Motor neuron regeneration accuracy: balancing trophic influences between pathways and end-organs

The key to recovery of function following peripheral nerve lesions is guiding axons back to their original target end-organs. The parent femoral nerve splits into two comparable terminal pathways: one to the muscle and the other to the skin. Normally, motor neurons only innervate the pathway to the muscle, but after the parent nerve is repaired regenerating motor neurons are often misrouted to the skin. When the muscle and skin pathways remain connected to their respective targets after the parent nerve is repaired, reinnervation favors the muscle pathway. If contact with the muscle is instead prevented, reinnervation favors the pathway to the skin. Here we examine whether shortening the skin pathway can alter motor reinnervation accuracy when the muscle pathway remains connected to the muscle. We demonstrate that reducing the influence of the skin pathway results in a more rapid and extensive reinnervation of the muscle pathway. These findings suggest that the relative balance of trophic influences from the pathways and their end-organs is an important determinant of motor neuron regeneration accuracy, and that the muscle pathway by itself is not the primary regulator for regeneration accuracy of motor neurons.     

Developmentally regulated changes in femoral nerve regeneration in the mouse and rat

The attractive influence of muscle on regenerating motor neuron axons is well-known. Less is known, however, about the intrinsic abilities of different nerve pathways to support these axons prior to end-organ contact. The age at which a nerve injury is sustained is also known to affect the relationship between regenerating motor axons and muscle. The femoral nerve model, with its distinct muscle and cutaneous pathways, is ideal to study intrinsic pathway properties because the influence of end-organs can easily be removed surgically. However, recent results using this model in adult mice are at odds with the same model in neonatal rats. To reconcile these discrepancies, we used the femoral nerve model to examine possible age differences in intrinsic pathway support for regenerating motor neurons in the mouse and rat. Rat motor neurons showed a preference to regenerate into the muscle pathway after axotomy at 3 weeks of age, but this preference was lost after axotomy at 6 weeks of age. Interestingly, mouse motor neurons showed no pathway preference after axotomy at 3 weeks of age but developed one for the cutaneous pathway after axotomy at 6 weeks of age. These results suggest that in the absence of end-organ contact there is no general preference for motor neurons to project to the muscle pathway.     

Effects of motor versus sensory nerve grafts on peripheral nerve regeneration

Autologous nerve grafting is the current standard of care for nerve injuries resulting in a nerve gap. This treatment requires the use of sensory grafts to reconstruct motor defects, but the consequences of mismatches between graft and native nerve are unknown. Motor pathways have been shown to preferentially support motoneuron regeneration. Functional outcome of motor nerve reconstruction depends on the magnitude, rate, and precision of end organ reinnervation. This study examined the role of pathway type on regeneration across a mixed nerve defect. Thirty-six Lewis rats underwent tibial nerve transection and received isogeneic motor, sensory or mixed nerve grafts. Histomorphometry of the regenerating nerves at 3 weeks demonstrated robust nerve regeneration through both motor and mixed nerve grafts. In contrast, poor nerve regeneration was seen through sensory nerve grafts, with significantly decreased nerve fiber count, percent nerve, and nerve density when compared with mixed and motor groups (P < 0.05). These data suggest that use of motor or mixed nerve grafts, rather than sensory nerve grafts, will optimize regeneration across mixed nerve defects.     

The impact of motor and sensory nerve architecture on nerve regeneration

Sensory nerve autografting is the standard of care for injuries resulting in a nerve gap. Recent work demonstrates superior regeneration with motor nerve grafts. Improved regeneration with motor grafting may be a result of the nerve's Schwann cell basal lamina tube size. Motor nerves have larger SC basal lamina tubes, which may allow more nerve fibers to cross a nerve graft repair. Architecture may partially explain the suboptimal clinical results seen with sensory nerve grafting techniques. To define the role of nerve architecture, we evaluated regeneration through acellular motor and sensory nerve grafts. Thirty-six Lewis rats underwent tibial nerve repairs with 5 mm double-cable motor or triple-cable sensory nerve isografts. Grafts were harvested and acellularized in University of Wisconsin solution. Control animals received fresh motor or sensory cable isografts. Nerves were harvested after 4 weeks and histomorphometry was performed. In 6 animals per group from the fresh motor and sensory cable graft groups, weekly walking tracks and wet muscle mass ratios were performed at 7 weeks. Histomorphometry revealed more robust nerve regeneration in both acellular and cellular motor grafts. Sensory groups showed poor regeneration with significantly decreased percent nerve, fiber count, and density (p < 0.05). Walking tracks revealed a trend toward improved functional recovery in the motor group. Gastrocnemius wet muscle mass ratios show a significantly greater muscle mass recovery in the motor group (p < 0.05). Nerve architecture (size of SC basal lamina tubes) plays an important role in nerve regeneration in a mixed nerve gap model.     

Manipulations of the mouse femoral nerve influence the accuracy of pathway reinnervation by motor neurons

Previous studies using the femoral nerve model in both mice and rats have shown that regenerating motor axons prefer to reinnervate the terminal nerve branch to muscle versus a terminal nerve branch to skin, a process that has been termed preferential motor reinnervation (PMR). If end organ contact with muscle and skin is prevented, this preferential motor reinnervation still occurs in the rat. To better understand the process of preferential motor reinnervation in the mouse, we examined motor neuron reinnervation of muscle and cutaneous pathways without any end organ contact as well as with only cutaneous end organ contact. Surprisingly, there was no preferential motor reinnervation: Motor neurons preferred the cutaneous pathway over the muscle pathway when all end organ contact was prevented and showed an even greater preference for the cutaneous pathway when it was attached to skin.     

Matching of motor-sensory modality in the rodent femoral nerve model shows no enhanced effect on peripheral nerve regeneration

The treatment of peripheral nerve injuries with nerve gaps largely consists of autologous nerve grafting utilizing sensory nerve donors. Underlying this clinical practice is the assumption that sensory autografts provide a suitable substrate for motoneuron regeneration, thereby facilitating motor endplate reinnervation and functional recovery.This study examined the role of nerve graft modality on axonal regeneration, comparing motor nerve regeneration through motor, sensory, and mixed nerve isografts in the Lewis rat. A total of 100 rats underwent grafting of the motor or sensory branch of the femoral nerve with histomorphometric analysis performed after 5, 6, or 7 weeks. Analysis demonstrated similar nerve regeneration in motor, sensory, and mixed nerve grafts at all three time points. These data indicate that matching of motor-sensory modality in the rat femoral nerve does not confer improved axonal regeneration through nerve isografts.     

Specific and nonspecific regeneration of motor axons after sciatic nerve injury and repair in the rat

The pattern of motor axon regeneration following unilateral sciatic nerve lesions (freezing or transection) was studied in adult rats. Transected nerves were repaired with epineurial or fascicular sutures. Four months after the lesion, the motor neuron cell body localization in the spinal cord of plantar or common peroneal nerve axons were examined bilaterally with retrograde transport of horseradish peroxidase. Motor neuron cell body localization was similar bilaterally after freezing, indicating that regenerating axons had reached their original peripheral innervation territory. However, after nerve transection, irrespective of whether epineurial or fascicular sutures were used, motor neuron cell body distribution on the operated side was abnormal with numerous labeled cell bodies located outside the area of the normal motor neuron pool. This finding indicates that after nerve transection the normal pattern of motor axon innervation is not restored even after fascicular nerve repair.     

Motor nerve biopsy studies in motor neuropathy and motor neuron disease

The clinical presentation of motor neuropathy often resembles that of motor neuron disease, sometimes leading to an erroneous diagnosis. Moreover, the underlying pathological process in motor neuropathy has been rarely investigated and there are no systematic studies of the affected motor nerves. We describe a new motor nerve biopsy procedure, performed in 15 patients: 6 with motor neuropathy and 9 with motor neuron disease. The motor branch from the anterior division of the obturator nerve to the gracilis muscle in the thigh was biopsied. In both groups of patients the motor nerves exhibited depletion of myelinated nerve fibers. In motor neuropathy there was a significantly higher density of regenerative clusters of small myelinated fibers in comparison to motor nerves from patients with motor neuron disease. In addition, in 3 patients with motor neuropathy there was evidence for demyelination with thinly myelinated axons and small onion bulb formations. These pathological studies of motor nerve biopsies can help to differentiate motor neuropathy from motor neuron disease. © 1997 John Wiley & Sons, Inc.     

Motor neurons can preferentially reinnervate cutaneous pathways

Previous work in the rat femoral nerve has shown that regenerating motor neurons preferentially reinnervate a terminal nerve branch to muscle as opposed to skin. This process has been termed preferential motor reinnervation (PMR) and has been interpreted as evidence that regenerating motor axons can differentiate between Schwann cell tubes that reside in muscle versus cutaneous terminal pathways. However, much of this previous work has been confounded by motor axons having access to target muscle during the regeneration period. The present experiments prevented muscle contact by regenerating motor axons. By 8 weeks under these conditions, significantly more motor neurons reinnervated the cutaneous pathway rather than the original muscle pathway. We propose that cutaneous and muscle terminal pathways are not inherently different in terms of their ability to support regeneration of motor neurons. Rather, we suggest that it is the relative level of trophic support provided by each nerve branch that determines whether motor axons will remain in that particular branch. Within the context of the femoral nerve model, our results suggest a hierarchy of trophic support for regenerating motor axons with muscle contact being the highest, followed by the length of the terminal nerve branch and/or contact with skin.     

Modification of motor neuron size and position in the central nervous system of adult snapping shrimps

Cell bodies of claw closer motor neurons in snapping shrimp are dimorphic. Snapper claw motor neurons are larger than corresponding pincer claw motor neurons, but the relative sizes of these cells are reversed during claw transformation. An additional neuronal modification occurs early within this period, in that the pincer claw dorsal inhibitor cell body migrates within the nervous system, from a dorsal to a ventral position. These findings are evidence of rapid, reversible changes in the nervous system following the trigger for the transformation process.     

Influence of aging on peripheral nerve function and regeneration

Aging deeply influences several morphologic and functional features of the peripheral nervous system (PNS). Morphologic studies have reported a loss of myelinated and unmyelinated nerve fibers in elderly subjects, and several abnormalities involving myelinated fibers, such as demyelination, remyelination and myelin balloon figures. The deterioration of myelin sheaths during aging may be due to a decrease in the expression of the major myelin proteins (P0, PMP22, MBP). Axonal atrophy, frequently seen in aged nerves, may be explained by a reduction in the expression and axonal transport of cytoskeletal proteins in the peripheral nerve. Aging also affects functional and electrophysiologic properties of the PNS, including a decline in nerve conduction velocity, muscle strength, sensory discrimination, autonomic responses, and endoneurial blood flow. The age-related decline in nerve regeneration after injury may be attributed to changes in neuronal, axonal, Schwann cell and macrophage responses. After injury, Wallerian degeneration is delayed in aged animals, with myelin remnants accumulated in the macrophages being larger than in young animals. The interaction between Schwann cells and regenerative axons takes longer, and the amount of trophic and tropic factors secreted by reactive Schwann cells and target organs are lower in older subjects than they are in younger subjects. The rate of axonal regeneration becomes slower and the density of regenerating axons decrease in aged animals. Aging also determines a reduction in terminal and collateral sprouting of regenerated fibers, further limiting the capabilities for target reinnervation and functional restitution. These age-related changes are not linearly progressive with age; the capabilities for axonal regeneration and reinnervation are maintained throughout life, but tend to be delayed and less effective with aging.     

运动神经元病162例的节段性运动神经传导测定分析

目的探讨运动神经元病(motor neuron d isease,MND)常规节段运动神经传导和位移技术检测的特点。方法对162例MND患者和60名健康对照进行常规节段运动神经传导测定,同时对部分神经采用位移技术测定,并进行分析比较。结果(1)健康人常规节段运动神经传导测定显示:近端与远端比较,波幅和面积下降程度均小于20%,时限增宽小于15%;(2)在MND患者,常规节段测定共有76个节段(5.57%)波幅下降超过20%,45个节段(3.30%)面积下降超过20%,76个节段(5.57%)时限延长超过15%。仅有4例(2.5%)患者4条神经的4个常规节段(0.29%)达到运动神经部分性传导阻滞标准,但采用位移技术测定时均未达到短节段传导阻滞的诊断标准。结论在大部分MND患者常规节段运动神经传导测定正常,在部分患者也可以出现“传导阻滞样”的电生理表现,但其发生率极低,进一步采用位移技术测定有助于鉴别。     

The reorganization of motor units in motor neuron disease

We studied 78 patients with motor neuron disease (MND) using concentric needle electromyography. Analysis on weak and maximal effort was performed using our own, fully automated, computer method, EMG-LAB. In addition to the conventional parameters of single motor unit action potentials (MUAPs) and interference pattern, new criteria were applied: the range of the acting motor units and the functional recruitment order. A total of 375 muscles of MND patients and 120 control muscles were investigated. The electromyographic data were analyzed separately in five groups of muscles, classified A, B, C, D, and E according to their clinical condition. Those results allowed us to discern six neurophysiological stages (N _{0, 1, 2, 3, 4, 5}) from the early to the most advanced phase. It has been confirmed that reinnervation in MND is adequate to compensate for the loss of over 50% of motor neurons but it is only a transitory phase in the morbid course. At stages N_0–5, the electrophysiological data reflect structural and functional integrity of the functioning motor units. Evaluation of not only single MUAPs but also of the full range of acting motor units and their recruitment order allowed a deeper look into the underlying pathophysiological mechanisms. © 1997 John Wiley & Sons, Inc. Muscle Nerve, 20, 306–315, 1997.     

End-to-side neurorrhaphy repairs peripheral nerve injury: sensory nerve induces motor nerve regeneration

End-to-side neurorrhaphy is an option in the treatment of the long segment defects of a nerve.It involves suturing the distal stump of the disconnected nerve(recipient nerve) to the side of the intimate adjacent nerve(donor nerve).However,the motor-sensory specificity after end-to-side neurorrhaphy remains unclear.This study sought to evaluate whether cutaneous sensory nerve regeneration induces motor nerves after end-to-side neurorrhaphy.Thirty rats were randomized into three groups:(1) end-to-side neurorrhaphy using the ulnar nerve(mixed sensory and motor) as the donor nerve and the cutaneous antebrachii medialis nerve as the recipient nerve;(2) the sham group:ulnar nerve and cutaneous antebrachii medialis nerve were just exposed;and(3) the transected nerve group:cutaneous antebrachii medialis nerve was transected and the stumps were turned over and tied.At 5 months,acetylcholinesterase staining results showed that 34% ± 16% of the myelinated axons were stained in the end-to-side group,and none of the myelinated axons were stained in either the sham or transected nerve groups.Retrograde fluorescent tracing of spinal motor neurons and dorsal root ganglion showed the proportion of motor neurons from the cutaneous antebrachii medialis nerve of the end-to-side group was 21% ± 5%.In contrast,no motor neurons from the cutaneous antebrachii medialis nerve of the sham group and transected nerve group were found in the spinal cord segment.These results confirmed that motor neuron regeneration occurred after cutaneous nerve end-to-side neurorrhaphy.     

Application of weak electric field to the hindpaw enhances sciatic motor nerve regeneration in the adult rat

Direct current (DC) electrical stimulation of the hindpaw is shown to enhance sciatic motor nerve regeneration in the adult rat. Cathodal stimulation, using weak currents (10 microA/cm2; field strength approximately 100 mV/cm) increased the reinnervation of the hindpaw muscles as measured by evoked electromyograms. This enhanced regeneration only occurred after cut-and-suture lesions, but not after crushing injury of the sciatic nerve. This enhancement of motor nerve regeneration by weak DC fields had been previously described in amphibians but we are the first to describe this phenomenon in mammals.     

Decremental responses to repetitive nerve stimulation (RNS) in motor neuron disease

Objective

To clarify the features of decremental responses following repetitive nerve stimulation in patients with motor neuron diseases (MNDs), in comparison with myasthenia gravis (MG).

Methods

The subjects consisted of 48 MND, 39 generalized MG and 19 ocular MG patients. Six muscles, both proximal and distal muscles, were tested.

Results

Significant decrements (>5%) in at least one muscle were observed in 83% of the MND patients, and 74% and 47% of the generalized MG and ocular MG patients, respectively. Decrements were more frequently observed in the proximal muscles both in MND and MG patients (deltoid 76% and 62%, and trapezius 71% and 51% for MND and generalized MG, respectively), suggesting lower safety factors in neuromuscular transmission in those muscles. Decrements in the nasalis were rare in MND (8%) in comparison with generalized MG (54%).

Conclusions

Decremental responses were frequently observed in MND patients. There were small differences between MND and MG regarding the distribution and other features of decrements, such as the degree of the U-shape or the responses to different stimulus frequencies and to brief exercise.

Significance

These results imply that the underlying mechanism regulating the decrements is common to MND and MG.     

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.     

The spectrum of lower motor neuron syndromes

Abstract. This review discusses the most important lower motor neuron syndromes. This relatively rare group of syndromes has not been well described clinically. Two subgroups can be distinguished: patients in whom motor neurons (lower motor neuron disease (LMND)) are primarily affected or motor axons and their surrounding myelin (multifocal motor neuropathy (MMN)), both leading to muscle atrophy and weakness.Both hereditary and sporadic forms of LMND have been described. The discussion of recent advances in the genetic knowledge of several hereditary forms of LMND may lead to a better understanding of the pathophysiology and the development of therapeutic strategies. By contrast, the pathogenesis of sporadic LMND is largely unknown. It is, therefore, difficult to consider the various sporadic forms of LMND, discussed in this review, as separate diseases. Because the diagnostic and therapeutic options may differ, it would seem rational to consider sporadic LMND as a spectrum of syndromes which can be distinguished from each other on the basis of clinical presentation.MMN is a lower motor neuron syndrome with presumed immunemediated pathogenesis. Evidence of motor conduction block on nerve conduction studies and a positive response to treatment with intravenous immunoglobulins (IVIg) are considered the most relevant criteria for the diagnosis of MMN. As it is treatable, it is important to distinguish MMN from LMND. Careful electrophysiological analysis in the search for conduction block is, therefore, required in all adult patients with pure lower motor neuron syndromes. For the individual patient, dist inction between the various lower motor neuron syndromes is important as it enables the physician to provide adequate information over the disease course in LMND and to facilitate early treatment in MMN.Supported by a grant from the Prinses Beatrix Fonds.     

运动神经元病患者骨骼肌纤维再生病理研究

目的观察运动神经元病患者骨骼肌纤维再生病理特点。方法选择经骨骼肌活检确诊为运动神经元病患者骨骼肌标本,冰冻连续切片,行HE染色、免疫组化染色,光镜观察肌纤维再生情况,部分留取电镜标本在电镜下观察骨骼肌纤维再生超微结构特点。结果活检骨骼肌标本在组化及免疫组化染色中未见明显再生肌纤维,电镜下观察到运动神经元病患者骨骼肌内可见静止、活化的肌卫星细胞。结论电镜下观察到运动神经元病患者肌肉内存在肌卫星细胞的活化、分裂及早期再生肌纤维。     

Experimental porta-caval anastomosis and motor nerve conduction velocity in the rat

Summary The influence of portal-systemic shunting on maximal motor nerve conduction velocity (MCV) was analyzed in rats with portacaval shunts. At 3 and 8 weeks after the shunting there was a marked but transient fall in MCV. This was not prevented by colectomy. At 16 weeks the MCV had almost normalized, despite sustained portal-systemic shunting. When examined at 3 weeks postoperatively, MCV was less affected by ischaemia than in control animals. The results favour hepatocellular failure as the more important pathophysiological mechanism in hepatic neuropathy.