Recovery of fibre numbers and diameters in the regeneration of peripheral nerves

In a normal mixed peripheral nerve it is possible to distinguish different groups of fibres, characterized by different diameters and degrees of medullation. These groups are responsible for different components in the conducted action potential of the nerve and subserve different functions [Grundfest, 1940; Zotterman, 1939]. When such a mixed nerve is interrupted all the axons and myelin in the peripheral stump degenerate, and reinnervation is accomplished by the outgrowth of axons from the central stump. Therefore, if the fibre-size groups ofnormal nerve are to be regarded as having any functional significance, regeneration should be directed towards the provision of an assemblage of fibres in which the numbers, diameters, medullation, and central and peripheral connexions of the fibres in each group approximate to those seen in normal nerve. In the regeneration of a particular nerve it should be possible to distinguish two processes: (1) outgrowth, which has as its result the arrival and connexion of fresh axons with the periphery, and (2) reconstitution, by which the numbers, diameters, and medullation of the fibres are restored. Although these two processes are largely coincidental, the reconstitutional aspect of regeneration has received relatively little attention. It is well known that the first fibres laid down in regeneration are thin and non-medullated, and that they later thicken, become medullated, and acquire nodes and incisures [Hentowa, 1933]. It has been stated that this process of thickening and medullation has a downward trend, spreading as a wave front along the nerve some distance behind the front of advance of axon tips [Howell & Huber, 1892; Kirk & Lewis, 1917; Sanders & Young, 1942], although in individual fibres it may proceed discontinuously [Speidel, 1932; Clark & Clark, 1938]. However, Davenport, Chor & Cleveland [1939] found that 3 months after division and suture of a nerve the ratio of the number of unmyelinated to myelinated fibres remained unchanged, and concluded that 'growth and