CLOSE APPOSITION AMONG NEIGHBORING AXONAL ENDINGS IN A NEUROMA

Axons in intact peripheral nerve trunks constitute independent afferen t and efferent communication channels. However, when nerves are severe d, several different forms of axon-axon cross-excitation develop in as sociation with the injury site. In this study we have examined experim ental sciatic nerve-end neuromas in rats with special interest in the compartmentalization of individual axons, and the barriers that separa te close neighbours. At postinjury times at which functional coupling is known to occur, neuromas were found to contain many examples of axo ns in which adjacent membrane faces come into close contact without an intervening Schwann cell process. These occur in bundles containing f rom two to as many as 30 individual nerve fibres wrapped in a common S chwann cell sheath. The surface area of close apposition between axon pairs ranges up to several tens of mum2. Closely apposed axon profiles may be outgrowing branches of a single parent axon, but anterograde t racer data indicate that many belong to independent neurons. Closely a pposed axons are separated from one another, and from associated Schwa nn cell processes, by a cleft about 130 angstrom wide. No synapses, ga p junctions or tight junctions were observed. Extracellular tracer stu dies using La3+ and Ruthenium Red indicated that the cleft system is p atent, permitting the free diffusion of small molecules between the sp ace adjacent to the axolemma and the bulk extracellular compartment. T ogether, these data provide a structural basis for interfibre interact ions based on local electrical current flow (ephaptic crosstalk), as w ell as coupling mediated by K+ ions and neurotransmitter molecules.