to progress slowly centrifugally and was correlated by him with Parker’s observations on centrifugal degeneration of motor fibers of the frog sciatic (Parker, 1933) and of myelin sheaths of the lateral line nerves of catfish (Parker and Paine, 1934). On the other hand, Titeca reported that action potentials disappeared simultaneously all along the degenerating frog nerve. Koch (1925) had earlier shown a uniform loss of resting potential throughout degenerating mammalian nerve. More recent experiments on rats and frogs (Holobut and Jalowy, 1936) again showed no change in chronaxie until loss of indirect excitability, and the uniform loss of action potential along a cut nerve (indirect excitability disappeared at 6-9 days at a temperature of lo-12°C.; Holobut, 1937). In view of the discrepancy between the action potential findings, which indicate simultaneous degeneration of frog nerve, and the evidence for centrifugal degeneration obtained from twitch heights, histology, and fatiguability, it seemed desirable to study several of these attributes on the same preparations .