A bioenergetic basis for peripheral nerve fiber dissociation

The selective vulnerability of myelinated axons in lesions of peripheral nerve is incompletely understood and appears somewhat at variance with the energy conservation attached to saltation in these fibers. We evaluated the relative energy requirement of resting A and C fibers in rabbit vagus nerve by measuring the amplitude of the components of the compound action potential at 5-10 min intervals during incubation in Ringer-bicarbonate solutions containing 0-20 mM glucose. In nerves in which the perineurial sheath was retained intact the A components remained at control amplitude with 20 mM glucose but, after a plateau period, declined increasingly rapidly with 5, 2, 1, 0.5 and 0 mM glucose. 2mM glucose sufficed to maintain control amplitude of the C fiber component. In desheathed nerves the A component remained at control amplitude with 5 mM glucose but declined increasingly rapidly with 2, 1, 0.5 and 0 mM glucose; 0.5 mM glucose sufficed to maintain control amplitude to C fibers. The depressed potentials generally recovered incompletely after transfer to glucose 5 mM (desheathed) or 20 mM (sheathed); however, the partial recovery was more rapid and more nearly complete in the C fiber group than in the A fiber group (P less than 0.05). The data demonstrate that resting A fibers are much more susceptible to energy lack in vitro than resting C fibers. This suggests that deprivation of energy may be a factor in the preferential destruction of large fibers, termed fiber dissociation, which characterized several syndromes of chronic pain.