Activity-independent neural influences on cat soleus motor unit phenotypes

Medial gastrocnemius muscles fatigue but do not atrophy in paralyzed cat hindlimb after long-term spinal cord hemisection and unilateral deafferentation

This study of medial gastrocnemius (MG) muscle and motor units (MUs) after spinal cord hemisection and deafferentation (HSDA) in adult cats, asked 1) whether the absence of muscle atrophy and unaltered contractile speed demonstrated previously in HSDA-paralyzed peroneus longus (PerL) muscles, was apparent in the unloaded HSDA-paralyzed MG muscle, and 2) how ankle unloading impacts MG muscle and MUs after dorsal root sparing (HSDA-SP) with foot placement during standing and locomotion. Chronic isometric contractile forces and speeds were maintained for up to 12 months in all conditions, but fatigability increased exponentially. MU recordings at 8-11½ months corroborated the unchanged muscle force and speed with significantly increased fatigability; normal weights of MG muscle confirmed the lack of disuse atrophy. Fast MUs transitioned from fatigue resistant and intermediate to fatigable accompanied by corresponding fiber type conversion to fast oxidative (FOG) and fast glycolytic (FG) accompanied by increased GAPDH enzyme activity in absolute terms and relative to oxidative citrate synthase enzyme activity. Myosin heavy chain composition, however, was unaffected. MG muscle behaved like the PerL muscle after HSDA with maintained muscle and MU contractile force and speed but with a dramatic increase in fatigability, irrespective of whether all the dorsal roots were transected. We conclude that reduced neuromuscular activity accounts for increased fatigability but is not, in of itself, sufficient to promote atrophy and slow to fast conversion. Position and relative movements of hindlimb muscles are likely contributors to sustained MG muscle and MU contractile force and speed after HSDA and HSDA-SP surgeries.

Functional overload in ground squirrel plantaris muscle fails to induce myosin isoform shifts

We performed 2 wk of mechanical overload by synergist ablation on plantaris muscles from a small rodent hibernator, Spermophilus lateralis. While this muscle displays prominent myosin heavy-chain (MyHC) isoform shifts during hibernation, sensitivity to mechanical loading as a stimulus for muscle mass and isoform plasticity has not been demonstrated. Squirrel muscles, whether during hibernation or not, potentially are less sensitive to mechanical unloading, but we hypothesized that increased loading would produce the typical mammalian response of greater plantaris mass and MyHC shifts. Mechanical overload produced a 50% increase in muscle mass but, surprisingly, no changes in MyHC isoform protein or mRNA expression, despite previously observed fast-to-slow MyHC isoform switching during hibernation. Citrate synthase enzyme activity, as well as mRNA expression of creatine kinase and the muscle growth factor myostatin, were all unchanged. The mRNA expression of critical muscle atrophy genes decreased by 50% during hypertrophy, including ubiquitin ligases MuRF1 and MAFbx, and the related transcription factor FOXO-1a. Insulin-like growth factor (IGF-1) and hypoxia-inducible factor (HIF-1alpha) mRNA expression was elevated by 400% and 150%. Fast-to-slow MyHC isoform shifts appear unnecessary to support the increased recruitment of the plantaris muscle, shifts which are seen in other rodent models. Our results are consistent with muscular activity during interbout arousals as a potential mechanism to preserve muscle mass, but illustrate the primary importance of other seasonal factors besides patterns of muscle activation which must act in concert to alter MyHC isoforms and muscle fiber type during hibernation.

The effect of passive movement on denervated soleus highlights a differential nerve control on SERCA and MyHC isoforms

The sarco-endoplasmic reticulum Ca2+ ATP-ase (SERCA) and myosin heavy chain (MyHC) levels were measured in hindlimb-denervated and selectively denervated rat soleus muscles. Selective denervation allowed passive movement of the soleus, whereas hindlimb denervation rendered it to passivity. To minimize chronic effects, we followed the changes only for 2 weeks. Selective denervation resulted in less muscle atrophy, a faster slow-to-fast transition of MyHC isoforms, and less coordinated expressions of the slow vs fast isoforms of MyHC and SERCA. Generally, expression of the slow-twitch type SERCA2a was found to be less dependent, whereas the slow-twitch type MyHC1 was the most dependent on innervation. Our study shows that passive movement is able to ameliorate denervation-induced atrophy of the soleus and that it also accentuates the dyscoordination in the expression of the corresponding slow and fast isoforms of MyHC and SERCA.

Persistence of motor unit and muscle fiber types in the presence of inactivity

The clarity of categorizing skeletal muscle fibers in individual motor units into phenotypes based on quantitative single fiber enzyme activities and as a function of neuromuscular activity level was examined. Neuromuscular activity was eliminated in adult cat hindlimb muscles by spinal cord isolation (SI), i.e. complete spinal cord transection at a low thoracic and a high sacral level with bilateral dorsal rhizotomy between the transection sites. One motor unit was isolated via ventral root teasing procedures from the tibialis anterior (TA) muscle of each hindlimb in control and SI cats, and physiologically tested and glycogen depleted through repetitive stimulation; fibers comprising each motor unit were visualized through glycogen staining. Each motor unit was composed of fibers of the same myosin immunohistochemical type. Myofibrillar adenosine triphosphatase, succinate dehydrogenase and alpha-glycerophosphate dehydrogenase activities were determined for a sample of motor unit and non-motor unit fibers, providing a measure of three enzyme activities often used to characterize fiber phenotype within a single unit. Although normal enzyme activities were altered after 6 months of inactivity, the relationships among the three enzymes were largely maintained. These data demonstrate that it is not the diversity in any single enzyme property but the profile of several metabolic pathways that underlies the significance of fiber phenotypes. These profiles must reflect a high level of coordination of expression of selected combinations of genes. Although neuromuscular activity level influences fiber phenotype, the present results demonstrate that activity-independent mechanisms remain important sources of the control of phenotype establishment in the near absence of activity.

Plasticity from muscle to brain

Recognition that the entire central nervous system (CNS) is highly plastic, and that it changes continually throughout life, is a relatively new development. Until very recently, neuroscience has been dominated by the belief that the nervous system is hardwired and changes at only a few selected sites and by only a few mechanisms. Thus, it is particularly remarkable that Sir John Eccles, almost from the start of his long career nearly 80 years ago, focused repeatedly and productively on plasticity of many different kinds and in many different locations. He began with muscles, exploring their developmental plasticity and the functional effects of the level of motor unit activity and of cross-reinnervation. He moved into the spinal cord to study the effects of axotomy on motoneuron properties and the immediate and persistent functional effects of repetitive afferent stimulation. In work that combined these two areas, Eccles explored the influences of motoneurons and their muscle fibers on one another. He studied extensively simple spinal reflexes, especially stretch reflexes, exploring plasticity in these reflex pathways during development and in response to experimental manipulations of activity and innervation. In subsequent decades, Eccles focused on plasticity at central synapses in hippocampus, cerebellum, and neocortex. His endeavors extended from the plasticity associated with CNS lesions to the mechanisms responsible for the most complex and as yet mysterious products of neuronal plasticity, the substrates underlying learning and memory. At multiple levels, Eccles' work anticipated and helped shape present-day hypotheses and experiments. He provided novel observations that introduced new problems, and he produced insights that continue to be the foundation of ongoing basic and clinical research. This article reviews Eccles' experimental and theoretical contributions and their relationships to current endeavors and concepts. It emphasizes aspects of his contributions that are less well known at present and yet are directly relevant to contemporary issues.

Rehabilitative Therapies after Spinal Cord Injury

We review some basic and highly relevant concepts in the effort to develop improved rehabilitative interventions for subjects with spinal cord injury (SCI). Interventions that are likely to contribute to improved sensorimotor function include (1) practice of the specific motor task that needs to be improved; and (2) combining the training with one or more interventions--such as pharmacological modulation of the excitability of spinal neural networks, implantation of selected cell types such as olfactory ensheathing glia (OEG), and/or modulation of the excitability of the spinal cord via epidural stimulation. Upon improvement of the neural control of the musculature following SCI, it will always be prudent to maximize the torque output from these activation patterns by assuring that muscle mass is maintained. Therefore, it seems quite feasible that considerable improvement in locomotor performance can be achieved by improved coordination of motor pools, as well as effective recovery of muscle mass, which will assist in the potential generation of normal forces among agonistic and antagonistic muscle groups.

Postfatigue potentiation of the paralyzed soleus muscle: evidence for adaptation with long-term electrical stimulation training

Understanding the torque output behavior of paralyzed muscle has important implications for the use of functional neuromuscular electrical stimulation systems. Postfatigue potentiation is an augmentation of peak muscle torque during repetitive activation after a fatigue protocol. The purposes of this study were 1) to quantify postfatigue potentiation in the acutely and chronically paralyzed soleus and 2) to determine the effect of long-term soleus electrical stimulation training on the potentiation characteristics of recently paralyzed soleus muscle. Five subjects with chronic paralysis (>2 yr) demonstrated significant postfatigue potentiation during a repetitive soleus activation protocol that induced low-frequency fatigue. Ten subjects with acute paralysis (<6 mo) demonstrated no torque potentiation in response to repetitive stimulation. Seven of these acute subjects completed 2 yr of home-based isometric soleus electrical stimulation training of one limb (compliance = 83%; 8,300 contractions/wk). With the early implementation of electrically stimulated training, potentiation characteristics of trained soleus muscles were preserved as in the acute postinjury state. In contrast, untrained limbs showed marked postfatigue potentiation at 2 yr after spinal cord injury (SCI). A single acute SCI subject who was followed longitudinally developed potentiation characteristics very similar to the untrained limbs of the training subjects. The results of the present investigation support that postfatigue potentiation is a characteristic of fast-fatigable muscle and can be prevented by timely neuromuscular electrical stimulation training. Potentiation is an important consideration in the design of functional electrical stimulation control systems for people with SCI.

Effects of inactivity on fiber size and myonuclear number in rat soleus muscle

The effects of short-term (4 days) and long-term (60 days) neuromuscular inactivity on myonuclear number, size, and myosin heavy chain (MHC) composition of isolated rat soleus fibers were determined using confocal microscopy and gel electrophoresis. Inactivity was produced via spinal cord isolation (SI), i.e., complete spinal cord transections at a midthoracic and a high sacral level and bilateral deafferentation between the transection sites. Compared with control, there was an increase in the percentage of fibers containing the faster MHC isoforms after 60, but not 4, days of SI. The mean sizes of type I and type I+IIa fibers were 41 and 27% and 66 and 56% smaller after 4 and 60 days of SI, respectively. Thus atrophy occurred earlier than the shift in myosin heavy chain (MHC) profile. The number of myonuclei was approximately 30% higher in type I than type I+IIa fibers in control soleus, but after 60 days of SI these values were similar. The number of myonuclei per millimeter in type I fibers was significantly lower than control after 60 days of SI, whereas there was no change in type I+IIa fibers. Thus myonuclei were eliminated from fibers containing only type I MHC. Because the magnitude of the loss of myonuclei was less than the level of atrophy, the myonuclear domains of both type I and type I+IIa fibers were significantly lower than control. Thus chronic (60 days) inactivity results in smaller, faster fibers that contain a higher than normal amount of DNA per unit of cytoplasm. The absence of activation of muscle fibers that are normally the most active (pure type I fibers) resulted in most, but not all, fibers expressing some fast MHC isoforms. The results also indicate that a loss of myonuclei is not a prerequisite for sustained muscle fiber atrophy.

Pathophysiology of spastic paresis. I: Paresis and soft tissue changes

Spastic paresis follows chronic disruption of the central execution of volitional command. Motor function in patients with spastic paresis is subjected over time to three fundamental insults, of which the last two are avoidable: (1) the neural insult itself, which causes paresis, i.e., reduced voluntary motor unit recruitment; (2) the relative immobilization of the paretic body part, commonly imposed by the current care environment, which causes adaptive shortening of the muscles left in a shortened position and joint contracture; and (3) the chronic disuse of the paretic body part, which is typically self-imposed in most patients. Chronic disuse causes plastic rearrangements in the higher centers that further reduce the ability to voluntarily recruit motor units, i.e., that aggravate baseline paresis. Part I of this review focuses on the pathophysiology of the first two factors causing motor impairment in spastic paresis: the vicious cycle of paresis-disuse-paresis and the contracture in soft tissues.

The calcineurin activity and MCIP1.4 mRNA levels are increased by innervation in regenerating soleus muscle

The level of active subunit of calcineurin and the calcineurin (Cn) enzyme activity are increased in innervated but not in denervated slow type regenerating skeletal soleus muscle. These nerve-dependent increases were not accompanied by similar increases in the mRNA levels. The changes in the mRNA level of the modulatory calcineurin interacting protein, MCIP1.4, reflected the calcineurin activity and did not increase in denervated regenerating muscles compared to the innervated regenerating controls. The increases in Cn activity and in MCIP1.4 mRNA levels occurred before the switch from fast to slow-type myosin heavy chain isoforms, a phenomenon similarly known to be dependent on innervation. This highlights the role of mediators, acting between the nerve and calcineurin, in the formation of slow fiber identity.

Expression of SERCA2a is independent of innervation in regenerating soleus muscle

The speed of contraction of a skeletal muscle largely depends on the myosin heavy chain isoforms (MyHC), whereas the relaxation is initiated and maintained by the sarcoplasmic reticulum Ca2+-ATPases (SERCA). The expression of the slow muscle-type myosin heavy chain I (MyHCI) is entirely dependent on innervation, but, as we show here, innervation is not required for the expression of the slow-type sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) in regenerating soleus muscles of the rat, although it can play a modulator role. Remarkably, the SERCA2a level is even higher in denervated than in innervated regenerating soleus muscles on day 7 when innervation is expected to resume. Later, the level of SERCA2a protein declines in denervated regenerated muscles but it remains expressed, whereas the corresponding mRNA level is still increasing. SERCA1 (i.e., the fast muscle-type isoform) expression shows only minor changes in denervated regenerating soleus muscles compared with innervated regenerating controls. When the soleus nerve was transected instead of the sciatic nerve, SERCA2a and MyHCI expressions were found to be even more uncoupled because the MyHCI nearly completely disappeared, whereas the SERCA2a mRNA and protein levels decreased much less. The transfection of regenerating muscles with constitutively active mutants of the Ras oncogene, known to mimic the effect of innervation on the expression of MyHCI, did not affect SERCA2a expression. These results demonstrate that the regulation of SERCA2a expression is clearly distinct from that of the slow myosin in the regenerating soleus muscle and that SERCA2a expression is modulated by neuronal activity but is not entirely dependent on it.

Adaptations in skeletal muscle disuse or decreased-use atrophy

Those factors that seem to play some role in inducing adaptations of skeletal muscle in vivo are discussed. The role of myogenesis in maintaining and repairing muscle during atrophic and hypertrophic states is discussed, including pointing out that the modulation of myonuclear number is one means of adapting to varying chronic levels of neuromuscular activity. Finally, we point out the potential consequences of muscle atrophy on the control of movement and the susceptibility to fatigue.

Influences of electromechanical events in defining skeletal muscle properties

Inactivity of the cat soleus muscle was induced via spinal cord isolation (SI), and the cats were maintained for 4 months. The soleus was electrically stimulated while lengthening (SI-L) or shortening (SI-S) during a simulated step cycle or during isometric (SI-I) contractions. For the SI, SI-S, SI-L, and SI-I groups, the soleus weights were 33, 55, 55, and 64% of the control, respectively, and the maximum tetanic tensions were 15, 30, 36, and 44% of the control, respectively. The specific tension was lower in all SI groups than in the control. Absolute forces at stimulation frequencies of 5-200 Hz were smaller in all SI groups than in the control. The SI-I group tended to have higher values for all force-related parameters than the other SI groups. Fatigue resistance was similar among all groups. The isometric twitch time-to-peak tension was shorter, and the frequency of the stimulation-tension response was shifted to the right in all SI groups with respect to the control. Maximum shortening velocities were 70, 59, and 73% faster for the SI, SI-S, and SI-L groups and similar to the control for the SI-I group. Inactivity resulted in an increased percentage of faster myosin heavy chains (MHCs) that was blunted in the SI-I and SI-L groups but not in the SI-S group. Pure type I MHC fibers atrophied by 80, 59, 58, and 47% in the SI, SI-S, SI-L, and SI-I groups. The data from the SI group quantify the contribution of activity-independent factors in maintaining the mechanical and phenotypic properties of the cat soleus. Relative to a fast-fatigable muscle, these results suggest that only 25% of the slowness (type I MHC) and none of the resistance to fatigue of the soleus muscle are dependent on activity-related factors. Short, daily bouts of electromechanical activation ameliorated several of these adaptations, with the isometric contractions being the most effective countermeasure. The clinical implications of these findings for rehabilitation strategies are discussed.