Close apposition among neighbouring axonal endings in a neuroma

Abstract

Axons in intact peripheral nerve trunks constitute independent afferent and efferent communication channels. However, when nerves are severed, several different forms of axon-axon cross-excitation develop in association with the injury site. In this study we have examined experimental sciatic nerve-end neuromas in rats with special interest in the compartmentalization of individual axons, and the barriers that separate close neighbours. At postinjury times at which functional coupling is known to occur, neuromas were found to contain many examples of axons in which adjacent membrane faces come into close contact without an intervening Schwann cell process. These occur in bundles containing from two to as many as 30 individual nerve fibres wrapped in a common Schwann cell sheath. The surface area of close apposition between axon pairs ranges up to several tens of μm². Closely apposed axon profiles may be outgrowing branches of a single parent axon, but anterograde tracer data indicate that many belong to independent neurons. Closely apposed axons are separated from one another, and from associated Schwann cell processes, by a cleft about 130 Å wide. No synapses, gap junctions or tight junctions were observed. Extracellular tracer studies using La³⁺ and Ruthenium Red indicated that the cleft system is patent, permitting the free diffusion of small molecules between the space adjacent to the axolemma and the bulk extracellular compartment. Together, these data provide a structural basis for interfibre interactions based on local electrical current flow (ephaptic crosstalk), as well as coupling mediated by K⁺ ions and neurotransmitter molecules.