End-structure of afferent axons injured in the peripheral and central nervous system

The end-structure of afferent axons chronically severed in the rat sciatic nerve or dorsal column (DC) was visualized by centrifugal transport of horseradish peroxidase (HRP) or wheatgerm agglutinin conjugated to HRP (WGA:HRP) injected into the L4 or L5 dorsal root ganglion. Nerve regeneration was prevented and neuroma formation encouraged by tightly ligating the cut nerve end. For the first few weeks postoperative, the time during which afferents trapped in a nerve-end neuroma generate their most intense ectopic impulse barrage, the developing neuroma was dominated by swollen terminal end-bulbs. There was some axonal dying-back, retrograde fiber growth, and terminal sprouting, but little preterminal branching. The rich tangle of fine preterminal branches usually thought of in relation to nerve-end neuromas did not elaborate until several months postoperative, a time when the neuroma is relatively quiescent electrically. Afferents cut in the DC, which never develop dramatic ectopic electrical activity, showed morphological peculiarities similar to nerve-end neuromas during the early postoperative period, including retrograde fiber growth and minimal sprouting. They did not, however, go on to form luxuriant branches. These data provide preliminary clues as to the structure of the ectopic impulse-generating mechanism thought to underlie paresthesias and pain associated with peripheral nerve injury.