Effects of chronic stimulation on the size and speed of long-term denervated and innervated rat fast and slow skeletal muscles

Increased hypertrophic response with increased mechanical load in skeletal muscles receiving identical activity patterns

It is often assumed that mechanical factors are important for effects of exercise on muscle, but during voluntary training and most experimental conditions the effects could solely be attributed to differences in electrical activity, and direct evidence for a mechanosensory pathway has been scarce. We here show that, in rat muscles stimulated in vivo under deep anesthesia with identical electrical activity patterns, isometric contractions induced twofold more hypertrophy than contractions with 50-60% of the isometric force. The number of myonuclei and the RNA levels of myogenin and myogenic regulatory factor 4 were increased with high load, suggesting that activation of satellite cells is mechano dependent. On the other hand, training induced a major shift in fiber type distribution from type 2b to 2x that was load independent, indicating that the electrical signaling rather than mechanosignaling controls fiber type. RAC-α serine/threonine-protein kinase (Akt) and ribosomal protein S6 kinase β-1 (S6K1) were not significantly differentially activated by load, suggesting that the differences in mechanical factors were not important for activating the Akt/mammalian target of rapamycin/S6K1 pathway. The transmembrane molecule syndecan-4 implied in overload hypertrophy in cardiac muscle was not load dependent, suggesting that mechanosignaling in skeletal muscle is different.

Daily Electrical Muscle Stimulation Enhances Functional Recovery Following Nerve Transection and Repair in Rats

Background. Incomplete recovery following surgical reconstruction of damaged peripheral nerves is common. Electrical muscle stimulation (EMS) to improve functional outcomes has not been effective in previous studies. Objective. To evaluate the efficacy of a new, clinically translatable EMS paradigm over a 3-month period following nerve transection and immediate repair. Methods. Rats were divided into 6 groups based on treatment (EMS or no treatment) and duration (1, 2, or 3 months). A tibial nerve transection injury was immediately repaired with 2 epineurial sutures. The right gastrocnemius muscle in all rats was implanted with intramuscular electrodes. In the EMS group, the muscle was electrically stimulated with 600 contractions per day, 5 days a week. Terminal measurements were made after 1, 2, or 3 months. Rats in the 3-month group were assessed weekly using skilled and overground locomotion tests. Neuromuscular junction reinnervation patterns were also examined. Results. Muscles that received daily EMS had significantly greater numbers of reinnervated motor units with smaller average motor unit sizes. The majority of muscle endplates were reinnervated by a single axon arising from a nerve trunk with significantly fewer numbers of terminal sprouts in the EMS group, the numbers being small. Muscle mass and force were unchanged but EMS improved behavioral outcomes. Conclusions. Our results demonstrated that EMS using a moderate stimulation paradigm immediately following nerve transection and repair enhances electrophysiological and behavioral recovery.

Electrical stimulation of transplanted motoneurons improves motor unit formation

Motoneurons die following spinal cord trauma and with neurological disease. Intact axons reinnervate nearby muscle fibers to compensate for the death of motoneurons, but when an entire motoneuron pool dies, there is complete denervation. To reduce denervation atrophy, we have reinnervated muscles in Fisher rats from local transplants of embryonic motoneurons in peripheral nerve. Since growth of axons from embryonic neurons is activity dependent, our aim was to test whether brief electrical stimulation of the neurons immediately after transplantation altered motor unit numbers and muscle properties 10 wk later. All surgical procedures and recordings were done in anesthetized animals. The muscle consequences of motoneuron death were mimicked by unilateral sciatic nerve section. One week later, 200,000 embryonic day 14 and 15 ventral spinal cord cells, purified for motoneurons, were injected into the tibial nerve 10-15 mm from the gastrocnemii muscles as the only neuron source for muscle reinnervation. The cells were stimulated immediately after transplantation for up to 1 h using protocols designed to examine differential effects due to pulse number, stimulation frequency, pattern, and duration. Electrical stimulation that included short rests and lasted for 1 h resulted in higher motor unit counts. Muscles with higher motor unit counts had more reinnervated fibers and were stronger. Denervated muscles had to be stimulated directly to evoke contractions. These results show that brief electrical stimulation of embryonic neurons, in vivo, has long-term effects on motor unit formation and muscle force. This muscle reinnervation provides the opportunity to use patterned electrical stimulation to produce functional movements.

Passive and active exercise improve regeneration and muscle reinnervation after peripheral nerve injury in the rat

INTRODUCTION

Lesions of peripheral nerves cause loss of motor and sensory function and also lead to hyperreflexia and hyperalgesia. Activity-dependent therapies promote axonal regeneration and functional recovery and may improve sensory-motor coordination and restoration of adequate circuitry at the spinal level.

METHODS

We compared the effects of passive (bicycle) and active (treadmill) exercise on nerve regeneration and modulation of the spinal H reflex after transection and repair of the rat sciatic nerve. Animals were evaluated during 2 months using electrophysiological, functional, and histological methods.

RESULTS

Moderate exercise for 1 hour/day, either active treadmill walking or passive cycling, improved muscle reinnervation, increased the number of regenerated axons in the distal nerve, and reduced the increased excitability of spinal reflexes after nerve lesion.

DISCUSSION

Maintenance of denervated muscle activity and afferent input, by active or passive exercise, may increase trophic factor release to act on regenerating axons and to modulate central neuronal plasticity.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

IP(3)-dependent, post-tetanic calcium transients induced by electrostimulation of adult skeletal muscle fibers

Tetanic electrical stimulation induces two separate calcium signals in rat skeletal myotubes, a fast one, dependent on Cav 1.1 or dihydropyridine receptors (DHPRs) and ryanodine receptors and related to contraction, and a slow signal, dependent on DHPR and inositol trisphosphate receptors (IP₃Rs) and related to transcriptional events. We searched for slow calcium signals in adult muscle fibers using isolated adult flexor digitorum brevis fibers from 5–7-wk-old mice, loaded with fluo-3. When stimulated with trains of 0.3-ms pulses at various frequencies, cells responded with a fast calcium signal associated with muscle contraction, followed by a slower signal similar to one previously described in cultured myotubes. Nifedipine inhibited the slow signal more effectively than the fast one, suggesting a role for DHPR in its onset. The IP₃R inhibitors Xestospongin B or C (5 µM) also inhibited it. The amplitude of post-tetanic calcium transients depends on both tetanus frequency and duration, having a maximum at 10–20 Hz. At this stimulation frequency, an increase of the slow isoform of troponin I mRNA was detected, while the fast isoform of this gene was inhibited. All three IP₃R isoforms were present in adult muscle. IP₃R-1 was differentially expressed in different types of muscle fibers, being higher in a subset of fast-type fibers. Interestingly, isolated fibers from the slow soleus muscle did not reveal the slow calcium signal induced by electrical stimulus. These results support the idea that IP₃R-dependent slow calcium signals may be characteristic of distinct types of muscle fibers and may participate in the activation of specific transcriptional programs of slow and fast phenotype.

Effects of functional electrical stimulation on dysphagia caused by radiation therapy in patients with nasopharyngeal carcinoma

PURPOSE

This study aimed to assess the effectiveness of functional electrical stimulation on the swallowing function of irradiated nasopharyngeal carcinoma patients with dysphagia.

MATERIALS AND METHODS

Twenty nasopharyngeal carcinoma subjects with dysphagia were divided into a functional electrical stimulation (FES) and a home rehabilitation program (HRP) group. Each subject completed a quality of life questionnaire on swallowing and a videofluoroscopic study at the commencement and the end of the procedures. The FES group received functional electrical stimulation of the supra-hyoid muscles over 15 sessions. The HRP group performed self-swallowing exercises at home. The evaluation parameters included the quality of life questionnaire scores, the penetration-aspiration scale (PAS), the movement of the hyoid bone, and the amount of pyriform sinuses stasis.

RESULTS

Most swallowing outcomes of the FES group improved after FES. The quality of life score (p=0.003), the duration of the movement of thin barium through the hyoid (p=0.001), the moving speed of paste barium through the hyoid (p=0.028), and the pyriform sinus stasis area of the paste barium (p=0.026) reached significant difference in the FES group. Most swallowing outcomes did not improve in the HRP group. The degree of improvement in the movement speed of the hyoid bone in the thin barium (p=0.018) and the PAS of the paste barium (p=0.016) were statistically significantly greater in the FES group than in the HRP group.

CONCLUSION

FES will improve the swallowing function of NPC patients with dysphagia and bring about better quality of life.

Acute molecular response of mouse hindlimb muscles to chronic stimulation

Stimulation of the mouse hindlimb via the sciatic nerve was performed for a 4-h period to investigate acute muscle gene activation in a model of muscle phenotype conversion. Initial force production (1.6 +/- 0.1 g/g body wt) declined 45% within 10 min and was maintained for the remainder of the experiment. Force returned to initial levels upon study completion. An immediate-early growth response was present in the extensor digitorum longus (EDL) muscle (FOS, JUN, activating transcription factor 3, and musculoaponeurotic fibrosarcoma oncogene) with a similar but attenuated pattern in the soleus muscle. Transcript profiles showed decreased fast fiber-specific mRNA (myosin heavy chains 2A and 2B, fast troponins T(3) and I, alpha-tropomyosin, muscle creatine kinase, and parvalbumin) and increased slow transcripts (myosin heavy chain-1beta/slow, troponin C slow, and tropomyosin 3y) in the EDL versus soleus muscles. Histological analysis of the EDL revealed glycogen depletion without inflammatory cell infiltration in stimulated versus control muscles, whereas ultrastructural analysis showed no evidence of myofiber damage after stimulation. Multiple fiber type-specific transcription factors (tea domain family member 1, nuclear factor of activated T cells 1, peroxisome proliferator-activated receptor-gamma coactivator-1alpha and -beta, circadian locomotor output cycles kaput, and hypoxia-inducible factor-1alpha) increased in the EDL along with transcription factors characteristic of embryogenesis (Kruppel-like factor 4; SRY box containing 17; transcription factor 15; PBX/knotted 1 homeobox 1; and embryonic lethal, abnormal vision). No established in vivo satellite cell markers or genes activated in our parallel experiments of satellite cell proliferation in vitro (cyclins A(2), B(2), C, and E(1) and MyoD) were differentially increased in the stimulated muscles. These results indicated that the molecular onset of fast to slow phenotype conversion occurred in the EDL within 4 h of stimulation without injury or satellite cell recruitment. This conversion was associated with the expression of phenotype-specific transcription factors from resident fiber myonuclei, including the activation of nascent developmental transcriptional programs.

In vivo time-lapse microscopy reveals no loss of murine myonuclei during weeks of muscle atrophy

Numerous studies have suggested that muscle atrophy is accompanied by apoptotic loss of myonuclei and therefore recovery would require replenishment by muscle stem cells. We used in vivo time-lapse microscopy to observe the loss and replenishment of myonuclei in murine muscle fibers following induced muscle atrophy. To our surprise, imaging of single fibers for up to 28 days did not support the concept of nuclear loss during atrophy. Muscles were inactivated by denervation, nerve impulse block, or mechanical unloading. Nuclei were stained in vivo either acutely by intracellular injection of fluorescent oligonucleotides or in time-lapse studies after transfection with a plasmid encoding GFP with a nuclear localization signal. We observed no loss of myonuclei in fast- or slow-twitch muscle fibers despite a greater than 50% reduction in fiber cross-sectional area. TUNEL labeling of fragmented DNA on histological sections revealed high levels of apoptotic nuclei in inactive muscles. However, when costained for laminin and dystrophin, virtually none of the TUNEL-positive nuclei could be classified as myonuclei; apoptosis was confined to stromal and satellite cells. We conclude that disuse atrophy is not a degenerative process, but is rather a change in the balance between protein synthesis and proteolysis in a permanent cell syncytium.

Nuclear domains during muscle atrophy: nuclei lost or paradigm lost?

According to the current paradigm, muscle nuclei serve a certain cytoplasmic domain. To preserve the domain size, it is believed that nuclei are injected from satellite cells fusing to fibres undergoing hypertrophy, and lost by apoptosis during atrophy. Based on single fibre observations in and ex vivo we suggest that nuclear domains are not as constant as is often indicated. Moreover, recent time lapse in vivo imaging of single fibres suggests that at least for the first few weeks, atrophy is not accompanied by any loss of nuclei. Apoptosis is abundant in muscle tissue during atrophy conditions, but in our opinion it has not been unequivocally demonstrated that such nuclei are myonuclei. As we see it, the preponderance of current evidence suggests that disuse atrophy is not accompanied by loss of nuclei, at least not for the first 2 months. Moreover, it has not been proven that myonuclear apoptosis does occur in permanent fibres undergoing atrophy; it seems more likely that it is confined to stromal cells and satellite cells. If muscle atrophy is not related to loss of nuclei, design of intervention therapies should focus on protein metabolism rather than regeneration from stem cells.

Physical therapy following peripheral nerve surgeries

Post-operative care of the surgical patient significantly contributes to the success of the surgical procedure. Post-operative physical therapy is directed at reducing pain and inflammation, preventing or minimizing scar tissue, and returning the patient to full function. An individualized and well-planned therapeutic exercise program is an integral part of the post-operative care. Manual therapy techniques are utilized to break up scar tissue and reduce joint stiffness. Pain and inflammation can be addressed by modalities such as ultrasound, laser, and electrical stimulation in addition to cryotherapy.

Chronic electrostimulation after nerve repair by self-anastomosis: effects on the size, the mechanical, histochemical and biochemical muscle properties

This study tests the effects of chronic electrostimulation on denervated/reinnervated skeletal muscle in producing an optimal restoration of size and mechanical and histochemical properties. We compared tibialis anterior muscles in four groups of rats: in unoperated control (C) and 10 weeks following nerve lesion with suture (LS) in the absence of electrostimulation and in the presence of muscle stimulation with either a monophasic rectangular current (LSEm) or a biphasic modulated current (LSEb). The main results were (1) muscle atrophy was reduced in LSEm (-26%) while it was absent in LSEb groups (-8%); (2) the peak twitch amplitude decreased in LS and LSEm but not in LSEb groups, whereas the contraction time was shorter; (3) muscle reinnervation was associated with the emergence of type IIC fibers and proportions of types I, IIA and IIB fibers recovered in the superficial portion of LSEb muscles; (4) the ratio of oxidative to glycolytic activities decreased in the three groups with nerve injury and repair; however, this decrease was more accentuated in LSEm groups. We conclude that muscle electrostimulation following denervation and reinnervation tends to restore size and functional and histochemical properties during reinnervation better than is seen in unstimulated muscle.

Does daily activity level determine muscle phenotype?

The activation level of a muscle is presumed to be a major determinant of many mechanical and phenotypic properties of its muscle fibers. However, the relationship between the daily activation levels of a muscle and these properties has not been well defined, largely because of the lack of accurate and sustained assessments of the spontaneous activity levels of the muscle. Therefore, we determined the daily activity levels of selected rat hindlimb muscles using intramuscular EMG recordings. To allow comparisons across muscles having varying activity levels and/or muscle fiber type compositions, we recorded EMG activity in a predominantly slow plantarflexor (soleus), a predominantly fast plantarflexor (medial gastrocnemius, MG), a predominantly fast ankle dorsiflexor (tibialis anterior, TA) and a predominantly fast knee extensor (vastus lateralis, VL) in six unanesthetized rats for periods of 24 h. EMG activity levels were correlated with the light:dark cycle, with peak activity levels occurring during the dark period. The soleus was the most active and the TA the least active muscle in all rats. Daily EMG durations were highest for soleus (11-15 h), intermediate for MG (5-9 h) and VL (3-14 h) and lowest for TA (2-3 h). Daily mean EMG amplitudes and integrated EMG levels in the soleus were two- to threefold higher than in the MG and VL and seven- to eightfold higher than in the TA. Despite the three- to fourfold difference in activation levels of the MG and VL vs the TA, all three predominantly fast muscles have been reported to have a similar, very low percentage of slow fibers. Comparing these relative EMG levels to the published fiber type profiles of these muscles yields a very poor relationship between daily activity level and fiber type composition in the same muscles across several species. Although it is clear that changing the levels of activity can modulate the expression of the myosin phenotype, these results indicate that factors other than activation must play critical roles in determining and maintaining normal phenotypic properties of skeletal muscle fibers.

Functional and histological effects of intravaginal electrical stimulation on the pelvic muscles: a study in the rat

We studied functional and histological effects of electrical stimulation (ES) on pelvic muscles of the rat. With intravaginal electrodes, the musculus pubococcygeus and musculus iliococcygeus in the awake animal were stimulated three times 6 min per day with 5 min of rest in between, 5 days per week, 7 consecutive weeks with a biphasic rectangular symmetrical current of 25 Hz, 400-mus pulse duration, on/off time of 5/10 and with an amplitude of 2-4 mA. A "sham group" received the same handling but no stimulation. Contraction measured with intra-rectal pressure during stimulation increased more in the stimulated than in the sham group, but did not reach statistical significance probably due to low power. The 2A fast fibres increased with 14% in the musculus iliococcygeus and with 6% in the musculus pubococcygeus. Type 1 slow fibres did not change. Increased capillary density was found after stimulation. Repeated intravaginal ES has mainly an influence on the fast fibres in the pelvic muscles. To influence slow fibres, another stimulation program or current parameters would seem necessary.

Electrical stimulation attenuates denervation and age-related atrophy in extensor digitorum longus muscles of old rats

Skeletal muscles of old rats and elderly humans lose muscle mass and maximum force. Denervation is a major cause of age-related muscle atrophy and weakness, because denervated fibers do not contract, and undergo atrophy. At any age, surgical denervation causes even more dramatic muscle atrophy and loss in force than aging does. Electrical stimulation that generates tetanic contractions of denervated muscles reduces the denervation-induced declines. We investigated whether a stimulation protocol that maintains mass and force of denervated extensor digitorum longus muscles of adult rats would also maintain these properties in denervated muscles of old rats during a 2-month period of age-induced declines in these properties. Contractile activity generated by the electrical stimulation eliminated age-related losses in muscle mass and reduced the deficit in force by 50%. These data provide support for the hypothesis that during aging, lack of contractile activity in fibers contributes to muscle atrophy and weakness.

Postnatal appearance of 5-HT2A receptors on fast flexor and slow extensor rat motor neurons

Motor neurons to the slowly contracting extensor soleus muscle in behaving rats begin to fire tonically in the 2nd week after birth. In the adult, tonic firing becomes predominant and appears to arise from plateau potentials under monoaminergic control. In the present work, motor neurons to slowly contracting extensor soleus and rapidly contracting extensor digitorum longus, a physiological flexor muscle, were retrogradely labeled with fluorescent dextran and examined for immunoreactivity to 5-HT(2A) receptors in 1 and 2 week old and adult rats. No reactivity was detected at 1 week. At 2 weeks, reactivity was detected on 67% slowly contracting extensor soleus (16 of 24) and 19% extensor digitorum longus (11 of 57) motor neurons. In the adult, the intensity of staining was higher and the percentage of labeled motor neurons 79 for slowly contracting extensor soleus (34 of 43) and 31 for extensor digitorum longus (11 of 35). On slowly contracting extensor soleus motor neurons, labeling appeared more often on soma and dendrites than on dendrites only, whereas on extensor digitorum longus motor neurons, labeling appeared more often on dendrites only. These results are consistent with the hypothesis that serotonergic innervation contributes to the appearance and subsequent increase in tonic firing of rat slowly contracting extensor soleus motor neurons in postnatal development.

"Fast" and "slow" muscle fibres in hindlimb muscles of adult rats regenerate from intrinsically different satellite cells

Myosin heavy chain (MyHC) expression was examined in regenerating fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles of adult rats. Myotoxic bupivacaine was injected into SOL and EDL and the muscles were either denervated or neuromuscularly blocked by tetrodotoxin (TTX) on the sciatic nerve. Three to 10 or 30 days later, denervated SOL or EDL, or innervated but neuromuscularly blocked EDL received a slow 20 Hz stimulus pattern through electrodes implanted on the muscles or along the fibular nerve to EDL below the TTX block. In addition, denervated SOL and EDL received a fast 100 Hz stimulus pattern. Denervated EDL and SOL stimulated with the same slow stimulus pattern expressed different amounts of type 1 MyHC protein (8% versus 35% at 10 days, 13% versus 87% at 30 days). Stimulated denervated and stimulated innervated (TTX blocked) EDL expressed the same amounts of type 1, 2A, 2X and 2B MyHC proteins. Cross-sections treated for in situ hybridization and immunocytochemistry showed expression of type 1 MyHC in all SOL fibres but only in some scattered single or smaller groups of fibres in EDL. The results suggest that muscle fibres regenerate from intrinsically different satellite cells in EDL and SOL and within EDL. However, induction by different extrinsic factors arising in extracellular matrix or from muscle position and usage in the limb has not been excluded. No evidence for nerve-derived trophic influences was obtained.

Neuromuscular rehabilitation by treadmill running or electrical stimulation after peripheral nerve injury and repair

Numerous studies have been devoted to the regeneration of the motor pathway toward a denervated muscle after nerve injury. However, the regeneration of sensory muscle endings after repair by self-anastomosis are little studied. In previous electrophysiological studies, our laboratory showed that the functional characteristics of tibialis anterior muscle afferents are differentially affected after injury and repair of the peroneal nerve with and without chronic electrostimulation. The present study focuses on the axonal regeneration of mechano- (fibers I and II) and metabosensitive (fibers III and IV) muscle afferents by evaluating the recovery of their response to different test agents after nerve injury and repair by self-anastomosis during 10 wk of treadmill running (LSR). Data were compared with control animals (C), animals with nerve lesion and suture (LS), and animals with lesion, suture, and chronic muscle rehabilitation by electrostimulation (LSE) with a biphasic current modulated in pulse duration and frequency, eliciting a pattern mimicking the activity delivered by the nerve to the muscle. Compared with the C group, results indicated that 1) muscle weight was smaller in LS and LSR groups, 2) the fatigue index was greater in the LS group and smaller in the LSE group, 3) metabosensibility remained altered in the LS and LSE groups, and 4) mechanosensitivity presented a large increase of the activation pattern in the LS and LSE groups. Our data indicated that chronic muscle electrostimulation partially favors the recovery of muscle properties (i.e., muscle weight and twitch response were close to the C group) and that rehabilitation by treadmill running also efficiently induced a better functional muscle afferent recovery (i.e., the discharge pattern was similar to the C group). The effectiveness of the chronic electromyostimulation and the treadmill exercise on afferent recovery is discussed with regard to parameters listed above.

Number of contractions to maintain mass and force of a denervated rat muscle

Within 5 weeks, denervated extensor digitorum longus (EDL) muscles of rats lose 66% of mass, 91% of force, and 76% of fiber cross-sectional area (CSA). We previously determined the parameters of electrical stimulation for denervated rat EDL muscles to generate tetanic contractions sufficient to maintain mass and force close to control values. Using these parameters, we tested the hypothesis that a range exists for number of contractions per day, below and above which values for mass, maximum force, and fiber CSA are lower than values for innervated control muscles. For 5 weeks, denervated EDL muscles were stimulated to generate between 25 and 5000 contractions daily with contractions separated by constant intervals of rest, repeated 24 h per day. Force was not maintained with fewer than 200 or more than 800 contractions daily, whereas mass and fiber CSA were not maintained with fewer than 50 contractions daily. Protocols of stimulation that maintain muscle mass and force during prolonged periods of denervation may minimize problems clinically associated with denervation atrophy.

Neosphincters in the management of faecal incontinence

BACKGROUND

Surgical treatment of end-stage faecal incontinence has its origin in the early 1950s. Interest has been revived as a result of technical advances achieved in the recent past. The purpose of this article is to review the principles that underlie the use of skeletal muscle transposition around the anal canal and of electrical stimulation in the treatment of incontinence, and to explore new methods of treatment of this condition.

METHODS

A literature search was performed using Pubmed and Medline, employing keywords related to treatment of faecal incontinence by neosphincter reconstruction. Basic science and clinical aspects of neosphincter reconstruction were gathered from relevant texts, original articles and recently published abstracts.

RESULTS

The electrically stimulated gracilis neoanal sphincter seems to be the popular choice of biological neosphincter. It is more likely to produce higher resting anal canal pressures than the unstimulated neosphincter, and hence improved continence. However, electrostimulator failure may result in explantation in a proportion of patients. Impairment of evacuation is a functional setback in approximately one-third of patients with the gracilis neosphincter. Overall, improvement of continence may be expected in up to 90 per cent of patients according to some reports. By contrast, experience with the artificial neosphincter, which is less expensive, has been limited to a few tertiary centres across the world. Reported continence of stool is 100 per cent, and that of gas and stool 50 per cent, following implantation of the artificial sphincter. Both of the above operations have been associated with implant-related infection and impaired evacuation.

CONCLUSION

Neoanal sphincter operations are technically demanding, require a considerable learning experience and should be confined to specialist colorectal centres. Patients are likely to benefit from a plan that incorporates preoperative counselling and a selective approach.

The value of continuous electrical muscle stimulation using a completely implantable system in the preservation of muscle function following motor nerve injury and repair: an experimental study

Functional recovery following motor nerve injury and repair is directly related to the degree of muscle atrophy that takes place during the period of nerve regeneration. The extent of this muscle atrophy is related to a number of factors including the accuracy of nerve repair; the distance through which the nerve must regenerate; the age of the patient; and the type of nerve injury and other associated tendon and soft tissue and bony damage. Atrophy of muscle that is always associated with nerve injury is a combination of disuse and degeneration. Our hypothesis proposed the following question: "Would continuous electrical stimulation of the denervated muscle during the period of nerve regeneration maintain the integrity of the muscle fibers and hence their potential functional capacity?" We have completed a series of animal studies (rabbit and canine models) in our laboratory using a completely implantable system to provide continuous muscle stimulation following nerve injury and microsurgical repair. In several different experiments, the nerves under study were cut and repaired at 4 and 12 cm from the muscles to study the effects of short- and long-term recovery. In all experiments, a beneficial effect was demonstrated with improved morphology and functional capacity of the reinnervated stimulated muscles when compared with nonstimulated controls. In addition, electrical stimulation using this implantable system could be applied for extended periods without evidence of discomfort in the experimental animals.

A clinical pilot study to assess functional return following continuous muscle stimulation after nerve injury and repair in the upper extremity using a completely implantable electrical system

This clinical pilot study evolved from a 10-year experience in the experimental laboratory using continuous muscle stimulation in a series of animal studies following nerve injury and microsurgical repair. A completely implantable system was developed (Medtronic) to provide electrical stimulation to the denervated muscles until nerve regeneration had occurred. Both peripheral nerve injuries in the extremities and facial nerve severances were studied, and a definite improvement in functional capacity was obtained as well as improved morphology compared with nonstimulated controls. In this study, 13 patients with peripheral nerve injuries in the upper extremity are included. All patients had muscle stimulation for extended periods until nerve regeneration was evident; a careful analysis of their functional capacity was then completed. There were five patients with median nerve injuries, four with ulnar nerve or combined median-ulnar nerve injuries, and four with severed radial nerves. All patients showed satisfactory nerve regeneration on clinical examination and electromyographic studies. Motor recovery was usually better than sensory return. Functional muscle analysis varied somewhat from patient to patient, but every patient had a satisfactory to excellent recovery. Patients with low nerve lesions had better results, but muscle recovery even in patients with mid-forearm or higher nerve injuries was most encouraging. Functional recovery in radial nerve injuries was close to normal in all cases. The results from this pilot study have clearly shown the benefits of continuous muscle stimulation using an implantable system following nerve injury and repair.

Electrical stimulation for stress incontinence

Electrical stimulation has been reported to be effective for stress incontinence, cure and improvement rates being reported to range from 30% to 50%, and from 6% to 90%, respectively. However, clinical application of this treatment is not common because there is little physiological and technical information. Electrodes for electrical stimulation are divided into two types: external (non-implantable) and internal (implantable), and there are two methods of stimulation: chronic (long-term, continuous) and short-term. Frequencies of 20-50 Hz, with a pulse duration of 1-5 ms, have been reported to be effective for urethral closure. The effectiveness of the treatment should be verified with placebo-controlled double-blinded trials, and four such studies using an active and a sham device have been reported. Two of these verified the superiority of the active device over the sham device, but the others did not demonstrate any significant difference between the two with regard to efficacy. Electrical stimulation has been reported to result in a long-term continuation of therapeutic effect. The effect has been explained as a re-education or a reactivation of lost functions of the pelvic floor muscles. As to adverse effects, there may be some complications in relation to anesthesia or surgical procedures, such as infection, pain and bleeding with implantable electrodes. The incidence of adverse effects in short-term electrical stimulation is less than 14%. In conclusion, short-term electrical stimulation using non-implantable anal or vaginal electrodes is the most recommendable because of safety and ease of use.

Determination of muscle contractile properties: the importance of the nerve

Contractile phenotype of muscle fibres is strongly influenced by hormones, stretch and influences from the motor neurones, although cell lineage probably also plays a role. Motor neurones can affect muscle fibres by releasing neurotrophic substances and by evoking electrical activity in the muscle. For regulating contractile properties such as speed, strength and endurance it has been demonstrated that electrical activity is crucial, while the role of putative neurotrophic substances remains unclear. The signal to change is coded in the pattern of electrical activity. Thus, high amounts of activity lead to slow shortening velocity and myosin heavy chains, while low amounts of activity lead to a fast phenotype. For regulation of twitch duration frequency also plays a role, and for preventing atrophy in denervated muscles high frequency seems to be beneficial, particularly in fast muscles. Little is known about the excitation-adaptation pathway linking action potentials to expression of genes that are relevant for contractile properties. Muscle specific transcription factors of the helix-loop-helix family such as myoD and myogenin could be important for regulating genes related to metabolic profile and fibre size/strength, while their role in determining myosin heavy chain expression and classical fibre type is more uncertain.

Paralysis of rat skeletal muscle equally affects contractile properties as does permanent denervation

The effects of long lasting (4-5 weeks) nerve conduction block and denervation were compared by investigating contractile, morphological and histochemical properties of slow (soleus) and fast (EDL) rat skeletal muscles. The block was based on improved perfusion techniques of the sciatic nerve with a tetrodotoxin (TTX) solution delivered at doses adequate to obtain maximal effects in the muscles. The TTX-inactivated axons retained normal histological and physiological properties such as the ability to evoke full contractile responses, to regenerate, and to completely reinnervate muscle. In spite of their intact innervation or of their full reinnervation, the TTX-paralysed muscles underwent weight loss, fibre atrophy and reduction in force, output quantitatively indistinguishable from those following denervation. The same was true for all other contractile parameters tested, that is, twitch speed, twitch to tetanus ratio, post-tetanic potentiation, endurance, and fibre type composition. The results indicate the fundamental role of activity as a regulatory signal for muscle contractile properties, while they do not support the notion of a participation of chemical, activity-independent factors in this regulation.

Effects of long-term phasic electrical stimulation on denervated soleus muscle: guinea-pig contrasted with rat

Guinea-pig soleus muscles were denervated and electrically stimulated for periods of 43 to 66 days. Stimuli were in 1 s bursts of 40 Hz pulses, repeated every 5 min. Other guinea-pigs were denervated for 82 days without stimulation and, in a third group, the soleus muscle was necrotized and allowed to regenerate without reinnervation for 13-15 days. Isometric and isotonic recordings were made in vivo. Denervated guinea-pig muscles were embedded in epoxy resin for light and electron microscopy. Chronic stimulation of denervated guinea-pig soleus had no effects on the prolonged twitch or on reduced maximal shortening velocity, maximal rate of rise of tension and tetanic force. This contrasts with the slow-to-fast conversion produced by denervation and denervation-stimulation of rat soleus. Loss of force was much greater in rat than guinea-pig after denervation, and chronic stimulation increased force in rat to the same level as in guinea-pig after denervation (with or without stimulation). Eighty-day denervated guinea-pig soleus did not reveal those morphological signs of fibre breakdown and regeneration which are prominent in denervated rat soleus muscles. Those changes in rat resembled aneurally regenerated muscles in several aspects, especially the increased incidence of fibres with internal myo-nuclei which did not appear in guinea-pig soleus after denervation. Aneurally regenerated guinea-pig soleus became fast like aneurally regenerated rat muscle. Our data are compatible with the hypothesis that slow-to-fast transformation of denervated rat soleus is not directly brought about by chronic stimulation but by de-novo formation of fast-contracting regenerated fibres. The persistence of fibrillation in guinea-pig but not rat after denervation may account for the species difference.

Electrical stimulation of denervated muscle: is it worthwhile?

Research conducted over the past 25 years has demonstrated that muscle activity, not neurotrophic substances, is the most important factor in the regulation of specific physiological and biochemical properties of muscle fibers. Application of this knowledge has led to considerable experimentation with chronic electrical stimulation as a possible clinical tool for the treatment of denervated muscles. Evidence accumulated from animal studies has indicated that direct electrical stimulation of denervated muscles can to a large extent substitute for innervation and preserve or restore the normal properties of the muscles. Appropriate stimulation parameters were critical for a successful intervention, and the best results were obtained when the stimulation pattern resembled the firing pattern of the normal motoneuron. Thus, fast muscles required intermittent, brief, high frequency stimulation and slow muscles needed continuous, low frequency stimulation. For human denervated muscles, critical questions still remain to be resolved before electrical stimulation will yield the optimum benefit. Research must be performed in human subjects to define the appropriate stimulation parameters the stimulation current, and the type and placement of electrodes.

Number of junctional acetylcholine receptors: control by neural and muscular influences in the rat

1. The number of acetylcholine receptors (AChRs) per neuromuscular junction in soleus muscles of adult rats was estimated from counts of 125I-alpha-bungarotoxin binding sites. The muscles were either denervated, denervated and electrically stimulated, paralysed by botulinum toxin (BoTX), or paralysed by tetrodotoxin (TTX). 2. After denervation, the number of junctional AChRs was normal after 18 days and then fell to 54 and 35% of normal after 33 and 57 days, respectively. 3. Direct high frequency muscle stimulation (100 Hz) maintained a normal number of junctional AChRs for at least 2 months when the stimulation started on the day of denervation. When the stimulation was started progressively later, the effect of the stimulation on AChR number disappeared within about a week. The disappearance was gradual and appeared to affect all the muscle fibres equally. 4. Stimulation at 100 Hz, starting on the day of denervation and stopping after 18 days, did not prevent the endplates from losing AChRs during the subsequent 15 days without stimulation. Thus 100 Hz stimulation and innervation are not equivalent in their effects on junctional AChR number. 5. Direct low frequency muscle stimulation from the day of denervation did not maintain a normal number of junctional AChRs, as the number of AChRs fell to 70 and 62% of normal after 33 days of stimulation at 20 and 10 Hz, respectively. 6. Endplates paralysed by BoTX or TTX for 33 days lost about as many junctional AChRs (54 and 55%) as endplates denervated for 33 days (46%). Direct stimulation at 100 Hz during the last 15 days of BoTX treatment reduced but did not prevent this AChR loss (36% loss at 33 days). 7. The results show that when motor nerve terminals in rat soleus muscles are removed by axotomy, they leave a 'trace' which, in conjunction with appropriate muscle stimulation, can maintain a normal number of AChRs in the postsynaptic region. In non-stimulated muscles the trace responsible for this maintenance disappears within about a week. In stimulated muscles it persists for at least 2 months. From indirect evidence it appears that the trace is a factor, or the postsynaptic effect of a factor, released by impulse activity in the nerve, and that its degradation after denervation is accelerated by the acute effects of nerve degeneration.

Factors causing different properties at neuromuscular junctions in fast and slow rat skeletal muscles

Neuromuscular junctions on fast and slow skeletal muscle fibers have different properties. Possible reasons for these differences were examined in adult rat soleus (SOL) muscle fibers reinnervated at new ectopic or old denervated sites by fast fibular (FIB) or slow SOL motoneurons. FIB motoneurons formed large ectopic junctions with a high density of nerve terminal varicosities (fast appearance), whereas SOL motoneurons formed small ectopic junctions with a low density of varicosities (slow appearance). Both FIB and SOL motoneurons formed small junctions with a slow appearance when reinnervating old SOL endplates. FIB nerves innervating ectopic sites and SOL nerves reinnervating old sites had the same appearance whether they contacted the SOL fibers alone (single innervation) or together (dual innervation). Continuous stimulation of the FIB nerve at 10 Hz for 3-4 months reduced the size of ectopic FIB and intact extensor digitorum longus (EDL) junctions and caused a modest reduction in density of terminal varicosities in EDL. Junction size and muscle fiber diameter were positively correlated, but the slope describing this relation was steeper for FIB junctions than for SOL junctions. It is concluded that in the present system (1) motoneuron type and not muscle fiber type determines the fast or slow character of the neuromuscular junction. (2) denervated endplates of one type place stable and severe constraints on the termination pattern of reinnervating axons of another type, (3) the appearance of fast EDL junctions undergoes a modest fast to slow transformation when exposed to long-term slow pattern stimulation, and (4) not only the size of the muscle fibers, but also the type and firing pattern of the motoneurons and the spatial constraints at preformed endplates influence the relation between junction size and muscle fiber diameter.

Nerve-evoked electrical activity regulates molecules and cells with immunological function in rat muscle tissue

Molecules coded by the major histocompatibility complex (MHC) are present on cell surfaces in most tissues, with the cells of the central nervous system and skeletal muscle as prominent exceptions. We show here that when rat skeletal muscles are rendered inactive by nerve impulse block, expression of MHC class I molecules occurs on the muscle fibres. In addition, the number of cells expressing MHC class II molecules in the muscle interstitium is increased by a factor of three after 2 weeks of impulse blockade. Similar effects obtained by denervation can be counteracted by direct electrical stimulation. Interferon-gamma-like immunoreactivity accumulates in inactive muscle fibres, and interferon-gamma or a related cytokine could be a link between inactivity and MHC up-regulation. These findings suggest that nerve-evoked muscle activity influences not only the phenotype of the muscle cells themselves, but also processes in the interstitium that may increase the immunoreactivity of inactive muscle tissue.

Id-1 as a possible transcriptional mediator of muscle disuse atrophy

Disuse of muscle leads to atrophy of the fibers. This atrophy is correlated with reduced transcription. We found that when muscle was denervated or paralyzed with a nerve impulse block, the mRNA for Id-1, a negative regulator of transcription, was increased 2- to 7-fold. To test the effect of high Id-1 levels in active muscles, we made transgenic mice in which Id-1 was overexpressed under control of regulatory elements which confer tissue- and fiber-type-specific expression in differentiated muscle cells. Fiber types with high transgene expression were atrophic compared to those in wild-type litter mates. In contrast, fiber types with low transgene expression displayed hypertrophy, presumably caused by an overload due to reduced strength in atrophic synergistic fibers. Apart from the selective effects on fiber caliber, the muscle tissue showed no signs of pathology, and apart from a characteristic slightly lower body weight, the transgenic animals looked and behaved normally. We suggest that in the mature muscle, Id-1 may be involved in regulating muscle fiber size at the transcriptional level during disuse.

Effects of chronic electrostimulation on rat soleus skinned fibers during hindlimb suspension

In order to counteract the changes of the contractile properties of the rat soleus occurring during 10 days of hypokinesia-hypodynamia, due to hindlimb suspension (HS), two different patterns of electrostimulation were applied to the tibial nerve. The contractile properties of single chemically skinned muscle fibers were investigated using the tension-pCa relationship characteristics, the similar or different calcium and strontium affinities, and by measuring the P/tmax kinetic parameters. Our results showed that a pattern similar to firing rates of motoneurons innervating slow twitch muscles inhibited the slow to fast fiber changes observed during HS, whereas a pattern similar to firing rates of motoneurons from fast twitch muscles seemed to favor these changes. Since neither pattern maintained the isometric contractile force developed by the soleus fibers, we concluded that the decrease in mechanical strains imposed on the muscle during unloading was the main factor for the development of atrophy, while the kinetic changes might be predominantly modulated by the nervous command.

Diverse firing properties of single motor units in the inner and outer portions of the guinea pig anterior digastric muscle

Microwire recordings from the histochemically heterogeneous inner compartment of the guinea pig anterior digastric muscle (ADG) revealed tonic firing of single motor units, which were spontaneously active and could also be recruited following orofacial afferent stimulation and during rhythmic jaw movements (RJM). As units with tonic firing were not observed in the homogeneously fast-twitch outer ADG, the tonic units were classified as slow-twitch motor units. Irregular patterns of motor-unit firing at variable frequencies were observed after orofacial stimulation and during RJM in the outer and inner compartments. The irregular firing pattern of units in the fast-twitch outer compartment was characterized by shorter and less variable bursts than that of units in the heterogeneous inner compartment. A phasic, centrally driven firing pattern was observed during RJM in outer and inner ADG units. The firing frequency of some of these units was modulated during the rhythmical bursts. It is suggested that, as in limb muscles, functionally specialized ADG motor units are recruited in an orderly sequence, starting with spontaneously active, slow-twitch units in the inner compartment, continuing with fast-twitch units recruited upon enhancement of the synaptic drive (as in the case of orofacial stimulation), and ending with massive, rhythmical recruitment of slow- and fast-twitch units during RJM.

The importance of frequency and amount of electrical stimulation for contractile properties of denervated rat muscles

Soleus (SOL) and extensor digitorum longus (EDL) muscles were denervated and directly stimulated for 23-69 days through implanted electrodes employing three different patterns. The stimulation was delivered in impulse trains where the pulse frequency differed (20, 75, and 150 Hz), while the train duration (0.3 s) and train repetition rate (1 min-1) were identical. Consequently, the number of pulses varied such that higher frequency was combined with a higher amount of stimulation. In both SOL and EDL the high-frequency pattern resulted in shorter twitch time-to-peak, greater post-tetanic potentiation, and greater tetanic force than the low frequency. Isotonic shortening velocity was increased to the same extent by all the patterns in SOL whereas in EDL fast intrinsic shortening velocity was maintained by the low-frequency pattern while it was decreased by the high-frequency pattern. We attribute this unexpected effect on the EDL to the larger number of pulses in the high-frequency pattern. By combining the present findings with previous data on directly stimulated rat muscles we conclude: in SOL the twitch duration is influenced by both the frequency and the amount of impulse activity, higher frequencies and smaller amounts leading to faster twitches. The EDL twitch duration is similarly dependent on the amount of activity, but the role of frequency is more unclear. In both SOL and EDL the isotonic shortening velocity is reduced by increasing amounts of activity and there is no evidence that impulse frequency plays a role. In EDL force output is strongly influenced by the impulse frequency, low frequencies resulting in low force outputs irrespective of the amount of activity.

Expression of superfast myosin in aneural regenerates of cat jaw muscle

We investigated whether innervation is necessary for the expression of superfast myosin in regenerating cat jaw-closing muscle. Strips of jaw muscle were permitted to regenerate bilaterally in the beds of a fast limb muscle with innervation on one side being prevented surgically. Immunocytochemical analyses using anti-myosin heavy chain antibodies were done at various times postoperatively, the latest being after 78 days. We found little difference between innervated and uninnervated regenerates up to 27 days postoperatively. All regenerating myotubes expressed fetal myosin. In addition, some myotubes stained for superfast or slow myosin, while others stained for both superfast and slow myosins. Subsequently, uninnervated myotubes became atrophic but continued to express fetal, slow, and superfast myosins while innervated myofibers suppressed fetal and slow myosin expression. These results are consistent with the notion that satellite cells of jaw-closing muscles are committed to express superfast myosin during myogenesis, and that the expression of this program is independent of innervation.

Restoration of paralyzed orbicularis oculi muscle function by controlled electrical current

A canine model of facial nerve paralysis was studied to apply controlled electrical current to the peripherally denervated orbicularis oculi muscle, in the attempt to effectively restore the absent function of this denervated muscle. After unilateral facial nerve neurotmesis was performed in eight dogs, the denervated orbicularis oculi muscles of four dogs were electrically stimulated for 75 postoperative days (40 min/day). Denervated and normal orbicularis oculi muscles were electrophysiologically studied and compared with the Student t test. During the study period, minimum closure of denervated treated orbicularis oculi muscles was evoked with average stimulus strength (80-ms duration) of 1.61 +/- 0.22 log mA x ms, not significantly different from that of denervated nontreated or normal orbicularis oculi muscles. From days 10 through 30 only, maximum closure of denervated treated orbicularis oculi muscles was achieved with mean pulse strength (80-ms duration) of 2.37 +/- 0.09 log mA x ms, significantly lower (P less than .01) than that evoking the same type of contraction from denervated nontreated muscles (80-ms duration, mean 2.83 +/- 0.10 log mA x ms). In addition, denervated treated muscle pulse strength eliciting maximum contraction was not significantly different from that of normal orbicularis oculi muscles during the same period. This finding was not observed, however, from day 40 through the end of the study. This investigation demonstrates (1) the transient reversal of denervation changes of paralyzed orbicularis oculi muscle by daily electrical stimulation, and (2) the feasibility of restoring orbicularis oculi muscle function by controlled electrical current.

Human skeletal muscle: phasic type of electrical stimulation increases its contractile speed

Skeletal muscles, exposed to a prolonged period of specific functional demands, respond adaptively. Electrical stimulation, when employed as a technique for subjecting selected muscles to altered use, enables precise entrainment of the pattern of functional activity. In this investigation, the vastus lateralis muscle in a group of volunteers was stimulated. The stimulation program typical of a phasic type of activity (high frequency, high current amplitude, short pulse duration) intermittently subjected the stimulated muscles to brief periods of intense activity, followed by relatively long pauses. The activation-relaxation time ratio chosen was 1 to 13. It was determined to prevent the muscles from fatiguing. The effects of the chronic stimulation program were established by measurements of the time course of contraction and relaxation and fatigue of the vastus lateralis muscle. Chronic phasic electrical stimulation increased the speed of muscle contraction by 15% while the fatigue characteristics remained unchanged.

Differences in H-reflex between athletes trained for explosive contractions and non-trained subjects

The efficacy of type Ia synapse on alpha-motoneurons of soleus and lateral gastrocnemius muscles has been investigated, using the H-reflex technique, in athletes engaged in sports requiring very rapid and intense contractions (sprinters and volley-ball players) as well as in non-trained subjects. It has been observed, in both muscles, that the ratio between the mean value of the maximal reflex response (Hmax) and the mean value of the maximal direct response (Mmax) elicited upon electrical stimulation of the tibial nerve is significantly smaller in athletes trained for explosive-type movements than in non-trained subjects. This difference in the Hmax: Mmax ratio was dependent on a smaller amplitude of Hmax and not on a greater amplitude of Mmax. No significant differences were observed between sprinters and volley-ball players. In both trained and non-trained subjects, soleus and lateral gastrocnemius muscles displayed significant differences in Hmax: Mmax ratio and Mmax amplitude but not in Hmax amplitude. Since the H-response is considered to be due mainly to activation of the smallest motoneurons in the motoneuronal pools, the difference in Hmax amplitude and Hmax: Mmax ratio between athletes and non-trained subjects could have been dependent on a lower incidence of these motoneurons in the athletes. This is in accord with the mechanical needs of muscles during explosive-type power training. Although this difference may have been wholly determined genetically, the possibility is discussed as to whether the lower incidence in sprinters and volley-ball players of small motoneurons could have been related to a training-induced transformation of small and slow motoneurons into large and fast ones.

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.

Electrical stimulation resembling normal motor-unit activity: effects on denervated fast and slow rat muscles

1. The slow-twitch soleus muscle and the fast-twitch extensor digitorum longus muscle (EDL) were denervated and stimulated directly with implanted electrodes for 33-82 days. Four different stimulation patterns were used in order to mimic important characteristics of the natural motor-unit activity in these muscles. In addition, to compare the effects of direct stimulation to other experimental models, some EDLs were stimulated through the nerve or cross-innervated by soleus axons. 2. After 33-82 days of stimulation the contractile properties were measured under isometric and isotonic conditions. 3. 'Native' stimulation patterns could maintain normal contractile speed in both EDL and soleus. In the EDL, normal isotonic shortening velocity was maintained only by a stimulation pattern consisting of very brief trains with an initial short interspike interval (doublet), and not by the other 'native' high-frequency patterns. 4. The contractile properties of both EDL and soleus muscles receiving a 'foreign' stimulation pattern were transformed in the direction of the muscle normally receiving that type of activity. The transformations were not complete, and soleus and EDL muscles stimulated with the same stimulation pattern remained different. This suggests that adult muscle fibres in rat EDL and soleus are irreversibly differentiated into different fibre types earlier in development. 5. The three high-frequency stimulation patterns used differed in their ability to change or maintain various contractile properties in the soleus and the EDL. The results indicate that the following qualities of a stimulation pattern might be of importance for the control of contractile properties: instantaneous frequency, total amount of stimulation, train length, interval between trains and presence of an initial doublet. 6. With the exception of the EDL shortening velocity, changes in contractile speed induced by a 'foreign' stimulation pattern were quantitatively similar to the effects of cross-innervation both in the EDL and the soleus. We thus suggest that the change in activity pattern is the mechanism behind most of the changes induced by cross-innervation.

Embryonic and neonatal myosin heavy chain in denervated and paralyzed rat skeletal muscle

Using immunofluorescence procedures with specific polyclonal and monoclonal antimyosin antibodies we have found that embryonic and neonatal myosin heavy chains (MHCs), which in rat skeletal muscle disappear during the first weeks after birth, are reexpressed in adult muscle after denervation. Reactivity for embryonic and neonatal MHCs was detected in some fibers as early as 3 days after denervation, became more evident by 7 days, and occurred exclusively in the type 2A fiber population. Paralysis of innervated muscles by tetrodotoxin block of the sciatic nerve also resulted in the reappearance of embryonic and neonatal MHCs in type 2A fibers. Significant variation in the degree of immunoreactivity was observed in different segments of the same muscle fiber, suggesting that coordination of muscle fiber nuclei in the control of myosin heavy chain gene expression is partially lost following denervation.