Reappraisal of the model for "bulk axoplasmic flow"

Axonal neurofilamentous accumulations: a comparison between human and canine giant axonal neuropathy and 2,5-HD neuropathy

The neuropathy produced by the hexacarbon 2,5-hexanedione (2,5-HD) resembles human and canine inherited giant axonal neuropathy (GAN) in the presence of giant axonal swellings that contain accumulations of neurofilaments. The accumulations are both paranodal and internodal in GAN and 2,5-HD induced neuropathy. Detailed morphometry on the neurofilaments reveals that the changes in human and canine GAN are closely similar and differ from those of 2,5-HD neuropathy, suggesting that the mechanisms underlying the formation of the axonal neurofilamentous accumulations differ between the two conditions. In both human and canine GAN, the neurofilaments are more closely spaced and are of greater diameter than in 2,5-HD neuropathy. The changes in the NF in GAN may be the consequence of flattening of the side-arms of the neurofilaments against the axis of the filaments.

Conduction studies in peripheral cat nerve using implanted electrodes: III. The effects of prolonged constriction on the distal nerve segment

Electrophysiological properties were monitored in detail in chronically constricted peripheral nerves by implanted, multicontact nerve cuff electrodes and correlated with morphometric histology in selected cases. The physiological and histological responses in nerve to a range of constricting cuffs of standard sizes were readily graded. The initial response to any significant constriction was a transient, focal conduction slowing or block at the constriction, followed by more protracted distal effects; the latter ranged from loss of excitability consistent with "dying-back" degeneration to reductions in conduction velocity consistent with histologically observed atrophy. Smaller myelinated fibers tended to have similar but less pronounced changes than larger diameter fibers. Recordings from ventral and dorsal roots showed that distal degeneration was more pronounced in motor than in sensory fibers of similar caliber. Electronmicroscopical measurements showed that basal laminas were relatively preserved around even the most atrophic and demyelinated axons. Perimeter measurements of the basal lamina could be used to estimate the diameter of the original nerve fiber.

Ultrastructural and morphometric analysis of long-term peripheral nerve regeneration through silicone tubes

Light and electron microscopy were used to investigate long-term regeneration in peripheral nerves regenerating across a 10 mm gap through silicone tubes. Schwann cells and axons co-migrated behind an advancing front of fibroblasts, bridging the 10 mm gap between 28 and 35 days following nerve transection. Myelination of regenerated fibres started between 14 and 21 days after transection and occurred in a manner similar to that reported during development. Although these early events were successful in producing morphologically normal-appearing regenerated fibres, complete maturation of many of these fibres was never achieved. Axonal distortion by neurofilaments, axonal degeneration and secondary demyelination were seen at 56 days following nerve transection. These changes progressed in severity with time as more axons advanced through the distal stump towards their peripheral target. Since regeneration occurs in the absence of endoneurial tubes, and because constrictive forces act on the nerve during regeneration, we suggest that these extrinsic factors limit the successful advancement of axons through the distal stump to their target organ.

Some effects of prolonged constriction on nerve regeneration in the rabbit

Serial nerve conduction studies have been used to study the time-course of regeneration after tibial nerve crush in the rabbit, and the effect on this of a constricting ligature 5 mm distal to the site of crush. Transverse sections from the region of the ligature showed that when constriction was severe, the regenerating fibers had difficulty in penetrating the constricted region, and that some took an aberrant course in superficial connective tissue outside the ligature. In 2 out of 6 animals no fibres succeeded in passing through the centre of ligature, but aberrant fibres were able to re-enter distal fascicles and to reinnervate distal muscles. When compared with findings after crush alone, the time taken to reinnervate distal muscles was increased by a ligature, and distal fibre diameter and motor conduction velocity remained low. Low voltage, long-duration, polyphasic muscle action potentials were evoked by distal nerve stimulation, and axon reflexes suggested excessive axon branching within nerve trunks. Since these changes were seen in the distal parts of both constricted and aberrant fibres, they cannot be attributed to constriction alone. Similar changes might be expected in other peripheral nerve lesions in which loss of fascicular continuity and neuroma formation occur.

Changes in peripheral nerve fibres distal to a constriction

Continued constriction of the tibial nerve of the rabbit by a ligature was accompanied by a reduction in maximal motor conduction velocity distal to the ligature, and by a reduction in axonal and total fibre diameter. From the presence of paranodal demyelination and distal fibre degeneration in severely affected nerves, it is suggested that in some instances the change in axonal calibre was part of a progressive distal atrophy which could lead to secondary demyelination and ultimately to "dying-back" of the affected axons.

Slow axoplasmic transport of mitochondria (MAO) and lactic dehydrogenase in mammalian nerve fibers

The axoplasmic transport of the enzyme monoamine oxidase (MAO) (monoamine:O2 oxidoreductase, (deaminating) EC 1.4.3.4), a marker for mitochondria, and lactic dehydrogenase (LDH) (L-lactate:NAD oxidoreductase, EC 1.1.1927), a soluble component of axoplasm, was studied in cat sciatic nerve. For both these enzymes a linear accumulation was found in the nerve proximal to ligations over a period of at least 20 h. In double-ligation experiments no evidence of a depletion of enzymes within the nerve segment was found over this period of time as would be the case if some portion of the enzymes was carried by fast axoplasmic transport. Both the soluble protein enzyme LDH and the mitochondria, shown by MAO, are thus considered to be moved down the nerve by slow axoplasmic transport. Some differences in the two materials were seen in the greater fall in the level of MAO compared to LDH within the double-ligated segment over the succeeding period from 20 to 48 h. These changes are considered with respect to the transport filament model as modified to take into account slow axoplasmic transport.

Studies on the control of myelinogenesis. I. Myelination of regenerating axons after entry into a foreign unmyelinated nerve

The proximal stump of a predominantly myelinated nerve (sternohyoid) was anastomosed in a tube to the distal stump of a largely unmyelinated nerve (cervical sympathetic) in order to explore the role of the axon in activating Schwann cells to produce myelin. The two nerves were examined after being united for periods between 3 and 15 weeks. At these times, regenerating myelinated and nonmyelinated fibres, originating from the proximal stump, were seen within the original fascicle in the distal stump of the sympathetic trunk. There was a significant (P less than 0.01) increase in the mean number of myelinated fibres in the anastomosed sympathetic nerves from the mean number of myelinated fibres in the normal sympathetic nerve. The histological and ultrastructural features of the anastomosed nerves are described and the differences between the morphology of the intratubal and extratubal regions are highlighted. The results of this study may indicate that the axon instructs the Schwann cell to produce myelin, but before this conclusion is reached, the origin of the myelinating cells has to be determined.

Slow axonal transport or proteins; blockade by interruption of contact between cell body and axon

The influence of ligation and colchicine treatment on the axonal transport of slowly migrating [3H]leucine-labelled proteins was studied in the vagus nerve of the rabbit. Two days after [3H]leucine labelling of the dorsal motor nucleus of the vagus nerve, ligation or local application of 60 mM colchicine immediately blocked the further progression of slowly migrating proteins distal to the site of treatment. Application of 50-100 mug colchicine to the nerve cell bodies 2 days after labelling blocked the transport of slowly migrating proteins within the next 24 h. It is suggested that contact between nerve cell body and the axon is necessary for the maintenance of the slow transport of proteins in these nerves.