Conduction velocity and myelin thickness in regenerating nerve fibres

All attempts to analyse the compound action potential of nerve in terms of the nerve fibres present depend upon the basic assumption that the largest fibre present conducts with the greatest velocity. While there is no reason to doubt the validity of this assumption, there is as yet no evidence to indicate that fibre diameter, and not some variable correlated with it, is the primary factor determining the conduction velocity of nerve fibres. For example, internodal distance varies linearly with fibre diameter in the nerves of adult animals (Boycott, 1904; Kubo & Yuge, 1938; Vizoso & Young, 1946), and therefore, as Hursh (1939) suggests, can be substituted for the fibre diameter in arguments about the control of velocity. Moreover, the myelin sheath thickness varies with the axon diameter (Sanders, 1946), and thus the total fibre diameter, which is equal to the axon diameter plus twice the sheath thickness, is also related to the axon diameter. Hence the assertion that any of these variablesaxon diameter, total fibre diameter, myelin sheath thickness, or internodal distance-controls conduction velocity is consistent with the available experimental evidence favouring a relationship between conduction velocity and fibre diameter. No attempt has been made so far to separate these variables by comparing the conduction velocities of, say, fibres with similar axon diameters but different sheath thicknesses or internodal distances. During nerve regeneration fibres are found which provide an opportunity of comparing, in the same animal, the conduction rates of normal medullated nerve fibres with those of similar fibres in which the normal relations between sheath thickness, axon diameter and internodal distance have been experimentally altered. The new fibres which invade the distal stump are at first thin and non-medullated, only later acquiring myelin sheaths and increasing in diameter (Young, 1942; Gutmann & Sanders, 1943). As these fibres medullate and increase in diameter auring the first 100 days of regeneration, those of the