Junction between parent and daughter axons in regenerating myelinated nerve: properties of structure and rapid axonal transport

Impact of peripheral nerve injury on sensorimotor control

Deficits in sensorimotor control are experienced immediately after nerve injury due to changes in the periphery and central nervous system. Muscle denervation and sensory loss often disrupt prehensile coordination requiring the use of alternative strategies. To effectively foster coordination postinjury clinicians should address not only impairments and function but motor control issues through the prescription of specific sensory and motor experiences. Engagement in carefully planned, therapeutic activity can take advantage of the nervous systems' ability to regenerate and reorganize following nerve lesions. This article reviews motor control issues and neural reorganization concepts that may influence the recovery of skilled prehension following upper limb nerve injury. It also provides clinical guidelines for examining and enhancing coordination.

Changes in water diffusion due to Wallerian degeneration in peripheral nerve

The authors report NMR measurements of the changes in water diffusion brought about by in vivo Wallerian degeneration due to either crush- or tie-injuries in the sciatic nerve of the frog. Using a pulsed-gradient spin-echo sequence with a diffusion measurement time of 28 ms, the degree of diffusion coefficient anisotropy ¿D(longitudinal)/D(transverse)¿ 4 weeks after injury in both crush- and tie-injured nerves (2.3 +/- 0.4 and 1.7 +/- 0.1, respectively) is significantly less than in normal frog sciatic nerve (3.9 +/- 0.4). The decrease of anisotropy in the degenerated nerves is due to both a decrease in longitudinal diffusion and an increase in transverse diffusion. The changes in diffusion coefficients are compared with the degree of axonal and myelin breakdown observed in light and electron micrographs of the nerves.

Multiexponential T2 relaxation in degenerating peripheral nerve

The multiexponential T2 relaxation spectrum of peripheral nerve undergoing Wallerian degeneration has been measured both in vivo and in vitro. Degeneration of the sciatic nerve of the amphibian Xenopus laevis was induced by crush injury, and T2 relaxation spectra of the nerve were measured at several times up to 35 days following injury. Histologic evidence verified that the nerve underwent Wallerian degeneration. Relaxation spectra were observed to undergo measurable changes as degeneration progressed, the most evident being a reduction from three well-resolved T2 components to one and a decline in the fraction of the spectra associated with the shortest T2 component. The former appears to reflect the collapse and loss of myelinated fibers, while the latter a combination of interstitial edema and myelin loss.

Anterograde to retrograde reversal of fast axonal transport within cold blocked and rewarmed intact axons

The possibility that anterograde to retrograde reversal of axonal transport might take place in mid axon at a site distant from any nerve termination was investigated in sciatic nerve preparations from Xenopus laevis. The nerve, containing a pulse of anterogradely transported protein labeled with [35S]methionine, was kept in a two-compartment temperature controlled chamber. One compartment containing the proximal nerve was maintained at room temperature throughout the duration of an experiment while the second compartment containing the distal nerve, and separated from the first by a thermal barrier, was initially cooled to 3-4 degrees C and later warmed to room temperature. Transport of labeled proteins in the nerve was detected with a position-sensitive detector of ionizing radiation. With the distal portion of the nerve cold, the pulse of labeled protein transported up to the thermal barrier and stopped. When the distal part of the nerve was warmed to room temperature, retrograde and anterograde pulses of label propagated away from the thermal barrier with no time delay. The retrograde pulse could be collected on the distal side of a proximally placed tie and could be eliminated by treatment of the proximal nerve with vinblastine or dinitrophenol. Functional and structural evidence indicated that the cold block and thermal barrier were not destructive to the axons. Electron microscopy showed that the numerical density of axonal microtubules distal to the cold block was decreased about seven fold during the cold treatment and that this decrease could be prevented by 10 mumol/l taxol.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationships between the rapid axonal transport of newly synthesized proteins and membranous organelles

Rapid axonal transport is generally viewed as being exactly analogous to the secretory process in nonneuronal cells. The cell biology of rapid axonal transport is reviewed, the central concern being to explore those aspects that do not fit into the general secretory model and which may thus represent specific neuronal adaptations. Particular attention is paid to the relationship between the transport of newly synthesized proteins and of the membranous organelles that act as carriers. Sites in the transport sequence at which the behavior of axonal transport may differ from the secretory model are at the initiation of axonal transport at the trans-side of the Golgi apparatus, within the axon where molecules are deposited from the moving phase to a stationary phase, and at nerve terminals or axonal lesions where transport reversal takes place.

Reversal of rapid axonal transport at a lesion: leupeptin inhibits reversed protein transport, but does not inhibit reversed organelle transport

The hypothesis that the reversal of rapid axonal transport requires a proteolytic conversion of anterograde transport vesicles was examined using sciatic nerve preparations from Xenopus laevis and leupeptin as an inhibitor of proteolysis. The transport of newly synthesized 35S-labeled proteins was studied with a position-sensitive detector of radiation. Organelle transport in isolated myelinated axons was studied by video microscopy. Leupeptin (0.1-0.4 mM) reduced the anterograde-to-retrograde reversal of protein transport adjacent to an axonal lesion. In experiments in which organelle transport was observed close to lesions in axons maintained in a medium compatible with intracellular function, 1.0 mM leupeptin inhibited neither the anterograde-to-retrograde nor the retrograde-to-anterograde reversal of organelle transport. In addition, in experiments in which conditions approximated those used to study protein transport, organelle transport away from the lesion was not inhibited by 1.0 mM leupeptin. A comparison of the morphology of rapidly transported organelles that underwent anterograde transport to the morphology of those that returned from a lesion (with or without the presence of leupeptin) provided no evidence that a morphological conversion was a necessary step in transport reversal.

Neurotoxic acrylamide and neurotrophic melanocortin peptides--can contrasting actions provide clues about modes of action?

Experimental acrylamide neuropathy has been studied as a model of degenerative neurological disorders of the 'dying-back' type for over 30 years. Functional, histological, ultrastructural, electrophysiological and biochemical aspects of acrylamide neuropathy have been described and several hypotheses concerning the mode of action proposed. However, the mechanism whereby acrylamide causes axonal degeneration and inhibits nerve sprouting remains unknown. By analogy with agonist/antagonist comparisons used by the pharmacologist, we have reconsidered the acrylamide problem in the light of the opposite effects summarized in Table 1, of neurotrophic peptides related to ACTH/MSH (collectively termed melanocortins). The contrasting effects on sprouting and the eventual quality of repair of mechanically lesioned nerves have suggested a mechanism whereby sprouting may regulate perikaryal adjustments to injury. We have also posed the question as to whether a common biochemical mechanism, namely selective proteolysis of neurofilament protein may underlie the opposing effects of acrylamide and melanocortins on nerve sprouting. This possibility implies a hitherto unknown role for neurofilament protein turnover in neuronal maintenance and repair, a suggestion that may provoke further research and discussion.