Numbers of regenerating axons in parent and tributary peripheral nerves in the rat

This study is concerned with numerical parameters of axonal regeneration in peripheral nerves. Our first finding is that the number of axons that regenerate into the distal stump of a somatic nerve at a particular time after transection is partially dependent on the type of lesion used to interrupt the axons. The second question concerns the proportion of axons that regenerate into the distal stump of a parent nerve compared to the proportions that regenerate into tributary nerves that arise from the parent. The proportions of regenerated myelinated axons in the nerve to the medial gastrocnemius muscle and myelinated and unmyelinated axons in the sural nerve are the same as the proportions of myelinated and unmyelinated axons that regenerate into the distal stump of the sciatic nerve for the crush, 0 and 4 mm gap transections. Proportionally fewer axons regenerate into the tributary nerves following the 8 mm gap transection, however. This implies that the length of the gap has an influence on whether or not axons in tributary nerves regenerate in concert with axons in the distal stump of the parent nerve. The unmyelinated fibers in the nerve to the medial gastrocnemius muscle are different because they do not regenerate in proportion to those in the distal stump of the sciatic nerve. We also provide evidence to indicate that myelinated axons branch whereas unmyelinated fibers end blindly when they enter the distal stump after crossing a sciatic nerve transection. Finally the normal arrangement of perineurial cells seems to be disrupted after the sciatic nerve regenerates across a gap.     

Single vs multiple somatic nerve crushes in the rat

Nerve lesions modify regenerative responses to subsequent lesions. Some of the modifications might be useful. To increase our understanding of these modifications, the present study determines myelinated and unmyelinated axon numbers in the distal part of rat sciatic nerve and in 2 smaller branches, the nerve to the medial gastrocnemius muscle and the sural nerve, 8 weeks and 9 months following either single or the last of 3 crushes to the rat sciatic nerve. For myelinated axons, there is a significant and proportional increase distal to the crush in the sciatic nerve and in its smaller tributaries following both single and triple crushes. These increased axons persist. We interpret these data to indicate that some of the regenerating myelinated axons branch at the site of lesion, pass without branching into the tributary nerves, and then presumably find attachments at the periphery. If true, single or multiple crushes might be useful in conditions where it would be desirable to increase numbers of processes from surviving neurons. The major differences between single and triple crushes are that myelinated axons are increased more after triple crush and increase significantly between 8 weeks and 9 months after triple crush but not after single crush. Thus not only myelinated axon numbers, but the timing of the myelination process seems to change if regeneration following single crush is compared to similar regeneration following multiple crushes. Unmyelinated axons do not regenerate in the same way as the myelinated axons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sciatic nerve regeneration after autologous sural nerve transplantation in the rat

The present study is concerned with the question as to whether the size of a nerve used as a transplant to bridge a gap between the stumps of transected nerves has a bearing on the number of axons and the cytological structure of the regenerate. The paradigm is rat sciatic nerve transection with 8 mm of nerve removed with the stumps placed in a silicone tube and two strands of the smaller sural nerve used as bridging transplants. The comparisons are with previously published results where the transplant, which is the removed piece of sciatic nerve, is exactly matched in size and with no transplant in the same regeneration paradigm. One surprising finding is that the size of the transplant does not seem to determine the size of the regenerated nerve. The cytological structure of the regenerated nerve is related to the size of the transplant, however, in that the proportion of axons that regenerate inside and outside the transplanted perineurial tubes differs in relation to the size of the transplant. In addition, although there is an increase in the number of blood vessels in all of these paradigms, the greatest increase is with the sural nerve transplants. The key finding in the study, however, is the similarity in numbers of regenerated axons in the gap, distal stump and tributary nerves when regeneration after sciatic nerve transplantation is compared with regeneration after sural nerve transplantation. Thus, notwithstanding the cytologic differences of the two types of regenerate, regenerated axon numbers are approximately the same. The conclusion is that the size of the transplant determines neither the size of the regenerate nor the numbers of regenerated axons in this paradigm. On the assumption that regeneration is better when axonal numbers are closer to normal, the non-matched sural nerve transplant is approximately equal to the matched sciatic nerve transplant and both are superior to the regeneration that takes place in the absence of a transplant in this paradigm.     

Regeneration of unmyelinated and myelinated sensory nerve fibres studied by a retrograde tracer method

Regeneration of myelinated and unmyelinated sensory nerve fibres after a crush lesion of the rat sciatic nerve was investigated by means of retrograde labelling. The advantage of this method is that the degree of regeneration is estimated on the basis of sensory somata rather than the number of axons. Axonal counts do not reflect the number of regenerated neurons because of axonal branching and because myelinated axons form unmyelinated sprouts. Two days to 10 weeks after crushing, the distal sural or peroneal nerves were cut and exposed to fluoro-dextran. Large and small dorsal root ganglion cells that had been labelled, i.e., that had regenerated axons towards or beyond the injection site, were counted in serial sections. Large and small neurons with presumably myelinated and unmyelinated axons, respectively, were classified by immunostaining for neurofilaments. The axonal growth rate was 3.7 mm/day with no obvious differences between myelinated and unmyelinated axons. This contrasted with previous claims of two to three times faster regeneration rates of unmyelinated as compared to myelinated fibres. The initial delay was 0.55 days. Fewer small neurons were labelled relative to large neurons after crush and regeneration than in controls, indicating that regeneration of small neurons was less complete than that of large ones. This contrasted with the fact that unmyelinated axons in the regenerated sural nerve after 74 days were only slightly reduced.     

Long-term patterns of axon regeneration in the sciatic nerve and its tributaries

The present study determines the numbers of axons that regenerate after sciatic nerve transection in the rat. The transections are done by removing either 4 mm or 8 mm of the nerve. The axons are counted in the gap and distal stump of the sciatic nerve and in 5 of its tributaries. Survival time is 9 months which we define as long-term to allow comparison with short-term data obtained after a much shorter survival. The first findings is that the numbers of axons in the gap and distal stump are different in the 2 transection paradigms. For the 4 mm paradigm, more axons than normal appear in the gap and only a fraction of these pass into the distal stump. For the 8 mm paradigm, the numbers of axons in the gap are normal and the numbers in the distal stump do not deviate far from these. Thus by changing only the length of the segment of removed nerve, one causes major differences in the numbers of axons that regenerate. Second the numbers of axons that regenerate in tributary nerves that innervate muscle have a different pattern than the numbers that regenerate into cutaneous nerves. Thus the factors control axonal numbers must be different in the 2 types of nerves. Finally, axons that regenerate into tributary nerves do not, by and large, regenerate in concert with those in the distal stump of the parent nerve. Thus the factors that control axonal numbers in the tributary nerves must be different from those that control the numbers in the distal stump of the parent nerve.     

Nerve regeneration through holey silicone tubes

Recent studies focus on regeneration where nerve stumps are placed in a silicone tube. Since the tube is impermeable, the fluid and cells that collect from the stumps bath the axons. This is presumably beneficial. Making the tube permeable by making holes in its walls should change the patterns of regeneration. If this is done, the major cytologic change is an increase in the fascicular perineurium. There are more individual fascicles, more cells line each fascicle and the lining cells are coated by more prominent external laminae than after similar regeneration in a regular silicone tube or in the normal untransected nerve. For axonal numbers, there are more myelinated and unmyelinated axons in the gap and more unmyelinated axons in the distal stump than after regeneration in a regular silicone tube. The numbers in the holey tube regenerate are statistically different from normal but they are closer to normal than after similar regeneration in a regular silicone tube. There are significantly fewer myelinated and unmyelinated axons than in the normal sural nerve after regeneration through a holey tube, but there are more than after regeneration through a regular tube. The numbers of axons in the nerve to the medial gastrocnemius muscle are not significantly different from normal or from the other regeneration paradigms. These data allow the suggestion that regeneration through a silicone tube with macroscopic holes in its walls may be superior in certain respects to regeneration through a regular impermeable silicone tube.     

Axonal branching following crush lesions of peripheral nerves of rat

Branching of myelinated and unmyelinated nerve fibers in normal and regenerating personal and soleus nerves was studied by light and electron microscopy. There were at most 2% more myelinated and 13% more unmyelinated axons in the distal as compared with the proximal nerve segments. Two to four weeks after a crush lesion the distal axons became 2-3 times more numerous; thereafter their number decreased. The number of axons in the proximal nerve segment did not change. The number of myelinated sprouts in most regenerated nerves equalled the number of myelinated fibers in the proximal nerve, while the number of unmyelinated axons after 12-19 weeks was 18-60% higher than normal. Branching was not restricted to the crush region. The results indicate that following a crush lesion all axons branch but only branches of unmyelinated fibers persist for a prolonged period of time. It is tentatively suggested that regenerating axons branch when searching for a target and that when contact is made with the target this prevents additional branching and eliminates redundant branches. Myelinated axons are guided by existing Schwann cells, whereas unmyelinated axons do not follow predetermined pathways; this may explain their greater tendency to form permanent branches.     

Changes in myelinated and unmyelinated axon numbers in the proximal parts of rat sural nerves after two types of injury

Counts of myelinated and unmyelinated axon profiles have been made from normal, uninjured rat sural nerves and from nerves injured 6 months earlier in one of two ways. In one group of rats the nerve was simply cut and left to regenerate, leading to the development of a neuroma in continuity, while in the second group the nerve was cut but then ligated as well to prevent regeneration; this led to stump neuroma formation. After nerve transection and regeneration, with subsequent formation of a neuroma in continuity, there was no change in the number of myelinated axon profiles found 25 mm proximal to the old injury site when compared with control, but there was an 18% reduction (P < 0.05) in the number of unmyelinated axon profiles. Immediately proximal to the injury site the picture was similar, with there still being the same number of myelinated axon profiles as in control material but here the reduction in unmyelinated axon numbers was slightly greater at 24% (P < 0.05). In the proximal part of nerves that had been cut and stump neuroma formation induced there was a large increase (33%) in myelinated axon profiles over and above control values (P < 0.001) but the number of unmyelinated profiles was the same as in controls. Closer to the stump neuroma the number of myelinated axon profiles had increased yet further to be 88% (P < 0.001) above control while the number of unmyelinated ones remained no different from control. Our interpretation of these results is that after nerve transection and regeneration there is no loss of peripheral neurons supporting myelinated axons but some loss of those supporting unmyelinated ones. If a cut nerve is prevented from regenerating and a stump neuroma forms, however, a vigorous sprouting response is triggered in neurons with myelinated axons while those supporting unmyelinated axons are possibly prevented from dying. The reaction of peripheral neurons to injury is such that the number of axons they support varies along the nerve as one goes disto-proximally away from the injury site. Thus discrepancies in results from different laboratories have come about because material for axon counting has been taken from different points along the nerve relative to the injury site and also because the material has been taken from nerves injured in different ways.     

The numbers of unmyelinated and myelinated axons in normal and regenerated rat saphenous nerves

Counts have been made of the numbers of unmyelinated and myelinated axons in the proximal and distal stumps of regenerated rat saphenous nerves and from equivalent sites in normal nerves. In the proximal part of normal nerves there were averages of 1 045 myelinated axons and 4 160 unmyelinated ones. Regenerated nerves contained the same number of myelinated axons in their proximal stumps but there was a 40% reduction in the unmyelinated axon count. In the distal stumps of these nerves the myelinated axon count had increased by an average of 620; this comes about because some regenerated myelinated axons support more than one process in the distal stump. In contrast, the number of unmyelinated axons was reduced further, from a mean of 2 476 in the proximal stump to one of 2 219.

The sizes of Schwann cell units in the normal and regenerated nerves were also noted. Schwann cell units in the proximal and distal stumps of the regenerated nerves were smaller than those in the normal ones.

These changes associated with unmyelinated axons in regenerated nerves are likely to contribute to the sensory, vasomotor and sudomotor abnormalities that sometimes occur after peripheral nerve injury and regeneration.     

Axonal regeneration in an articular branch following rat sciatic nerve lesions

This study examines the outcome of axonal regeneration in the posterior articular nerve of the adult rat knee joint (PAN), after sciatic nerve lesions. Some animals had previously been subjected to chemical sympathectomy with guanethidine. In crushed cases the number of myelinated PAN axons was 50% above control level. The occurrence of C-fibers was doubled, mainly due to an increased number of sympathetic efferents. In neurotomy/suture cases the number of myelinated fibers was clearly elevated, but the number of C-fibers was close to normal. Most C-fibers were sensory. Similar, but less marked, post-regeneration abnormalities were seen in the nerve to the lateral gastrocnemius muscle. The sural nerve exhibited moderately increased numbers of myelinated and unmyelinated fibers in crushed cases. In neurotomy cases, the myelinated axons had increased and the C-fibers had decreased in number. The size distribution of myelinated PAN axons was less abnormal in crushed cases than after neurotomy, like in the other nerves. These results show that the outcome of axon regeneration in an articular branch of the lesioned rat sciatic nerve differs from that in non-articular branches, and suggest that joints may become hyperinnervated by C-fibers after nerve crush lesions.     

Unmyelinated nerve fibres in feline acrylamide neuropathy

Electronmicroscope studies have been performed on the greater splanchnic nerve and the nerve to the medial head of gastrocnemius muscle of control and acrylamide poisoned cats. Degeneration of unmyelinated as well as of myelinated fibres was observed in both nerves. In cats severely poisoned with acrylamide, some very large unmyelinated axons undergoing early degeneration were seen in the splanchnic nerve. In the nerve to medial head of gastrocnemius, there was a decrease in the proportion of large diameter unmyelinated axons.     

Axonal regeneration in the foot branch of the superficial peroneal nerve and the lateral plantar nerve of the rat after sciatic nerve lesions

Hand injuries with nerve lesions often leave the patient with a persistent sensory deficit, particularly with respect to glabrous skin. The present study examines axonal regeneration in the foot branch of the superficial peroneal nerve (fSPN) and the lateral plantar nerve (LPN), supplying hairy skin and glabrous skin together with some intrinsic muscles, respectively, after sciatic nerve lesions in the rat. Following crush lesions, the number of myelinated axons is normal in the fSPN, and the occurrence of C-fibers appears slightly reduced. In the LPN, the numbers of myelinated axons and C-fibers are both significantly increased. Post-crush regenerated myelinated fSPN and LPN axons show normal size ranges, but the proportion of small myelinated axons is increased. After neurotomy and suture, the numbers of myelinated axons and C-fibers in the fSPN are not significantly different from normal. The LPN exhibits a significantly increased number of myelinated axons, but the number of C-fibers is not significantly abnormal. In both nerves, the myelinated axons present an abnormally narrow size range. These findings show that the quantitative outcome of regeneration in a nerve innervating glabrous skin (and some intrinsic muscles) differs significantly from that of branches to hairy skin of the foot, with respect to myelinated as well as unmyelinated axons. To what extent these differences mirror functional differences awaits elucidation.     

Unmyelinated nerve fiber degeneration in chronic inflammatory demyelinating polyneuropathy

To determine whether unmyelinated nerve fibers escape degeneration as one might expect in an immune response exclusively directed at myelin, we performed a morphometric examination of unmyelinated axons and myelinated nerve fibers in sural nerve biopsy specimens of 14 patients with a chronic inflammatory demyelinating polyneuropathy (CIDP) and of 12 age-matched normal controls. The numbers of unmyelinated axons, myelinated nerve fibers, denervated Schwann cell units and collagen pockets were quantified and related to the clinical and electrophysiological data of the patients with CIDP. In 4 patients with a rapid onset of the neuropathy and a highly elevated CSF protein, the numbers of both unmyelinated axons and myelinated nerve fibers were decreased equally. In 8 patients we found that the unmyelinated axons were relatively spared compared with the loss of myelinated nerve fibers. In these patients, however, the presence of denervated Schwann cell units and of collagen pockets was increased. We conclude that unmyelinated nerve fibers are affected in patients with CIDP.     

Numerical patterns of axon regeneration that follow sciatic nerve crush in the neonatal rat

Different proportions of axons regenerate in cutaneous nerves compared with muscle nerves after sciatic nerve crush in the rat. The questions here are whether or not these differences reflect the proportions of axons that regenerate in the different nerves from which these nerves arise and do the differences persist. The findings indicate that the numbers of axons that regenerate in the muscle nerves do not reflect the numbers in the nerve from which the muscle nerves originate and that these differences persist. This leads to the conclusion that some factors determining numbers of axons in the muscle nerves must operate distal to the lesion. Thus when one is considering mechanisms that control regenerated axon numbers after neonatal nerve crush, one must consider not only the lesion site but also branching and axon diversion far distally, presumably at the origin of the muscle nerves.     

Ultrastructural analysis of NMDA,AMPA, and kainate receptors on unmyelinated and myelinated axons in the periphery

The present study determines the proportions of unmyelinated cutaneous axons at the dermal–epidermal junction in glabrous skin and of myelinated and unmyelinated axons in the sural and medial plantar nerves that immunostain for subunits of the ionotropic glutamate receptors. Approximately 20% of the unmyelinated cutaneous axon profiles at the dermal–epidermal junction immunostain for either N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), or kainate receptor subunits. These findings are consistent with previous observations that NMDA and non-NMDA antagonists ameliorate nociceptive behaviors that result from noxious peripheral stimulation. In the sural nerve, where the large majority of myelinated fibers are sensory, approximately half of the myelinated axon profiles immunostain for the NMDA receptor 1 (R1) subunit, 28% immunostain for the glutamate receptor 1 (GluR1) AMPA subunit, and 11% for the GluR5,6,7 kainate subunits. Even higher proportions immunostain for these receptors in the medial plantar nerve, a mixed sensory and motor nerve. In the sural nerve, 20% of the unmyelinated axon profiles immunostain for NMDAR1 and only 7% label for GluR1 or GluR5,6,7. Because the sural nerve innervates hairy skin, these data suggest that glutamate will activate a higher proportion of unmyelinated axons in glabrous skin than in hairy skin. Measurements of fiber diameters indicate that all sizes of myelinated axon profiles, including Aδ and Aβ, are positively labeled for the ionotropic receptors. The presence of glutamate receptors on large-diameter myelinated axons suggests that these mechanosensitive receptors, presumably transducing touch and pressure, may also respond to local glutamate and thus be chemosensitive. J. Comp. Neurol. 391: 78–86, 1998. © 1998 Wiley-Liss, Inc.     

Directional specificity of regenerating primary sensory neurons after peripheral nerve crush or transection and epineurial suture A sequential double-labeling study in the rat

Clinical and experimental observations have demonstrated that peripheral nerve transection generally results in lasting disturbed sensory discrimination whereas nerve crush is followed by more or less complete functional restoration. This has been explained by an increased misdirection of regenerating fibers after transection as compared to crush injury. In the present study, sequential double-labeling was used to investigate the relative proportions of peripherally misdirected sensory fibers in the sural and tibial nerve branches after crush or transection of the parent sciatic nerve in the rat. Control experiments showed that 0.21% ± 0.12 (mean ± S.D.) of all labeled tibial and sural neurons normally send axons to both nerves. After sciatic nerve crush or transection, 1.31% ± 0.78 and 3.79% ± 3.01, respectively, of all labeled tibial and sural axons were double-labeled indicating previously sural axons now having an axon in the tibial. Statistically significant differences in the percentages of bidirectional sciatic sensory neurons were found between the normal controls and after crush injury (P < 0.01) or transection injury (P < 0.001), respectively, but not between transection and crush (P > 0.05). The results indicate that the number of sensory neurons having an axon in two peripheral nerves is normally very small, that a substantial number of sensory axons become misdirected after both crush and transection with resuture, and that the number of misdirected fibers in the major sciatic branches after these types of injury is similar.     

Multipotentiality of Schwann cells in cross-anastomosed and grafted myelinated and unmyelinated nerves: quantitative microscopy and radioautography.

Cross-anastomoses and autogenous grafts of unmyelinated and myelinated nerves were examined by electron microscopy and radioautography to determine if Schwann cells are multipotential with regard to their capacity to produce myelin or to assume the configuration seen in unmyelinated fibres. Two groups of adult white mice were studied. (A) In one group, the myelinated phrenic nerve and the unmyelinated cervical sympathetic trunk (CST) were cross-anastomosed in the neck. From 2 to 6 months after anastomosis, previously unmyelinated distal stumps contained many myelinated fibres while phrenic nerves joined to proximal CSTs became largely unmyelinated. Radioautography of distal stumps indicated that proliferation of Schwann cells occurred mainly in the first few days after anastomosis but was also present to a similar extent in isolated stumps. (B) In other mice, CSTs were grafted to the myelinated sural nerves in the leg. One month later, the unmyelinated CSTs became myelinated and there was no radioautographic indication of Schwann cell migration from the sural nerve stump to the CST grafts. Thus, Schwann cell proliferation in distal stumps is an early local response independent of axonal influence. At later stages, axons from the proximal stumps cause indigenous Schwann cells in distal stumps from the previously unmyelinated nerves to produce myelin while Schwann cells from the previously unmyelinated nerves to produce myelin while Schwann cells from the previously myelinated nerves become associated with unmyelinated fibres. Consequently, the regenerated distal nerve resembled the proximal stump. It is suggested that this change is possible because Schwann cells which divide after nerve injury reacquire the developmental multipotentiality which permits them to respond to aoxonal influences.     

Sciatic nerve regeneration across gaps within silicone chambers: long-term effects of NGF and consideration of axonal branching

We examined whether the short-term beneficial effects of nerve growth factor (NGF) upon regeneration are sustained over a prolonged period of time across 8-mm gaps within silicone chambers. Rat sciatic nerve regeneration both with and without NGF was examined after 10 weeks. Myelinated counts from the regenerated sciatic and distal tributary nerves were correlated to the numbers of motor and sensory neurons retrogradely labeled with horseradish peroxidase (HRP) applied distal to the regenerated segment. Regenerated sciatic and sural nerves were examined ultrastructurally for morphological analysis. Both regenerated groups by 10 weeks achieved essentially complete counts of myelinated axons in the distal tributary nerves and the regenerated segment of the sciatic nerve compared to the uninjured controls. There were similar numbers of retrogradely labeled sensory and motor neurons in the dorsal root ganglia (DRG) and lumbar spinal cord of both groups and, surprisingly, of the uninjured normal control group. Ultrastructural analysis demonstrated no difference in the distribution of axonal diameters or myelin thickness between the regenerated groups. In evaluating regeneration in experimental silicone chamber models, it is important to determine such parameters as the percentage of neurons that grow across the gap and the incidence of axonal sprouting. One can then make accurate assessments of experimental perturbations and predict whether they improve the naturally occurring regeneration through chambers. These results must ultimately be compared with equivalent determinations in the uninjured nerve. At 10 weeks there was essentially complete regeneration of both the NGF and control regenerative groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sex-related difference in collateral sprouting of nociceptive axons after peripheral nerve injury in the rat

Possible sex-related differences in the extent of collateral sprouting of noninjured nociceptive axons after peripheral nerve injury were examined. In the first experiment, peroneal, tibial, and saphenous nerves were transected and ligated in female and male rats. Eight weeks after nerve injury, skin pinch tests revealed that the nociceptive area of the noninjured sural nerve in the instep skin expanded faster in females; the final result was a 30% larger increase in females than in males. In the second experiment, the end-to-side nerve anastomosis was used as a model for axon sprouting. In addition to the previous procedure, the end of an excised peroneal nerve segment was sutured to the side of the intact sural nerve. Eight weeks later, collateral sprouting of nociceptive axons into the anastomosed peroneal nerve segment was assessed by the nerve pinch test and axon counting. There was no significant difference with respect to the percentages of male and female rats with a positive nerve pinch test. The number of myelinated axons in the anastomosed nerve segment was significantly larger in female (456 +/- 217) than in male (202 +/- 150) rats, but the numbers of unmyelinated axons were not significantly different. In normal sural nerves, the numbers of either all myelinated axons or thin myelinated axons did not significantly differ between the two sexes. Therefore, the more extensive collateral axon sprouting observed in female than in male rats is probably due to the higher sprouting capacity of thin myelinated sensory axons in females.     

Topography of unmyelinated axons in regenerated soleus nerves of the rat.

The topography of unmyelinated axons on cross sections of normal and regenerated soleus nerves of rat was studied by electron microscopy. The experimental nerves were crushed and assessed after 1-19 weeks. Unmyelinated axons in normal nerves were arranged in few groups. Nerve crush did not alter the arrangement of unmyelinated axons in the proximal nerve. Distal to the crush lesion, however, the unmyelinated axons became scattered throughout the entire cross section. The grouping of unmyelinated axons within the cross section was quantitated by means of a "clustering factor", defined as the percentage of unmyelinated axons in those 10% of the cross-sectional area which had the highest density of unmyelinated axons. The results indicate that unmyelinated axons during regeneration do not follow their original pathways.