THE DEGENERATION OF PERIPHERAL NERVE FIBRES

After section of a peripheral nerve a series of processes occurs in the distal stump involving all of its constituents and leading to a breakdown of the axons and their myelin sheaths. This process was first studied by Waller (1852) and is generally known as " Wallerian degeneration ". The first part of Waller's law states that after cutting a nerve degeneration occurs in the distal stump and that degeneration is due to disconnection of the nerve trunk from its " trophic " centre, i.e., the nerve cell. Although it is almost 100 years since this law was conceived, the mechanism of degeneration is still not clear. In order to understand the process it is necessary to investigate the nature of this " trophic influence ". Only in this way can the question, Why does a nerve fibre degenerate ? be solved. Wallerian degeneration results after a certain time in failure of transmission of nerve impulses. The time at which this failure of transmission of nerve impulses can be recorded is apparently not a constant value. It is for instance known that both the metabolic rate and temperature influence the rate of degeneration (Muralt, 1945). Additional information about the factors affecting the rate of degeneration should help in understanding the mechanism of "Wallerian degeneration". It is necessary first to define clearly the criteria of degeneration used. After nerve section degeneration in the physiological sense appears primarily when the nerve fibre will no longer transmit impulses. As a physiological criterion we are therefore using the time at which failure of transmission of nerve impulses can be recorded. A much more complicated problem is the evaluation of the criteria of degeneration in the morphological sense. Muralt (1945) states that morphological changes generally appear only a long time after loss of excitability of the nerve fibres. The first changes in the axons, however, are observed much earlier (Ram6n y Cajal, 1928) and changes in the myelin sheaths studied by the polarized light method are observed, within 24 hours of nerve section (Prickett and Stevens, 1939). The criterion of degeneration in