A comparison of the effects of denervation on the mechanical properties of rat and guinea-pig skeletal muscle

Denervation-Associated Change in the Palatinus and Levator Veli Palatini Muscles of Dogs with Elongated Soft Palate

Muscle lesions and decreased numbers of peripheral nerve branches have been reported in the soft palates of dogs presenting with brachycephalic airway obstruction syndrome (BAOS). Myosin adenosine triphosphatase staining was employed to investigate whether muscle lesions in the elongated soft palate (ESP) of dogs with BAOS reflect the presence of denervation. Soft palates were collected from nine brachycephalic dogs during surgical intervention for BAOS and from five healthy beagle dogs as controls. In the control soft palates, myofibres with relatively uniform diameters and a random mosaic pattern of type I and II myofibres were observed in the palatinus muscle (PM), while almost all of the myofibres in the levator veli palatini muscle (LVPM) were of type II. In the ESPs, small group atrophy, large group atrophy and angular-shaped atrophy were observed in myofibres of the PM and rarely in the LVPM. Fibre type grouping and an increase in type IIC myofibres were found only in the PM. Morphometric analysis of ESPs revealed a significant increase in the number of type I and II myofibres in the PM showing atrophy or hypertrophy compared with controls. A significant increase in atrophic type II myofibres was found in the LVPM of affected dogs. Myopathy consistent with denervation was observed in the PM, but rarely in the LVPM, of ESP specimens. The results suggest that the myopathy seen in dogs with ESP may partly reflect atrophy of myofibres resulting from damage to peripheral nerve branches, with subsequent reinnervation of myofibres.

HIF-1-driven skeletal muscle adaptations to chronic hypoxia: molecular insights into muscle physiology

Skeletal muscle is a metabolically active tissue and the major body protein reservoir. Drop in ambient oxygen pressure likely results in a decrease in muscle cells oxygenation, reactive oxygen species (ROS) overproduction and stabilization of the oxygen-sensitive hypoxia-inducible factor (HIF)-1α. However, skeletal muscle seems to be quite resistant to hypoxia compared to other organs, probably because it is accustomed to hypoxic episodes during physical exercise. Few studies have observed HIF-1α accumulation in skeletal muscle during ambient hypoxia probably because of its transient stabilization. Nevertheless, skeletal muscle presents adaptations to hypoxia that fit with HIF-1 activation, although the exact contribution of HIF-2, I kappa B kinase and activating transcription factors, all potentially activated by hypoxia, needs to be determined. Metabolic alterations result in the inhibition of fatty acid oxidation, while activation of anaerobic glycolysis is less evident. Hypoxia causes mitochondrial remodeling and enhanced mitophagy that ultimately lead to a decrease in ROS production, and this acclimatization in turn contributes to HIF-1α destabilization. Likewise, hypoxia has structural consequences with muscle fiber atrophy due to mTOR-dependent inhibition of protein synthesis and transient activation of proteolysis. The decrease in muscle fiber area improves oxygen diffusion into muscle cells, while inhibition of protein synthesis, an ATP-consuming process, and reduction in muscle mass decreases energy demand. Amino acids released from muscle cells may also have protective and metabolic effects. Collectively, these results demonstrate that skeletal muscle copes with the energetic challenge imposed by O2 rarefaction via metabolic optimization.

Key changes in denervated muscles and their impact on regeneration and reinnervation

The neuromuscular junction becomes progressively less receptive to regenerating axons if nerve repair is delayed for a long period of time. It is difficult to ascertain the denervated muscle's residual receptivity by time alone. Other sensitive markers that closely correlate with the extent of denervation should be found. After a denervated muscle develops a fibrillation potential, muscle fiber conduction velocity, muscle fiber diameter, muscle wet weight, and maximal isometric force all decrease; remodeling increases neuromuscular junction fragmentation and plantar area, and expression of myogenesis-related genes is initially up-regulated and then down-regulated. All these changes correlate with both the time course and degree of denervation. The nature and time course of these denervation changes in muscle are reviewed from the literature to explore their roles in assessing both the degree of detrimental changes and the potential success of a nerve repair. Fibrillation potential amplitude, muscle fiber conduction velocity, muscle fiber diameter, mRNA expression levels of myogenic regulatory factors and nicotinic acetylcholine receptor could all reflect the severity and length of denervation and the receptiveness of denervated muscle to regenerating axons, which could possibly offer an important clue for surgical choices and predict the outcomes of delayed nerve repair.

History, Mechanisms and Clinical Value of Fibrillation Analyses in Muscle Denervation and Reinnervation by Single Fiber Electromyography and Dynamic Echomyography

This work reviews history, current clinical relevance and future of fibrillation, a functional marker of skeletal muscle denervated fibers. Fibrillations, i.e., spontaneous contraction, in denervated muscle were first described during the nineteenth century. It is known that alterations in membrane potential are responsible for the phenomenon and that they are related to changes in electrophysiological factors, cellular metabolism, cell turnover and gene expression. They are known to inhibit muscle atrophy to some degree and are used to diagnose neural injury and reinnervation that are occurring in patients. Electromyography (EMG) is useful in determining progress, prognosis and efficacy of therapeutic interventions and their eventual change. For patients with peripheral nerve injury, and thus without the option of volitional contractions, electrical muscle stimulation may be helpful in preserving the contractility and extensibility of denervated muscle tissue and in retarding/counteracting muscle atrophy. It is obvious from the paucity of recent literature that research in this area has declined over the years. This is likely a consequence of the decrease in funding available for research and the fact that the fibrillations do not appear to cause serious health issues. Nonetheless, further exploration of them as diagnostic tools in long-term denervation is merited, in particular if Single Fiber EMG (SFEMG) is combined with Dynamic Echomyography (DyEM), an Ultra Sound muscle approach we recently designed and developed to explore denervated and reinnervating muscles.

Time-dependent changes in caspase-3 activity and heat shock protein 25 after spinal cord transection in adult rats

Chronic reductions in muscle activation and loading are associated with decreased heat shock protein 25 (Hsp25) expression and phosphorylation (pHsp25) which, in turn, may contribute to elevated caspase-3-mediated muscle protein breakdown. Thus, the purpose of the present study was to determine whether there are any changes in Hsp25, pHsp25 and caspase-3 activity among rat muscles having different fibre type compositions and functions [soleus, adductor longus (AL), plantaris and tibialis anterior (TA)] at 0 (control), 1, 8 or 28 days after a complete spinal cord transection (ST). The Hsp25 levels were unaffected on days 1 and 8 in all muscles, except for a significant reduction on day 8 in plantaris. The Hsp25 levels were lower than control values in all muscles except TA on day 28. The pHsp25 levels were lower than control values after 8 and 28 days in plantaris and AL and after 28 days in soleus, but higher than control in TA after 8 and 28 days. Caspase-3 activity was higher in ST than control rats on day 8 in all muscles except TA. Caspase-3 activity was negatively correlated with muscle mass for all muscles. In plantaris, Hsp25 and pHsp25 were negatively correlated with caspase-3 activity and Hsp25 was correlated with muscle mass. These relationships were not observed in other muscles. Thus, the effects of ST on Hsp25 and caspase-3 are muscle specific and time dependent, factors that should be considered in developing any intervention to maintain muscle mass after a spinal cord injury.

The denervated muscle: facts and hypotheses. A historical review

Denervation changes in skeletal muscle (atrophy; alterations of myofibrillar expression, muscle membrane electrical properties, ACh sensitivity and excitation-contraction coupling process; fibrillation), and their possible causes are reviewed. All changes can be counteracted by muscle electrostimulation, while denervation-like effects can be caused by the complete conduction block in muscle nerve. These results do not support the hypothesis that the lack of neurotrophic, non-motor factors plays a role in denervation phenomena. Instead they support the view that the lack of neuromotor discharge is the only cause of the phenomena and that neuromotor activity is an essential factor in regulating muscle properties. However, some experimental results cannot apparently be explained by the lack of neuromotor impulses, and may still suggest that neurotrophic influences exist. A hypothesis is that neurotrophic factors, too feeble to maintain a role in completely differentiated, adult muscles, can concur with neuromotor activity in the differentiation of immature, developing muscles.

Plasticity of the transverse tubules following denervation and subsequent reinnervation in rat slow and fast muscle fibres

We have studied the effects of short term denervation followed by reinnervation on the ultrastructure of the membrane systems and on the content of and distribution of key proteins involved in calcium regulation of fast-twitch (FT) extensor digitorum longus (EDL) and slow-twitch (ST) soleus (SOL) muscle fibres. Ischiadic nerve freezing resulted in total lack of neuromuscular transmission for 3 days followed by a slow recovery, but no decline in twitch force elicited by direct stimulation. The latter measurements indicate no significant atrophy within this time frame. The membrane systems of skeletal muscle fibres were visualized using Ca92+)-K3Fe(CN)6-OsO4 techniques and observed using a high voltage electron microscope. [3H]nitrendipine binding was used to detect levels of dihydropyridine receptor (DHPR) expression. The Ca2+ pumping free sarcoplasmic reticulum domains were not affected by the denervation, but the Ca2+ release domains were dramatically increased, particularly in the FT-EDL muscle fibres. The increase is evidenced by a doubling up of the areas of contacts between SR and transverse (t-) tubules, so that in place of the normal triadic arrangement, pentadic and heptadic junctions, formed by multiple interacting layers of ST and t-tubules are seen. Frequency of pentads and heptads increases and declines in parallel to the denervation and reinnervation but with a delay. Immunofluorecence and electron microscopy observations show presence of DHPR and ryanodine receptor clusters at pentads and heptads junctions. A significant (P < 0.01) positive correlation between the level of [3H]nitrendipine binding component and the frequency pentads and heptads was observed in both the FT-EDL and ST-SOL muscle fibres indicating that overexpression of DHPRs accompanies the build up extra junctional contacts. The results indicate that denervation reversibly affects the domains of the membrane systems involved in excitation-contraction coupling.

Denervation-activated inward rectifier in frog slow skeletal muscle fibers

We tested whether the absence of an inward rectifier channel in slow skeletal muscle fibers of the frog is regulated by innervation. Normal and denervated slow fibers were identified according to their passive electrical properties. In current-clamp experiments, anomalous rectification was quantified as the ratio of effective resistances for hyperpolarizing and depolarizing pulses. In isotonic potassium solution, this ratio was 0.45 +/- 0.1 (n = 14) for twitch fibers, whereas slow fibers displayed linear behavior [ratio = 1.0 +/- 0.05 (n = 15)]. However, denervated slow fibers showed anomalous rectification (ratio, 0.48 +/- 0.07; n = 5). This finding was supported by voltage-clamp experiments in which denervated slow fibers displayed (1) an inward rectifier current during hyperpolarizing pulses, (2) an increase in this current when [K(+)](o) was increased, and (3) a current inhibition after application of Ba(2+). These results suggest that frog slow fibers, which normally do not possess inward rectifier channels, can express them after denervation.

Mechanical properties of the electrically silent adult rat soleus muscle

The isometric and isotonic in situ mechanical properties of the soleus muscle of adult female rats were determined after 60 days of inactivity induced by spinal cord isolation (SI). Compared to control, the absolute muscle mass, physiological cross-sectional area, and maximum tetanic tension of the soleus in SI rats were reduced by 69%, 66%, and 77%, respectively. Isometric twitch time-to-peak-tension and half-relaxation times were 41% and 60% shorter in SI than control rats. The maximum velocity of shortening (mm/s), as determined using the afterloaded technique, was 66% faster in SI than control rats, whereas unloaded shortening velocity was similar in the two groups (9% faster in SI rats). Peak power was 48% lower in SI than control rats. The SI soleus was 39% more fatigable than control. Thus, the soleus became a smaller, faster, and more fatigable muscle following 60 days of inactivity. In general, the results indicate that the adaptations are of a lesser magnitude than those reported previously following denervation for the same duration. These data provide a baseline for future efforts to experimentally define the mechanisms of neurally mediated, but activity-independent, regulation of the mechanical properties of the rat soleus muscle.

A specific force deficit exists in skeletal muscle after partial denervation

Skeletal muscle demonstrates a specific force deficit after repair of injured peripheral nerves, microneurovascular muscle transfer, and normal aging. Because atrophy cannot account for deficits in specific force, other, unknown, mechanisms are responsible for the resulting muscle contractile dysfunction under these circumstances. We tested the hypothesis that a subpopulation of denervated fibers is partially or completely responsible for the specific force deficit after partial denervation of the rat extensor digitorum longus muscle (EDL). Adult Fisher rats underwent either sham exposure or partial transection of 80% of the cross-sectional area of the left deep peroneal nerve. After a 2-week recovery period, maximum isometric force (F(0)) was measured in situ and maximum specific force (sF(0)) was calculated for EDL from both control (n = 8) and partial denervation (n = 7) groups. Innervated fiber cross-sectional area (CSA(inn)) was measured directly from whole EDL cross sections after immunohistochemical labeling for neural cell adhesion molecule (NCAM), a marker of muscle fiber denervation. A corrected specific force value (sF(0-inn)) was calculated by normalizing F(0) to CSA(inn). Partial skeletal muscle denervation resulted in significant reductions in muscle mass, F(0), and sF(0). The percentage of muscle fibers expressing NCAM in the extrajunctional sarcolemma increased from 1.0 +/- 0.8% in control to 49 +/- 15% in partially denervated EDL muscles. A 62.7% deficit in EDL specific force was observed after partial denervation. Denervated muscle fibers accounted for 59.3% of this deficit, but sF(0-inn) still differed significantly between control and partially denervated muscles, with a 25.5% difference between groups. In partially denervated muscles, the specific force deficit is partially but not fully explained by a subpopulation of noncontractile, denervated fibers.

Effects of denervation on Ca2+ channels in slow skeletal muscle fibers of the frog

Effects of denervation on calcium channels in slow skeletal muscle fibers in the frog (Rana pipiens) were studied using the three-microelectrode voltage-clamp technique in intact fibers. Ca2+, Ba2+, and Sr2+ currents were all significantly reduced in amplitude during the first 2 weeks after denervation. After nerve section the selectivity sequence Ba congruent with Ca > Sr was changed to Ba > Sr > Ca and the values for relative ratio increased from 1.04 to 2.65 for Ba2+ and from 0.58 to 1.20 for Sr2+ (with respect to Ca2+). Barium current saturation was more obvious in denervated fibers than in non-denervated fibers. The values obtained with the Michaelis-Menten type expression, I = Imax/(1+Kd/[Ba]e) were Kd = 2.7 mM and Imax = 20 microA/cm2 in fibers 2 weeks after nerve section compared with the values Kd = 4.4 mM and Imax = 60 microA/cm2 obtained in non-denervated fibers. Additionally, the effects of two calcium channel blockers (cobalt and nifedipine) were greater by a factor of two in denervated fibers than in non-denervated fibers. Three weeks or so after nerve section, all the biophysical properties studied began to show a tendency to recover toward the values obtained in non-denervated muscles (controls). These results suggest that calcium channels are modified or that there is a change in the types of calcium channels present in frog slow skeletal muscle fibers after denervation.

Dynamics of nuclei of muscle fibers and connective tissue cells in normal and denervated rat muscles

The mitotic activity in muscles of growing rats and the effect of denervation were studied by means of continuous infusion of 5-bromo-2-deoxyuridine (BRDU). Denervated muscles after 10 weeks contained 20 to 60% fewer muscle nuclei than normal; BRDU labeled about 25% of the nuclei of normal soleus and extensor digitorum longus (EDL) and of denervated EDL muscles but only 5% in the denervated soleus muscle. Labeled nuclei persisted in denervated but not in normal muscles. After the main growth period, the turnover of myonuclei was at most 1 to 2% per week. The behavior of connective tissue nuclei was similar to that in muscle fibers. Infusion of BRDU had no effect on contractile properties. It is suggested that the exceptionally rapid atrophy of the denervated rat soleus associated with loss of satellite cells was due to loss of myonuclei and differentiation and fusion of satellite cells. The cause may possibly be that the phase of postdenervation fibrillation is shorter than in other muscles.

Effects of immobilization and subsequent low- and high-intensity exercise on morphology of rat calf muscles

After a cast immobilization of 3 weeks, the effects of 4-week remobilization by free cage activity or treadmill running on the morphology of the rat soleus and gastrocnemius muscles were studied. The studied morphometric parameters were: percentage volume of intramuscular connective tissue, capillary density, muscle fiber size, number of fibers with a pathological structural alteration, and fiber type distribution. In both muscles, immobilization of 3 weeks produced a significant increase in the connective tissue volume and number of fibers with pathological alterations, with a similar decrease in the capillary number and fiber size. At the same time, the relative amount of type I fibers decreased and type IIA fibers increased. Free remobilization and especially intensified remobilization by treadmill running significantly restored these values towards controls. These findings indicate that in rat soleus and gastrocnemius muscles immobilization-induced accumulation of intramuscular connective tissue, capillary loss, reduction in fiber size, accumulation of fibers with pathological structural alterations, and changes in fiber type distribution are to a great extent reversible phenomena, especially if remobilization is intensified by physical training. In clinical practice, this suggests that in patients with musculoskeletal injuries the postimmobilization rehabilitation should be early and effective.

Dihydropyridine receptor and ryanodine receptor gene expression in long-term denervated rat muscles

Following disruption of the nerve supply, extensor digitorum longus (EDL) and soleus (SOL) muscles in rats are known to exhibit alterations in excitation-contraction coupling. After total RNA isolation from the denervated and the contralateral control muscles performed at 25 and 50 days following denervation, RNase protection assays were carried out with four cDNA probes specific for the skeletal and cardiac isoforms of both the DHPR alpha 1-subunit and the RyR. Longterm denervation increased the expression of the mRNA for skeletal DHPR and skeletal RyR in SOL muscle, but it also significantly increased the expression of the mRNA for the cardiac isoform of the DHPR alpha 1 subunit in EDL muscle.

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.

Electron microscopic study of long-term denervated rat skeletal muscle

BACKGROUND

This study describes the ultrastructure of long-term denervated rat extensor digitorum longus and tibialis anterior muscles, with particular emphasis on understanding the cellular basis for the reduced restorative capacity of long-term denervated muscle.

METHODS

In 30 male WI/HicksCar rats, the right hindleg was denervated for periods of 1, 2, 4, 5.5, 6, 7, 12, 14, and 18 months before tissues were prepared for electron microscopy.

RESULTS

Atrophy of muscle fibers was prominent by the second month post-denervation. At this time, type II fibers showed greater atrophy than type I fibers. At further periods of denervation, atrophy of all fibers was seen; and with increasing times of denervation the muscle fibers became surrounded by dense mats of collagen fibers. Muscle spindles persisted for the duration of this study. At two and four months, satellite cells showed signs of activation, such as elongated cytoplasmic processes and an increased concentration of cytoplasmic organelles. As denervation progressed, activated satellite cells became more widely separated from their associated muscle fibers, and basal lamina material was deposited between the satellite cells and muscle fibers. Some satellite cells broke free from their muscle fibers, and others acted as bridges between two muscle fibers. Evidence was seen of both muscle fiber degeneration and the regeneration of new muscle fibers, often more than one regenerating fiber beneath a single basal lamina. Loose folds of basal lamina were often present around atrophic muscle fibers. As denervation progressed, the morphology of individual muscle fibers varied. Some contained well-ordered lattice arrays of myofilaments, whereas in others considerable sarcomeric disorganization was evident. Mitochondria became smaller and rounded; elements of the sarcoplasmic reticulum proliferated and became more disorganized; lipid droplets, glycogen deposits, and autophagic vesicles were all present in the cytoplasm of atrophic muscle fibers.

CONCLUSIONS

In addition to muscle fiber atrophy, long-term denervated muscles show evidence of myofiber and capillary death, as well as the deposition of massive amounts of interstitial collagen. These changes, all of which would appear to reduce the restorative capacity of the denervated muscle, take place concurrently with the morphological activation of satellite cells. The latter indicates that even in the denervated condition, restorative processes occur concurrently with regressive processes.

Regulation of myosin heavy chain expression in adult rat hindlimb muscles during short-term paralysis: comparison of denervation and tetrodotoxin-induced neural inactivation

The extent to which myosin profiles within adult fast and slow muscles are altered by short-term paralysis remains equivocal. We used an array of specific antibodies to identify adult and developmental MHC isoforms within EDL and soleus muscle fibers, and show a marked multiple expression of MHCs with a general shift towards slower and more energy efficient MHC profiles after 2 weeks of denervation or TTX nerve conduction block. Paralysis also induced marked expression of an embryonic MHC within most EDL cell types, and a subtle, paralysis-sensitive, expression of alpha-cardiac MHC within specific EDL and soleus extrafusal fibers. Comparison of treatment groups also permitted assessment of the relative influence of neural activity versus trophic factors on these isoforms, and confirmed activity as a major, but not sole, regulator of MHC expression.

Morphological changes in the triads and sarcoplasmic reticulum of rat slow and fast muscle fibres following denervation and immobilization

We observed the morphological features of the membrane systems (sarcoplasmic reticulum, transverse tubules and triads) involved with the excitation-contraction coupling in rat soleus and extensor digitorum longus muscle following two disuse protocols: denervation and immobilization. The immobilized positions were: maximum dorsal flexor (soleus were stretched and extensor digitorum longus were shortened), maximum plantar flexor (soleus were shortened and extensor digitorum longus were stretched), and midway between the dorsal flexor and plantar flexor. The arrangement of the membrane systems was disordered following both disuse conditions. Increases in transverse tubule network were apparent; there were clearly more triads than in normal fibres, and pentadic and heptadic structures (i.e., a close approximation of two or three transverse tubule elements with three or four elements of terminal cisternae of sarcoplasmic reticulum) were frequently appeared following both denervation and immobilization. The most notable difference between the influence of denervation and immobilization on the membrane systems is the time at which the pentads and heptads appeared. They appeared much earlier (1 week after denervation) in denervated than in immobilized (3 or 4 weeks after immobilization) muscle fibres. On the other hand, the frequency of pentads and heptads is clearly related to the fibre type (significantly higher in extensor digitorum longus) and to extent of atrophy. The different influences of immobilization in each leg position suggest that disuse, but with neurotrophic factor(s), influences on the membrane systems were affected by sarcomere length, and the neurotrophic factor(s) and muscle activity were not always necessary to form new membrane systems in disuse skeletal muscle fibres.

Satellite cells and myonuclei in long-term denervated rat muscles

BACKGROUND

The percentage of satellite cells rapidly decreases in aneurally regenerating soleus muscles of rat. Also denervation of intact muscles causes fiber loss and regeneration, but the fate of satellite cells is unknown; myonuclei have been suggested to undergo changes resembling those in apoptotic cells.

METHODS

Rat soleus and extensor digitorum longus (EDL) muscles were denervated at birth or at age 5 weeks and investigated after periods of up to 38 weeks. At least 400 myonuclei in each muscle were assessed by electron microscopy, and satellite cell nuclei were counted. In situ nick translation and tailing were performed after 30 weeks denervation in order to demonstrate DNA breaks associated with apoptosis.

RESULTS

Myotubes indicating regeneration were prominent in the adult denervated soleus and deep layers of EDL muscles after 7 weeks and in the superficial parts of EDL muscle after 16 weeks. The percentage of satellite cell nuclei slowly decreased to less than one fifth of normal after 20-30 weeks. Almost all satellite cells had vanished 10 weeks after neonatal denervation. Degenerating myonuclei in adult, but not in neonatally denervated muscles, remotely resembled apoptotic nuclei of lymphocytes, but no evidence of DNA breaks was found.

CONCLUSION

Denervation of rat skeletal muscles causes, in addition to fiber atrophy, loss of fibers with subsequent regeneration. Proliferation of satellite cells under aneural conditions may lead to exhaustion of the satellite cell pool. This process is more rapid in growing than in adult muscles. Myonuclei in denervated muscles do not show DNA breaks which can be demonstrated by in situ nick translation.

Relation of recurrent laryngeal nerve compound action potential to laryngeal biomechanics

This study was designed to investigate the compound action potential (CAP) of the recurrent laryngeal nerve (RLN) and to correlate this electrophysiologic signal to laryngeal biomechanics and phonatory function. Four adult mongrel canines were anesthetized. The RLN was isolated and stimulated, and recording electrodes were applied. The electromyographic (EMG) electrode was placed in the thyroarytenoid (TA) muscle. The RLN CAP and the EMG of the TA muscle were recorded and compared to the stimulation intensity, subglottic pressure (Psub), and each other. The CAP peak-to-peak and EMG peak-to-peak amplitudes demonstrated a sigmoidal relation to stimulus intensity and a linear relation to Psub and to each other. On the basis of these findings, the RLN CAP appears to be a reliable physiologic measure of laryngeal function.

Contractile properties of aneurally regenerated compared with denervated muscles of rat

The time course of aneural regeneration in slow-twitch soleus muscles of young adult rats was studied and compared with the changes following denervation in soleus and fast extensor digitorum longus muscles. Regeneration was induced by auto-grafting after treatment with bupivacaine; isometric contractions were recorded from 5 to 70 days later. Force was detected at 5 days; at 12 days force was maximal (at least 20% of original) and thereafter fell exponentially. Force varied normally with total fibre area, except at 5 and 71 days when force generating capacity was low. Contraction and relaxation in the twitch were longer than normal (maximally at 5 days), and were closer to denervated soleus than EDL; in contrast, the maximal rate of rise of force was as high as that of denervated EDL and much higher than in denervated soleus. It is suggested that the muscle was fundamentally fast contracting, but the twitches were probably slow because of greater than normal activation following a single stimulus--a hypothesis supported by twitch:tetanus ratios that were higher than in denervated muscles. Tetanic force was much more sensitive than normal to changes of muscle length from optimum, despite the fact that the lengths of regenerated muscles were similar to those of contralateral muscles. The properties of denervated soleus gradually approached those of regenerated soleus, probably because of replacement of original fibres by regenerated ones.

Differences between contractions in vitro of slow and fast rat skeletal muscle persist after random reinnervation

1. The medial popliteal nerve was sectioned in adult rats, and reinnervation was permitted after nerve reunion. Reinnervation was observed in soleus (slow twitch) and plantaris (fast twitch) muscles for periods of 4-21 months after the initial operation. 2. In the reinnervated muscles, isometric twitch time to peak (contraction time) and time to half-relaxation were longer in soleus than plantaris in every muscle examined. 3. Some of the muscles were stained for actomyosin ATPase activity. The proportion of type I fibres (presumed slow) was at least twice as high in soleus compared with plantaris muscles. The proportion in the latter was similar to that predicted from random reinnervation. 4. The effects of differing proportions of fast and slow fibres on the twitch contraction and half-relaxation times were modelled, and the experimental twitch data were found to be compatible with the proportions of fibre types in individual muscles. 5. It is speculated that the mechanism accounting for the higher proportion of slow fibres in soleus muscle probably does not involve selective reinnervation. It is suggested that fast motoneurones may be transformed to slow by a trophic chemical released by slow muscle fibres; such a process would be more extensive in predominantly slow twitch soleus than a fast muscle, and the resulting decrease in the proportion of fast motoneurones would account for the less extensive conversion in soleus.

Effects of chronic electrical stimulation on contractile properties of long-term denervated rat skeletal muscle

1. The contractile properties of fast-twitch (extensor digitorum longus or EDL) and slow-twitch (soleus) muscles in the rat were followed for periods of between 4 and 10 months after denervation. The effects of chronic electrical stimulation during the last 3-8 weeks of denervation were investigated. 2. The fall in tetanic tension that follows axotomy ended after about 4 months' denervation. The equilibrium tension was about 0.75% of control tension in EDL and 0.2-0.3% in soleus. 3. The low tension in soleus was due partly to the small diameter of the muscle fibres (atrophy) and partly to their necrosis that resulted in an 8-fold fall in specific tension (the force per unit cross-sectional area). Similar but less extreme changes occurred in EDL. 4. It is speculated that the final level of tension reached by unstimulated denervated muscles is an equilibrium between decrease in force due to atrophy and necrosis and increase due to regeneration. Differences between the final tension levels in soleus and EDL cannot be accounted for quantitatively by known differences in atrophy alone. Therefore, the rate of necrosis in soleus and of regeneration in EDL may be higher. 5. Chronic stimulation of long-term denervated muscle increased force generation by about 7-fold in EDL and between 20 and 55 times in soleus. The final tension reached was between 4 and 5% of normal in both muscles. Specific tension of fibres was almost completely restored by stimulation and the number of fibres was normal. The failure to recover full tension was largely due to failure to reverse denervation atrophy completely. 6. Twitch contraction and relaxation times were identical in denervated-stimulated soleus and EDL. There was no evidence for dependence on duration of stimulation or tension of the muscle. The normalized maximum rate of rise of tetanic tension remained higher in EDL than soleus.

A comparison of fibrillation in denervated skeletal muscle of the anaesthetized rat and guinea-pig

We have used intracellular recording in vivo to study fibrillation (spontaneous repetitive membrane activity) in extensor digitorum longus (fast twitch) and soleus (slow twitch) muscles of the anaesthetized rat and guinea-pig denervated for periods of about 10 to 60 days. The proportion of fibres fibrillating in the guinea-pig soleus was greater than 50% in most animals up to the longest period of denervation (65 days). Fibrillation was rarely found in rat soleus after three weeks of denervation. Its incidence in the extensor digitorum longus muscles of both species was intermediate. The mean frequency of fibrillation was higher in guinea-pig extensor digitorum longus (16 Hz) and soleus (8 Hz) than in the rat extensor digitorum longus (3 Hz) and soleus (2 Hz). The resting membrane potentials of the denervated muscles were less than normal and correlated inversely with the frequency of fibrillation but not with the incidence of fibrillation: in rat soleus, many fewer fibers were fibrillating at a given membrane potential than in the other three muscles. The incidence of fibrillation was compared with previously reported tensions of the four denervated muscles and was found to have the same rank order. We suggest that fibrillation may reduce atrophy (and hence tension loss) of denervated muscle, which may have implications for artificial stimulation. Fibrillation frequency was directly related to changes in twitch speed of the four muscles after denervation.