Ultrastructural alterations of primary afferent axons in the nucleus gracilis after peripheral nerve axotomy.

Alterations of primary afferent axons in the nucleus gracilis were studied at three weeks and four, 12, and 24 months after peripheral nerve axotomy by hindlimb amputation at the hip joint in 19 female cats. The contralateral side and two cats not subjected to amputation were controls. We observed two major types of fiber alterations. One, fibers showed changes of axonal atrophy, myelin remodelling, and degeneration. Adaxonal invagination occurred more frequently (p less than 0.005) at three weeks postamputation, in territories known to contain centrally directed axons of primary afferent neurons of the lower limb, than in controls. Perhaps adaxonal sequestration contributed to axonal attenuation. Two, there were filamentous, granular, central core, and other types of axonal swellings found in territories containing central axons of primary afferent neuron terminals of the lower limb. These changes occurred most frequently at 12 and 24 months and were significantly more frequent than in control tissue. These reactive/dystrophic axons therefore were associated with permanent axotomy, but we have not established that they occur in central axons of primary afferent neurons, or whether they are degenerative or abortive regenerative changes.