Incomplete rematching of nerve and muscle properties in motor units after extensive nerve injuries in cat hindlimb muscle

Outcomes after Anterior Interosseous Nerve to Ulnar Motor Nerve Transfer

Background  Ulnar nerve lesions proximal to the elbow can result in loss of intrinsic muscle function of the hand. The anterior interosseous nerve (AIN) to deep motor branch of the ulnar nerve (DBUN) transfer has been demonstrated to provide intrinsic muscle reinnervation, thereby preventing clawing and improving pinch and grip strength. The purpose of this study was to evaluate the efficacy of the AIN to DBUN transfer in restoring intrinsic muscle function for patients with traumatic ulnar nerve lesions. Methods  We performed a prospective, multi-institutional study of outcomes following AIN to DBUN transfer for high ulnar nerve injuries. Twelve patients were identified, nine of which were enrolled in the study. The mean time from injury to surgery was 15 weeks. Results  At final follow-up (mean postoperative follow-up 18 months + 15.5), clawing was observed in all nine patients with metacarpophalangeal joint hyperextension of the ring finger averaging 8.9 degrees (+ 10.8) and small finger averaging 14.6 degrees (+ 12.5). Grip strength of the affected hand was 27% of the unaffected extremity. Pinch strength of the affected hand was 29% of the unaffected extremity. None of our patients experienced claw prevention after either end-to-end ( n  = 4) or end-to-side ( n  = 5) AIN to DBUN transfer. Conclusion  We conclude that, in traumatic high ulnar nerve injuries, the AIN to DBUN transfer does not provide adequate intrinsic muscle reinnervation to prevent clawing and normalize grip and pinch strength.

Hemihypoglossal-facial nerve anastomosis: results and electromyographic characterization

INTRODUCTION

The facial nerve surgery belongs to the basic procedures during lateral skull base approaches. Its damage has serious medical and psychological consequences, and therefore mastery of reconstruction and correction techniques should belong to the repertoire of skull base surgeons. The goal of this study was to demonstrate usefulness of electromyographic follow-up in facial nerve reconstruction.

MATERIAL AND METHODS

A total of 16 patients underwent hemihypoglossal-facial anastomosis between 2005 and 2017. Most of the primary lesions came from vestibular schwannoma surgery. All patients were examined with electromyography and scored according to the House-Brackmann and IOWA grading scales. Function of the tongue has been evaluated.

RESULTS

Ten patients achieved definitive House-Brackmann grade 3 score (62.5%). We did not observe any association with the patient's age, previous irradiation and the etiology of the damage. Electromyography showed pathological spontaneous activity after the first surgery. Incipient regeneration potentials were detected in 4-17 months (average 7.6) and reached maximum in 6.5-18 months (average 16). Electromyographic assessment of the effect of tongue movement showed better mimic voluntary activity by swallowing or by moving the tongue up. There was no relationship between the start of activity and the interval to achieving maximal activity.

CONCLUSION

Hemihypoglossal-facial nerve anastomosis is a safe procedure and it is an optimal solution for cases lacking a proximal stump or in the case of reconstruction in the second stage. Electromyography can predict initial reinnervation activity after reconstructive procedures. During subsequent follow-up it can help to discover insufficiently recovering patients, however clinical characteristics are crucial.

Peripheral nerves preferentially regenerate in intramuscular endoneurial tubes to reinnervate denervated skeletal muscles

Schwann cells are essential for peripheral nerve regeneration but, over short distances in acellular nerve grafts, extracellular matrix (ECM) molecules can support growth. The ECM molecules are present also on denervated muscle surfaces where they can support nerve growth. In this study, we addressed the efficacy of the ECM molecules of denervated muscle to support nerve fiber regeneration and muscle reinnervation. In the hindlimb of Sprague-Dawley rats, the proximal stump of the transected posterior tibial nerve, was cross-sutured to the distal nerve stump (NN) of each of three denervated muscles, tibialis anterior, extensor digitorum longus, and soleus, or implanted onto the denervated muscles' surfaces (N-M), proximal or distal to the endplate zone. Recordings of muscle and motor unit (MU) isometric forces and silver/cholinesterase histochemical staining of longitudinal muscle cryosections were used to determine the numbers of reinnervated MUs and the spatial course of regenerating nerve fibers, respectively. MU numbers declined significantly after N-M (>50%) as compared to those after NN. Muscle forces were reduced despite each nerve reinnervating up to three times the normal MU muscle fiber number. Regenerating nerves 'streamed' from the N-M site either proximal or distal to endplate zones toward the denervated intramuscular endoneurial tubes, with reduced numbers reinnervating endplates. We conclude that there is preferential reinnervation through the endoneurial tube and that it is important to drive implanted nerve fibers to enter endoneurial tubes for optimal muscle reinnervation. Schwann cells play the essential role in guiding regenerating nerve fibers to reinnervate denervated muscle fibers.

Postsurgical Electrical Stimulation Enhances Recovery Following Surgery for Severe Cubital Tunnel Syndrome: A Double-Blind Randomized Controlled Trial

BACKGROUND

Patients with severe cubital tunnel syndrome often have poor functional recovery with conventional surgical treatment. Postsurgical electrical stimulation (PES) has been shown to enhance axonal regeneration in animal and human studies.

OBJECTIVE

To determine if PES following surgery for severe cubital tunnel syndrome would result in better outcomes compared to surgery alone.

METHODS

Patients with severe cubital tunnel syndrome in this randomized, double-blind, placebo-controlled trial were randomized in a 1:2 ratio to the control or stimulation groups. Control patients received cubital tunnel surgery and sham stimulation, whereas patients in the stimulation group received 1-h of 20 Hz PES following surgery. Patients were assessed by a blinded evaluator annually for 3 yr. The primary outcome was motor unit number estimation (MUNE) and secondary outcomes were grip and key pinch strength and McGowan grade and compound muscle action potential.

RESULTS

A total of 31 patients were enrolled: 11 received surgery alone and 20 received surgery and PES. Three years following surgery, MUNE was significantly higher in the PES group (176 ± 23, mean + SE) compared to controls (88 ± 11, P < .05). The mean gain in key pinch strength in the PES group was almost 3 times greater than in the controls (P < .05). Similarly, other functional and physiological outcomes showed significantly greater improvements in the PES group.

CONCLUSION

PES enhanced muscle reinnervation and functional recovery following surgery for severe cubital tunnel syndrome. It may be a clinically useful adjunct to surgery for severe ulnar neuropathy, in which functional recovery with conventional treatment is often suboptimal.

Peripheral Nerve Regeneration and Muscle Reinnervation

Injured peripheral nerves but not central nerves have the capacity to regenerate and reinnervate their target organs. After the two most severe peripheral nerve injuries of six types, crush and transection injuries, nerve fibers distal to the injury site undergo Wallerian degeneration. The denervated Schwann cells (SCs) proliferate, elongate and line the endoneurial tubes to guide and support regenerating axons. The axons emerge from the stump of the viable nerve attached to the neuronal soma. The SCs downregulate myelin-associated genes and concurrently, upregulate growth-associated genes that include neurotrophic factors as do the injured neurons. However, the gene expression is transient and progressively fails to support axon regeneration within the SC-containing endoneurial tubes. Moreover, despite some preference of regenerating motor and sensory axons to “find” their appropriate pathways, the axons fail to enter their original endoneurial tubes and to reinnervate original target organs, obstacles to functional recovery that confront nerve surgeons. Several surgical manipulations in clinical use, including nerve and tendon transfers, the potential for brief low-frequency electrical stimulation proximal to nerve repair, and local FK506 application to accelerate axon outgrowth, are encouraging as is the continuing research to elucidate the molecular basis of nerve regeneration.

Nerve Regeneration: Understanding Biology and Its Influence on Return of Function After Nerve Transfers

Poor functional outcomes are frequent after peripheral nerve injuries despite the regenerative support of Schwann cells. Motoneurons and, to a lesser extent, sensory neurons survive the injuries but outgrowth of axons across the injury site is slow. The neuronal regenerative capacity and the support of regenerating axons by the chronically denervated Schwann cells progressively declines with time and distance of the injury from the denervated targets. Strategies, including brief low-frequency electrical stimulation that accelerates target reinnervation and functional recovery, and the insertion of cross-bridges between a donor nerve and a recipient denervated nerve stump, are effective in promoting functional outcomes after complete and incomplete injuries.

Synkinesis following recurrent laryngeal nerve injury: A computer simulation

OBJECTIVES/HYPOTHESIS

When the recurrent laryngeal nerve (RLN) is injured, functional recovery may be limited by the number of axons that regrow across the site of injury, and by the proportions of these axons that reinnervate the antagonistic muscle (synkinesis). This process was investigated in a computer model of RLN recovery.

STUDY DESIGN

Computer simulation.

METHODS

The developed computer program accepted as inputs: number of RLN axons; proportions of axons originally innervating adductor versus abductor, fraction of axons transected, fraction of axons that grow back, and width of 1 standard deviation about the mean. The program employed random sampling from a normal distribution to model various degrees of recovery, using random numbers to assign each axon to the correct muscle, an incorrect muscle, or no recovery. Each simulation was run 1,000×, and the mean, highest, and lowest degrees of synkinesis were determined.

RESULTS

More severe injuries were associated with greater degrees of synkinesis. Extremes of synkinesis were possible but were rare. One example result, for a 50% injury with a 50% recovery rate, found: in the adductor muscles, 74.8% of axons will be innervated, of which 49.2% are the original uninjured axons, 19.4% are recovered adductor axons, and 6.2% are misdirected abductor axons. In the posterior cricoarytenoid (PCA), these values were 75.7%, 50.8%, 6.1%, and 18.8%, respectively. Results of many such simulations are plotted.

CONCLUSIONS

Laryngeal synkinesis can be simulated based on known anatomic ratios and estimated recovery rates. The PCA is invariably much more affected by synkinetic reinnervation than are the adductor muscles.

LEVEL OF EVIDENCE

NA Laryngoscope, 126:1600-1605, 2016.

Changes in motor unit properties in SOD1 (G93A) rats

INTRODUCTION

In amyotrophic lateral sclerosis (ALS), progressive death of motor neurons results in denervation and reinnervation of muscles. It is not clear how ALS affects the properties of motor units (MUs).

METHODS

Properties of single MUs in the medial gastrocnemius (MG) muscle of rats bearing the human mutated superoxide dismutase gene type 1 (SOD1) were determined at 3 stages: asymptomatic (ALS I); early symptomatic (ALS II); and terminal (ALS III).

RESULTS

In ALS II, higher proportions of FF (fast fatigable) and S (slow) MUs were observed, whereas in ALS III higher percentages of S and lower percentages of FF MUs were noted compared with controls. S motor neurons reinnervated fast muscle fibers, and those MUs gained some properties of fast MUs, including lower fatigue resistance, greater force generation, and higher action potential amplitudes.

CONCLUSION

Changes in MU properties of SOD1 rats have progressive and multidirectional character and speed depending on the MU type.

S-nitrosoglutathione reductase deficiency-induced S-nitrosylation results in neuromuscular dysfunction

AIMS

Nitric oxide (NO) production is implicated in muscle contraction, growth and atrophy, and in the onset of neuropathy. However, many aspects of the mechanism of action of NO are not yet clarified, mainly regarding its role in muscle wasting. Notably, whether NO production-associated neuromuscular atrophy depends on tyrosine nitration or S-nitrosothiols (SNOs) formation is still a matter of debate. Here, we aim at assessing this issue by characterizing the neuromuscular phenotype of S-nitrosoglutathione reductase-null (GSNOR-KO) mice that maintain the capability to produce NO, but are unable to reduce SNOs.

RESULTS

We demonstrate that, without any sign of protein nitration, young GSNOR-KO mice show neuromuscular atrophy due to loss of muscle mass, reduced fiber size, and neuropathic behavior. In particular, GSNOR-KO mice show a significant decrease in nerve axon number, with the myelin sheath appearing disorganized and reduced, leading to a dramatic development of a neuropathic phenotype. Mitochondria appear fragmented and depolarized in GSNOR-KO myofibers and myotubes, conditions that are reverted by N-acetylcysteine treatment. Nevertheless, although atrogene transcription is induced, and bulk autophagy activated, no removal of damaged mitochondria is observed. These events, alongside basal increase of apoptotic markers, contribute to persistence of a neuropathic and myopathic state.

INNOVATION

Our study provides the first evidence that GSNOR deficiency, which affects exclusively SNOs reduction without altering nitrotyrosine levels, results in a clinically relevant neuromuscular phenotype.

CONCLUSION

These findings provide novel insights into the involvement of GSNOR and S-nitrosylation in neuromuscular atrophy and neuropathic pain that are associated with pathological states; for example, diabetes and cancer.

Fatigue of muscles weakened by death of motoneurons

Weakness is a characteristic of muscles influenced by the postpolio syndrome (PPS), amyotrophic lateral sclerosis (ALS), and spinal cord injury (SCI). The strength deficits relate to changes in muscle use and to the chronic denervation that can follow the spinal motoneuron death common to these disorders. PPS, ALS, and SCI also involve variable amounts of supraspinal neuron death, the effects of which on muscle weakness remains unclear. Nevertheless, weakness of muscle itself defines the functional consequences of these disorders. A weaker muscle requires an individual to work that muscle at higher than usual intensities relative to its maximal capacity, inducing progressive fatigue and an increased sense of effort. Little evidence is available to suggest that the fatigue commonly experienced by individuals with these disorders relates to an increase in the intrinsic fatigability of the muscle fibers. The only exception is when SCI induces chronic muscle paralysis. To reduce long-term functional deficits in these disorders, studies must identify the signaling pathways that influence neuron survival and determine the factors that encourage and limit sprouting of motor axons. This may ensure that a greater proportion of the fibers in each muscle remain innervated and available for use.

Fatigue properties of human thenar motor units paralysed by chronic spinal cord injury

Human muscles paralysed chronically by spinal cord injury (SCI) fatigue excessively. Whether these reductions in force reflect a decrease in the fatigue resistance of the motor units is unknown. Our aim was to determine the fatigability of thenar motor units paralysed chronically (10 +/- 2 years) by cervical SCI. Surface electromyographic activity (EMG) and force were recorded from 17 paralysed motor units (n = 7 subjects) in response to intraneural motor axon stimulation (13 pulses at 40 Hz, 1 s(-1) for 2 min). Unit force decreased progressively, reaching 8-60% of initial after 2 min, whereas both the amplitude and area of the first EMG potentials in the trains increased significantly (both P < 0.05). Thus, transmission of neural signals to the sarcolemma was effective and the reduction in force must reflect impaired processes in the muscle fibres. The median fatigue index for paralysed units (0.31), the ratio of the force at 2 min compared to the initial force, was significantly lower than that for units from control subjects (0.85, P < 0.05), but the distribution of fatigue indices for each population had a similar shape (ranges: 0.08-0.60 and 0.41-0.95, respectively). Hence, chronic paralysis did not limit the range of fatigability typically found for thenar units, only its magnitude. These findings suggest that all paralysed units underwent similar reductions in fatigue resistance. After fatigue, paralysed unit forces were reduced at all frequencies (1-100 Hz, P < 0.05). Twitch contraction and half-relaxation times were increased, as was the frequency needed to produce half maximal force (P < 0.05). Thus, stimulation protocols used to produce functional movements in paralysed muscles need to accommodate the significant and rapid fatigue of the motor units.

Short- or long-term effects of adult myoblast transfer on properties of reinnervated skeletal muscles

Skeletal muscle demonstrates a force deficit after repair of injured peripheral nerves. Data from the literature indicate that myoblast transfer enhances recovery of muscle function. Thus, we tested the hypothesis that transfer of adult myoblasts improves the properties of reinnervated rabbit tibialis anterior (TA) muscles in both the short term (4 months) and long term (14 months). Two months after transection and immediate suture of the common peroneal nerve, TA muscles were made to degenerate by cardiotoxin injection and then transplanted with adult myoblasts cultured for 13 days. Under these conditions, muscles studied at 4 months were heavier, contained larger fibers, and developed a significantly higher maximal force than muscles that had only been denervated-reinnervated. In the long term, although muscles made to degenerate were heavier and developed a significantly higher maximal force than denervated-reinnervated muscles, myoblast transfer failed to improve these parameters. However, the overall characteristics of long-term operated muscles tended clearly to approach those of the controls. Taken together, these results may have significant implications in certain orthopedic contexts, particularly after immediate or delayed muscle reinnervation.

Polysialylated neural cell adhesion molecule is necessary for selective targeting of regenerating motor neurons

It is well established that peripheral nerves regenerate after injury. Therefore, incomplete functional recovery usually results from misguided axons rather than a lack of regeneration per se. Despite this knowledge very little is known about the molecular mechanisms regulating axon guidance during regeneration. In the developing neuromuscular system the neural cell adhesion molecule (NCAM) and its polysialic acid (PSA) moiety are essential for proper motor axon guidance. In this study we used a well established model of nerve transection and repair to examine whether NCAM and/or PSA promotes selective regeneration of femoral motor nerves in wild-type and NCAM (-/-) mice. We found that regenerating axons innervating the muscle pathway and, to a lesser extent, cutaneous axons in the sensory pathway reexpress high levels of PSA during the time when the cut axons are crossing the lesion site. Second, we found that motor neurons in wild-type mice preferentially reinnervated muscle pathways, whereas motor neurons in NCAM (-/-) mice reinnervated muscle and cutaneous pathways with equal preference. Preferential regeneration was not observed in wild-type mice when PSA was removed enzymatically from the regenerating nerve, indicating that this form of selective motor axon targeting requires PSA. Finally, transgenic mice were used to show that the number of collateral sprouts, their field of arborization, and the withdrawal of misprojected axons were all attenuated significantly in mice lacking PSA. These results indicate that regenerating motor axons must express polysialylated NCAM, which reduces axon-axon adhesion and enables motor neurons to reinnervate their appropriate muscle targets selectively.

Transection of peripheral nerves, bridging strategies and effect evaluation

Disruption of peripheral nerves due to trauma is a frequently occurring clinical problem. Gaps in the nerve are bridged by guiding the regenerating nerves along autologous grafts or artificial guides. This review gives an overview on the different methods of nerve repair techniques. Conventional suturing techniques are discussed as well as the use of e.g. biological, synthetic, non-degradable or degradable nerve guides. Functional assessment showed that repair of a gap with a bio-degradable guide is superior to that with autologous grafts. But still, long lasting changes were observed in the Sciatic Function Index (SFI), abnormal walking patterns, disturbed Electro Myo Graphic (EMG) patterns, next to shifts in the histochemical properties of the muscles and longlasting abnormalities in neuromuscular contacts. These phenomena are explained by an at-random reinnervation. When transecting the nerve at young ages, this did not lead to enhanced recovery. Rearing rats operated at adult age in an enriched environment, also had no beneficial effect. Future research should aim at developing longer guides, possibly lined with Schwann cells, or additives, improving specific reinnervation of the former target areas.

Measuring myoelectric fatigue of the serratus anterior in healthy subjects and patients with long thoracic nerve palsy

Proper function of serratus anterior plays a vital role in the movement and stability of the scapula, and thus of the glenohumeral joint and the upper limb. The unique anatomy of this muscle makes direct measurements of its fatigue properties impossible. Here we describe for the first time indirect measurements of the myoelectric manifestations of fatigue in the serratus anterior. Eight healthy volunteers (29-35 years) were tested, four of them on two different occasions, using two exercise protocols (60 s isometric maximum upward force in 120 degrees arm flexion, and 60 s maximum forward force at 90 degrees arm flexion) with simultaneous recording by surface and wire electrodes applied according to established methods. Signals were analysed to obtain the rate of fall of median EMG frequency and the rate of rise of amplitude. Both exercise protocols gave similar results. Frequency-slope measurements (mean rate of fall 0.6+/-0.1% initial value per second (% s(-1)) with both surface and wire electrodes) were more precise than those of amplitude (mean rate of rise 2.6+/-0.3% s(-1) with surface electrodes, only 1.3+/-0.2% s(-1) with wire electrodes). Surface electrodes gave much lower variation than fine wires, the coefficient of variation of slopes for surface electrodes being approximately 20-40% both between studies in a single subject and between subjects. In 5 patients (aged 22-59 years) recovering from long thoracic nerve palsy studied using surface electrodes the frequency slopes was normal (0.6+/-0.1% s(-1)), while the amplitude slope was reduced (0.9+/-0.4% s(-1), P = 0.01). This shows abnormal fatigue properties of the reinnervated muscle and a dissociation between the frequency and amplitude manifestations of fatigue.

The resilience of the size principle in the organization of motor unit properties in normal and reinnervated adult skeletal muscles

Henneman's size principle relates the input and output properties of motoneurons and their muscle fibers to size and is the basis for size-ordered activation or recruitment of motor units during movement. After nerve injury and surgical repair, the relationship between motoneuron size and the number and size of the muscle fibers that the motoneuron reinnervates is initially lost but returns with time, irrespective of whether the muscles are self- or cross-reinnervated by the regenerated axons. Although the return of the size relationships was initially attributed to the recovery of the cross-sectional area of the reinnervated muscle fibers and their force per fiber, direct enumeration of the innervation ratio and the number of muscle fibers per motoneuron demonstrated that a size-dependent branching of axons accounts for the size relationships in normal muscle, as suggested by Henneman and his colleagues. This same size-dependent branching accounts for the rematching of motoneuron size and muscle unit size in reinnervated muscles. Experiments were carried out to determine whether the daily amount of neuromuscular activation of motor units accounts for the size-dependent organization and reorganization of motor unit properties. The normal size-dependent matching of motoneurons and their muscle units with respect to the numbers of muscle fibers per motoneuron was unaltered by synchronous activation of all of the motor units with the same daily activity. Hence, the restored size relationships and rematching of motoneuron and muscle unit properties after nerve injuries and muscle reinnervation sustain the normal gradation of muscle force during movement by size-ordered recruitment of motor units and the process of rate coding of action potentials. Dynamic modulation of size of muscle fibers and their contractile speed and endurance by neuromuscular activity allows for neuromuscular adaptation in the context of the sustained organization of the neuromuscular system according to the size principle.Key words: motor unit size, motor unit recruitment, innervation ratio, reinnervation.

Re-innervation and recovery of rat soleus muscle and motor unit function after nerve crush

In this study we have investigated the effects of peripheral nerve crush on the contractile properties of the adult rat soleus muscle. The soleus nerve was crushed close to the muscle and functional re-innervation was assessed by the measurement of the force produced by contraction induced by electrical nerve stimulation. Whole soleus muscle and single motor unit (MU) properties were studied at increasing re-innervation times 7-56 days after crush. Results showed progressive re-innervation as 50% of the axotomized motoneurones had re-innervated their muscle 7 days after crush, 72% at 14 days and re-innervation was complete at 28 days. The force parameters recovered more slowly. Tetanic contractions faded at high frequency stimulation, which did not occur in the control muscle. This disruption in the tetanic response was more pronounced in single MUs. Our results demonstrate for the first time a process of progressive axonal re-innervation by the axotomized motoneurones and provide a functional picture of the effective restoration of the neuromuscular function.

Denervated Muscle Fibers Explain the Deficit in Specific Force following Reinnervation of the Rat Extensor Digitorum Longus Muscle

The authors tested the hypothesis that, after denervation and reinnervation of skeletal muscle, observed deficits in specific force can be completely attributed to the presence of denervated muscle fibers. The peroneal nerve innervating the extensor digitorum longus muscle in rats was sectioned and the distal stump was coapted to the proximal stump, allowing either a large number of motor axons (nonreduced, n = 12) or a drastically reduced number of axons access to the distal nerve stump (drastically reduced, n = 18). A control group of rats underwent exposure of the peroneal nerve, without transection, followed by wound closure (control, n = 9). Four months after the operation, the maximum tetanic isometric force (Fo) of the extensor digitorum longus muscle was measured in situ and the specific force (sFo) was calculated. Cross-sections of the muscles were labeled for neural cell adhesion molecule (NCAM) protein to distinguish between innervated and denervated muscle fibers. Compared with extensor digitorum longus muscles from rats in the control (295 +/- 11 kN/m2) and nonreduced (276 +/- 12 kN/m2) groups, sFo of the extensor digitorum longus muscles from animals in the drastically reduced group was decreased (227 +/- 15 kN/m2, p < 0.05). The percentage of denervated muscle fibers in the extensor digitorum longus muscles from animals in the drastically reduced group (18 +/- 3 percent) was significantly higher than in the control (3 +/- 1 percent) group, but not compared with the nonreduced (9 +/- 2 percent) group. After exclusion of the denervated fibers, sFo did not differ between extensor digitorum longus muscles from animals in the drastically reduced (270 +/- 20 kN/m2), nonreduced (301 +/- 13 kN/m2), or control (303 +/- 10 kN/m2) groups. The authors conclude that, under circumstances of denervation and rapid reinnervation, the decrease in sFo of muscle can be attributed to the presence of denervated muscle fibers.

Histochemical alterations of re-innervated rat extensor digitorum longus muscle after end-to-end or graft repair: a comparative histomorphological study

Changes in the histochemical profile of 43 rat extensor digitorum longus muscles undergoing de-innervation and re-innervation were recorded. Assessment of fibre type composition and muscle fibre cross-sectional area was performed at 15, 30, 90 and 180 days post operative (p.o.) after either primary end-to-end repair or autologous graft repair of the common peroneal nerve (n = 5 per time point and type of repair). The size and histochemical profile of single muscle fibres were analysed by computer-assisted quantification on the basis of their myofibrillar ATPase (pH 4.3) and succinate dehydrogenase (SDH) activities in serial, whole-muscle cross-sections. Accordingly, four muscle-fibre types could be functionally identified: (1) slow oxidative (SO, type I); (2) fast-oxidative glycolytic (FOG, type IIA); (3) fast glycolytic (FG, type IIB); and (4) succinate dehydrogenase intermediate (SDH-INT). At 15 days following end-to-end repair, the SDH-INT muscle fibre type was observed. By contrast, 15 days following graft repair, no changes in fibre type composition were observed (vs. control). At 30 days p.o. in the group that received end-to-end repair, type SDH-INT reached its maximum and was significantly higher than in the group that underwent graft repair. At 90 days p.o., the amount of SDH-INT fibres declined after end-to-end repair, but it was still significantly higher than in the group treated with a nerve graft. The increase of the SDH-INT fibre type was mirrored by a proportional disappearance of FG and FOG fibres. These changes were time-dependent, not reversible at 180 days p.o and largely blunted after nerve graft. Muscle-fibre size decreased at 15 and 30 days after both types of nerve repair. This decrease was transient and reversible within 90 days p.o. These findings reflect the fact that the reorganization of the histochemical profile in re-innervated muscles is both time dependent and long lasting. The degree of this reorganization is significantly higher after end-to-end repair than after graft repair.

Recovery of type I fiber regionalization in gastrocnemius medialis of the rat after reinnervation along original and foreign paths, with and without muscle rotation

After reinnervation following transection of the sciatic nerve, normal patterns of regional type I fiber distribution are known to return in rat hindlimb muscles. Here we investigate how this recovery is influenced by experimental conditions. In an initial operation, the nerve of gastrocnemius medialis (GM) of adult rats was cut close to the muscle and reinserted either (i) close to the original nerve entry, or (ii) at a more medial 'foreign' site. In other groups of animals, these nerve operations were combined with a rotation of the GM muscle around its longitudinal axis, trying to ascertain whether the position of the muscle within the limb was of importance for the reinnervation processes. In a control group the muscle was rotated but innervation remained intact. After 21 weeks, the GM muscles were removed from both hindlimbs. Cross-sections were cut at seven different levels along each muscle, and 'slow' type I fibers were identified after staining for myofibrillar ATPase. The topographical positions were mapped out for all type I fibers. In all reinnervated muscles, an extensive type I fiber grouping was seen, indicating a widespread respecification of muscle fiber properties by ingrowing 'slow' axons. Normal topographical directions of type I fiber regionalization were about equally well restored in groups with the nerve inserted at the original or at the foreign site. In rotated muscles, the direction of type I fiber regionalization was significantly less rotated than the muscle as a whole. The results suggest that ingrowing 'slow' motor axons are guided toward their normal 'slow' regions by clues which are largely independent of the i.m. path of regeneration (original vs. foreign nerve entry site) but partly dependent on the position of the target muscle within the limb (rotated vs. non-rotated cases).

Functional impact of axonal misdirection after peripheral nerve injuries followed by graft or tube repair

Accuracy of reinnervation is one of the main factors conditioning functional recovery after brain, spinal, or peripheral axonal damage. Using the peripheral nerve as an experimental model, we studied the amount of inaccurate muscle reinnervation and its consequences on walking. Adult rats were submitted to an 8-mm resection of the sciatic nerve repaired by autograft (AG, n = 9), silicone (SIL, n = 13) or poly-L-lactate-epsilon-caprolactone (PLC, n = 11) single guides, and fascicular tubulization of peroneal and tibial branches with a dual silicone tube (FSIL, n = 9). At the end of follow-up, the sciatic nerve and its tibial and peroneal fascicles were dissected and stimulated by means of a suction electrode. In control rats, gastrocnemius and plantar muscles are fully innervated by the tibial fascicle and the tibialis anterior muscle by the peroneal nerve. None of the groups had noticeable recovery of locomotion assessed by the walking track index (SFI around -70 in all groups). After resection, all animals of groups AG, SIL, and PLC showed aberrant muscle reinnervation by axons from a non-corresponding fascicle, whereas in group FSIL only one of six regenerated animals showed misdirected activity. The proportion of inaccurate muscle activation was similar in group AG (47% for gastrocnemius, 54% for tibialis anterior, and 44% for plantar muscles) and in group SIL (42%, 42%, and 42%), and reduced in group PLC (26%, 38%, and 27%). In conclusion, fascicular silicone tubulization allowed the highest degree of accuracy but the lowest recovery, whereas resorbable PLC guides provided for the best balance between amount and accuracy of reinnervation after nerve resection.

Regional distribution of slow-twitch muscle fibers after reinnervation in adult rat hindlimb muscles

In adult rats, the sciatic nerve was unilaterally sectioned and reunited above the knee. Following a survival time of 21 weeks, five muscles were removed from both lower hindlimbs after determining their intra-limb positions. In each muscle, cryostat sections from seven equidistant proximo-distal levels were stained for myofibrillar ATPase. Intramuscular positions were determined for all slow-twitch type I fibers. Within each muscle, type I fibers were heterogeneously distributed, and the direction of type I fiber accumulation was, on average, almost identical in reinnervated muscles and contralateral controls. Furthermore, as in controls, a proximo-distal decline of type I fiber density was found in reinnervated muscles. Compared to contralateral controls, reinnervated muscles consistently showed a very high number of type I fibers at close interfiber distances, indicating respecification of muscle fiber types by the ingrowing nerve fibers. The results suggest that slow-twitch motor axons preferentially grew back toward the original slow-twitch muscle regions.

The effect of two episodes of denervation and reinnervation on skeletal muscle contractile function

Sensory or motor "baby-sitting" has been proposed as a clinical strategy to preserve muscle integrity if motion-specific axons must regenerate over a long distance to reach denervated target muscles. Denervated muscles are innervated temporarily by using axons from nearby sensory or motor nerves. After motion specific motor axons have reached the target, the baby-sitter nerve is severed and motion-specific axons are directed to the target. Although this strategy minimizes denervation time, the requisite second episode of denervation and reinnervation might be deleterious to muscle contractile function. This study was designed to test the hypothesis that two sequential episodes of skeletal muscle denervation and reinnervation result in greater force and power deficits than a single peripheral nerve injury and repair. Adult Lewis rats underwent either transection and epineurial repair or sham exposure of the left peroneal nerve. After a 4-month recovery period, the contractile properties of the extensor digitorum longus muscle of the sham exposure group (control, n = 9) and one of the nerve division and repair groups (repair group 1, n = 9) were evaluated with measurements of the maximum tetanic isometric force, peak power, and maximal sustained power. A third group of rats underwent a second cycle of nerve division and repair (repair group 2, n = 9) at this same time point. Four months postoperatively, contractile properties of the extensor digitorum longus muscles were evaluated. Maximum tetanic isometric force and peak power were significantly reduced in repair group 2 rats as compared with repair group 1 and control rats. Maximal sustained power was not significantly different between the groups. These data support our working hypothesis that skeletal muscle contractile function is adversely affected by two cycles of denervation and reinnervation as compared with a single episode of nerve division and repair.

Effects of chronic nerve cuff and intramuscular electrodes on rat triceps surae motor units

In order to assess the long-term effects of implanted electrodes on motor unit properties, we studied triceps surae (TS) motor units in rats implanted for 3-10 months with a tibial nerve cuff electrode for H-reflex elicitation and intramuscular electrodes for recording TS electromyographic activity. Motor units with sag from implanted rats displayed greater tetanic force than those from unimplanted rats. Motor units without sag had shorter twitch contraction times. This disrupted the relationship between sag and contraction time that was always present in unimplanted rats. These differences were consistent with a small degree of muscle denervation and subsequent reinnervation. Further analyses ascribed this effect to the nerve cuff rather than to the intramuscular electrodes. Comparable changes in motor unit properties may occur in humans with implanted nerve cuffs.

Superior muscle reinnervation after autologous nerve graft or poly-L-lactide-epsilon-caprolactone (PLC) tube implantation in comparison to silicone tube repair

Recovery after peripheral nerve injury depends not only on the amount of reinnervation, but also on its accuracy. The rat sciatic nerve was subjected to an 8 mm long gap lesion repaired either by autograft (AG, n = 6) or tubulization with impermeable silicone tube (SIL, n = 6) or permeable tube of poly-L-lactide-epsilon-caprolactone (PLC, n = 8). Recordings of the compound muscle action potential (CMAP) from gastrocnemius (mGC), tibialis anterior (mTA) and plantar (mPL) muscles were performed 90 days after injury to assess the amount of muscle reinnervation. The CMAP amplitude achieved in mGC, mTA and mPL was similar in after nerve autograft (39%, 42%, 22% of control values) and PLC tube implantation (37%, 36%, 24%) but lower with SIL tube (29%, 30%, 14%). The nerve fascicles projecting into each of these muscles were then transected and retrograde tracers (Fluoro Gold, Fast Blue, DiI) were applied to quantify the percentage of motoneurons with single or multiple branches to different targets. The total number of labeled motoneurons for the three muscles did not differ in autografted rats (1186 +/- 56; mean +/- SEM) with respect to controls (1238 +/- 82), but was reduced with PLC tube (802 +/- 101) and SIL tube (935 +/- 213). The percentage of neurons with multiple projections was lower after autograft and PLC tube (6%) than with SIL tube (10%). Considering the higher CMAP amplitude and lower number of neurons with multiple projections, PLC nerve conduits seem superior to SIL tubes and a suitable alternative to autografts for the repair of long gaps.

Skeletal muscle reinnervation by reduced axonal numbers results in whole muscle force deficits

Patients sustaining a peripheral nerve injury will frequently experience residual muscle weakness after muscle reinnervation, even if the nerve repair is performed under optimal circumstances to allow rapid muscle reinnervation. The mechanisms responsible for this contractile dysfunction remain unclear. It is hypothesized that after peripheral nerve injury and repair, a reduced number of axons are available for skeletal muscle reinnervation that results in whole muscle force and specific force deficits. A rat model of peroneal nerve injury and repair was designed so that the number of axons available for reinnervation could be systematically reduced. In adult rats, the peroneal nerve to the extensor digitorum longus muscle was either left intact (sham group, n = 8) or divided and repaired with either 50 percent (R50 group, n = 7) or 100 percent (R100 group, n = 8) of the axons in the proximal stump included in the repair. Four months after surgery, maximal tetanic isometric force was measured and specific force was calculated for each animal. Mean tetanic isometric force for extensor digitorum longus muscles from R50 rats (2765.7 +/- 767.6 mN) was significantly lower than sham (4082.8 +/- 196.5 mN) and R100 (3729.0 +/-370.2 mN) rats (p < 0.003). Mean specific force calculations revealed significant deficits in both the R100 (242.1 +/- 30 kN/m2) and R50 (190.6 +/- 51.8 kN/m2) rats compared with the sham animals (295.9 +/- 14 kN/m2) (p < 0.0005). These data support our hypothesis that after peripheral nerve injury and repair, reinnervation of skeletal muscle by a reduced number of axons results in a reduction in tetanic isometric force and specific force. The greater relative reduction in specific force compared with absolute force production after partial nerve repair may indicate that a population of residual denervated muscle fibers is responsible for this deficit.

Differential effects of NT-3 on reinnervation of the fast extensor digitorum longus (EDL) and the slow soleus muscle of rat

Previous studies of gastrocnemius muscle reinnervation showed specific normalization of the proportion and diameter of fast type 2b muscle fibres following NT-3 delivery to the proximal stump of the cut sciatic nerve. Here, we investigate if normalization was related to greater improvement of muscle reinnervation of fast (extensor digitorum longus; EDL) than slow (soleus) motor units. NT-3-impregnated (NT-3 group) or plain fibronectin (FN group) mats were inserted into a sciatic nerve gap. Neuromuscular junctions (NMJs) labelled with TRITC-alpha-bungarotoxin were colabelled with calcitonin gene-related peptide (CGRP) or 4E2 antisera and imaged using confocal microscopy. CGRP and 4E2 were used as markers for newly reinnervated and structurally mature NMJs, respectively. At 40 days postsurgery, denervated NMJs in EDL and soleus muscles of both groups presented a 50% decrease of surface area due to decreased width. At day 80 in EDL, more NMJs were reinnervated by CGRP-immunoreactive terminals in the NT-3 (7.1%) than in the FN group (4.2%); there was no difference between groups for soleus. At 120 days, 4E2-immunoreactive NMJs were more numerous in EDL of the NT-3 (40.0%) than in the FN group (7.3%), unlike in soleus (NT-3, 1. 6%; FN, 1.8%), and presented a partial size recovery. These results indicate that NT-3 preferentially improves reinnervation of fast muscles over slow muscle, although the mechanism of this improvement is still unclear.

Sag during unfused tetanic contractions in rat triceps surae motor units

Contractile properties and conduction velocity were studied in 202 single motor units of intact rat triceps surae muscles activated by intra-axonal (or intra-myelin) current injection in L5 or L6 ventral root to assess the factors that determine the expression of sag (i.e., decline in force after initial increase during unfused tetanic stimulation). Sag was consistently detected in motor units with unpotentiated twitch contraction times <20 ms. However, the range of frequencies at which sag was expressed varied among motor units such that there was no single interstimulus interval (ISI), with or without adjusting for twitch contraction time, at which sag could be detected reliably. Further analysis indicated that using the absence of sag as a criterion for identifying slow-twitch motor units requires testing with tetani at several different ISIs. In motor units with sag, the shape of the force profile varied with tetanic frequency and contractile properties. Simple sag force profiles (single maximum reached late in the tetanus followed by monotonic decay) tended to occur at shorter ISIs and were observed more frequently in fatigue-resistant motor units with long half-relaxation times and small twitch amplitudes. Complex sag profiles reached an initial maximum early in the tetanus, tended to occur at longer ISIs, and were more common in fatigue-sensitive motor units with long half-relaxation times and large twitch amplitudes. The differences in frequency dependence and force maximum location suggested that these phenomena represented discrete entities. Successive stimuli elicited near-linear increments in force during tetani in motor units that never exhibited sag. In motor units with at least one tetanus displaying sag, tetanic stimulation elicited large initial force increments followed by rapidly decreasing force increments. That the latter force envelope pattern occurred in these units even in tetani without sag suggested that the factors responsible for sag were expressed in the absence of overt sag. The time-to-peak force (TTP) of the individual contractions during a tetanus decreased in tetani with sag. Differences in the pattern of TTP change during a tetanus were consistent with the differences in force maximum location between tetani exhibiting simple and complex sag. Tetani from motor units that never exhibited sag did not display a net decrease in TTP during successive contractions. These data were consistent with the initial force decrement of sag resulting from a transient reduction in the duration of the contractile state.