Ultrastructure of afferent axon endings in a neuroma

Injured sensory axons with endings trapped in a nerve-end neuroma become a source of abnormal impulse discharge and neuropathic pain. We have examined the ultrastructure of such endings using anterogradely transported WGA-HRP and freeze-fracture replication, with emphasis on the postinjury period during which the abnormal neural discharge is maximal. Most axons ended in a terminal swelling, depleted of myelin but surrounded by Schwann cell processes. These 'neuroma endbulbs' were richly packed with membrane-bound organelles, and had a smoothly undulating surface with (in neuromas of several weeks standing) a moderate number of short filopodia. Massive sprouting did not occur until several months postinjury. Both p- and e-faces of endbulb axolemma had larger intramembranous particles, on average, than corresponding internodal membrane of control axons. This change, interpreted as indicating remodelling of axolemmal channel (and perhaps receptor) content, may be related to the abnormal electrical behavior of neuroma afferents.