Type of sensory nerve fibre sprouting to form a neuroma

Abstract

WHEN a peripheral nerve is cut across, the axons in the central end send out sprouts. If these sprouts fail to reach the distal part of the cut nerve they form a tangled mass called a neuroma¹. Neuromas which occur in man after amputation or after failure of regeneration of a damaged nerve are always tender on pressure and are sometimes the source of ongoing pain². When experimental neuromas are stimulated, it is known that a delayed volley of nerve impulses arrives at the central nervous system³ and it was assumed that the delay was occurring in the fine sprouts. Now that we have examined single fibres originating from a neuroma, we find that this conclusion was wrong and that there is a much more surprising explanation: only the small diameter, slowly conducting axons within the peripheral nerve seem to send sprouts into the neuroma and the delay in arrival of the afferent volley is primarily due to the fact that it runs only in slowly conducting fibres. This selective outgrowth of small fibres may be of considerable practical interest. These fibres include those which normally carry signals from injured tissue. In the normal situation, however, injury detection signals are accompanied by impulses in large diameter nerve fibres which decrease the central effect of the injury signals. The neuroma, as a pure culture of small fibres, may generate pain producing impulses³ where the pain has an intensity and peculiarly unpleasant quality due to the absence of the normal modulating impulses⁴ in the larger fibres which have failed to regenerate.