The loss of sensation and muscle power after a nerve transection has been known from antiquity. In the nineteenth century, when the microscopic structure of the nerve fiber became known, the amputated stump of transected nerves was seen to undergo the characteristic breakdown called Wallerian degeneration. The phenomenon led to a major advance in understanding the different functions of the spinal cord roots; sensory fibers carried in the dorsal roots, motor fibers in the ventral roots, the Bell-Magendie law. On cutting a root, degeneration was seen only in that portion of its fibers separated from the cells in the ganglia. The inference of these results was that the cell bodies are required to maintain viability of their fibers. The pursuit of how this comes about led to the recognition of the neuron doctrine and the need for some means by which materials from the cells are carried out into their fibers, the mechanism of axoplasmic transport, which will be discussed in detail in Chapters 11 and 12. In this chapter, the analysis of Wallerian degeneration is presented and shown to be a two-stage process in which the earliest phase is a beading of the fibers.

THE BELL-MAGENDIE LAW

In a paper he had privately printed in 1811 and that was privately circulated, and only much later publically revealed, the famous English anatomist Charles Bell (1774–1842) reported that injury to the anterior (ventral) portion of the spinal cord marrow caused convulsive muscular movements in vivisected animals, more so than an injury to the posterior (dorsal) portions of the cord.