Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. I. Muscle and motor unit properties

Analysis of Biomechanical and Biochemical Markers of Rat Muscle Soleus Fatigue Processes Development during Long-Term Use of C60 Fullerene and N-Acetylcysteine

The development of an effective therapy aimed at restoring muscle dysfunctions in clinical and sports medicine, as well as optimizing working activity in general remains an urgent task today. Modern nanobiotechnologies are able to solve many clinical and social health problems, in particular, they offer new therapeutic approaches using biocompatible and bioavailable nanostructures with specific bioactivity. Therefore, the nanosized carbon molecule, C₆₀ fullerene, as a powerful antioxidant, is very attractive. In this study, a comparative analysis of the dynamic of muscle soleus fatigue processes in rats was conducted using 50 Hz stimulation for 5 s with three consistent pools after intraperitoneal administration of the following antioxidants: C₆₀ fullerene (a daily dose of 1 mg/kg one hour prior to the start of the experiment) and N-acetylcysteine (NAC; a daily dose of 150 mg/kg one hour prior to the start of the experiment) during five days. Changes in the integrated power of muscle contraction, levels of the maximum and minimum contraction force generation, time of reduction of the contraction force by 50% of its maximum value, achievement of the maximum force response, and delay of the beginning of a single contraction force response were analyzed as biomechanical markers of fatigue processes. Levels of creatinine, creatine phosphokinase, lactate, and lactate dehydrogenase, as well as pro- and antioxidant balance (thiobarbituric acid reactive substances, hydrogen peroxide, reduced glutathione, and catalase activity) in the blood of rats were analyzed as biochemical markers of fatigue processes. The obtained data indicate that applied therapeutic drugs have the most significant effects on the 2nd and especially the 3rd stimulation pools. Thus, the application of C₆₀ fullerene has a (50–80)% stronger effect on the resumption of muscle biomechanics after the beginning of fatigue than NAC on the first day of the experiment. There is a clear trend toward a positive change in all studied biochemical parameters by about (12–15)% after therapeutic administration of NAC and by (20–25)% after using C₆₀ fullerene throughout the experiment. These findings demonstrate the promise of using C₆₀ fullerenes as potential therapeutic nanoagents that can reduce or adjust the pathological conditions of the muscular system that occur during fatigue processes in skeletal muscles.

Peripheral Nerve Regeneration and Muscle Reinnervation

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

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.

Adaptation of motor unit contractile properties in rat medial gastrocnemius to treadmill endurance training: Relationship to muscle mitochondrial biogenesis

This study aimed at investigating the effects of 2, 4 and 8 weeks of endurance training on the contractile properties of slow (S), fast fatigue resistant (FR) and fast fatigable (FF) motor units (MUs) in rat medial gastrocnemius (MG) in relation to the changes in muscle mitochondrial biogenesis. The properties of functionally isolated MUs were examined in vivo. Mitochondrial biogenesis was judged based on the changes in mitochondrial DNA copy number (mtDNA), the content of the electron transport chain (ETC) proteins and PGC-1α in the MG. Moreover, the markers of mitochondria remodeling mitofusins (Mfn1, Mfn2) and dynamin-like protein (Opa1) were studied using qPCR. A proportion of FR MUs increased from 37.9% to 50.8% and a proportion of FF units decreased from 44.7% to 26.6% after 8 weeks of training. The increased fatigue resistance, shortened twitch duration, and increased ability to potentiate force were found as early as after 2 weeks of endurance training, predominantly in FR MUs. Moreover, just after 2 weeks of the training an enhancement of the mitochondrial network remodeling was present as judged by an increase in expression of Mfn1, Opa1 and an increase in PGC-1α in the slow part of MG. Interestingly, no signs of intensification of mitochondrial biogenesis assessed by ETC proteins content and mtDNA in slow and fast parts of gastrocnemius were found at this stage of the training. Nevertheless, after 8 weeks of training an increase in the ETC protein content was observed, but mainly in the slow part of gastrocnemius. Concluding, the functional changes in MUs’ contractile properties leading to the enhancement of muscle performance accompanied by an activation of signalling that controls the muscle mitochondrial network reorganisation and mitochondrial biogenesis belong to an early muscle adaptive responses that precede an increase in mitochondrial ETC protein content.

Rapid activity-dependent modulation of the intrinsic excitability through up-regulation of KCNQ/Kv7 channel function in neonatal spinal motoneurons

Activity-dependent changes in the properties of the motor system underlie the necessary adjustments in its responsiveness on the basis of the environmental and developmental demands of the organism. Although plastic changes in the properties of the spinal cord have historically been neglected because of the archaic belief that the spinal cord is constituted by a hardwired network that simply relays information to muscles, plenty of evidence has been accumulated showing that synapses impinging on spinal motoneurons undergo short- and long-term plasticity. In the brain, brief changes in the activity level of the network have been shown to be paralleled by changes in the intrinsic excitability of the neurons and are suggested to either reinforce or stabilize the changes at the synaptic level. However, rapid activity-dependent changes in the intrinsic properties of spinal motoneurons have never been reported. In this study, we show that in neonatal mice the intrinsic excitability of spinal motoneurons is depressed after relatively brief but sustained changes in the spinal cord network activity. Using electrophysiological techniques together with specific pharmacological blockers of KCNQ/Kv7 channels, we demonstrate their involvement in the reduction of the intrinsic excitability of spinal motoneurons. This action results from an increased M-current, the product of the activation of KCNQ/Kv7 channels, which leads to a hyperpolarization of the resting membrane potential and a decrease in the input resistance of spinal motoneurons. Computer simulations showed that specific up-regulations in KCNQ/Kv7 channels functions lead to a modulation of the intrinsic excitability of spinal motoneurons as observed experimentally. These results indicate that KCNQ/Kv7 channels play a fundamental role in the activity-dependent modulation of the excitability of spinal motoneurons.

Electrical Stimulation of Low-Threshold Proprioceptive Fibers in the Adult Rat Increases Density of Glutamatergic and Cholinergic Terminals on Ankle Extensor α-Motoneurons

The effects of stimulation of low-threshold proprioceptive afferents in the tibial nerve on two types of excitatory inputs to α-motoneurons were tested. The first input is formed by glutamatergic Ia sensory afferents contacting monosynaptically α-motoneurons. The second one is the cholinergic input originating from V0_c—interneurons, located in lamina X of the spinal cord, modulating activity of α-motoneurons via C-terminals. Our aim was to clarify whether enhancement of signaling to ankle extensor α-motoneurons, via direct electrical stimulation addressed predominantly to low-threshold proprioceptive fibers in the tibial nerve of awake rats, will affect Ia glutamatergic and cholinergic innervation of α-motoneurons of lateral gastrocnemius (LG). LG motoneurons were identified with True Blue tracer injected intramuscularly. Tibial nerve was stimulated for 7 days with continuous bursts of three pulses applied in four 20 min sessions daily. The Hoffmann reflex and motor responses recorded from the soleus muscle, LG synergist, allowed controlling stimulation. Ia terminals and C-terminals abutting on LG-labeled α-motoneurons were detected by immunofluorescence (IF) using input-specific anti- VGLUT1 and anti-VAChT antibodies, respectively. Quantitative analysis of confocal images revealed that the number of VGLUT1 IF and VAChT IF terminals contacting the soma of LG α-motoneurons increased after stimulation by 35% and by 26%, respectively, comparing to the sham-stimulated side. The aggregate volume of VGLUT1 IF and VAChT IF terminals increased by 35% and by 30%, respectively. Labeling intensity of boutons was also increased, suggesting an increase of signaling to LG α-motoneurons after stimulation. To conclude, one week of continuous burst stimulation of proprioceptive input to LG α-motoneurons is effective in enrichment of their direct glutamatergic but also indirect cholinergic inputs. The effectiveness of such and longer stimulation in models of injury is a prerequisite to propose it as a therapeutic method to improve inputs to selected group of α-motoneurons after damage.

IH activity is increased in populations of slow versus fast motor axons of the rat

Much is known about the electrophysiological variation in motoneuron somata across different motor units. However, comparatively less is known about electrophysiological variation in motor axons and how this could impact function or electrodiagnosis in healthy or diseased states. We performed nerve excitability testing on two groups of motor axons in Sprague–Dawley rats that are known to differ significantly in their chronic daily activity patterns and in the relative proportion of motor unit types: one group innervating the soleus (“slow motor axons”) and the other group innervating the tibialis anterior (“fast motor axons”) muscles. We found that slow motor axons have significantly larger accommodation compared to fast motor axons upon application of a 100 ms hyperpolarizing conditioning stimulus that is 40% of axon threshold (Z = 3.24, p = 0.001) or 20% of axon threshold (Z = 2.67, p = 0.008). Slow motor axons had larger accommodation to hyperpolarizing currents in the current-threshold measurement (-80% Z = 3.07, p = 0.002; -90% Z = 2.98, p = 0.003). In addition, we found that slow motor axons have a significantly smaller rheobase than fast motor axons (Z = -1.99, p = 0.047) accompanied by a lower threshold in stimulus-response curves. The results provide evidence that slow motor axons have greater activity of the hyperpolarization-activated inwardly rectifying cation conductance (I_H) than fast motor axons. It is possible that this difference between fast and slow axons is caused by an adaptation to their chronic differences in daily activity patterns, and that this adaptation might have a functional effect on the motor unit. Moreover, these findings indicate that slow and fast motor axons may react differently to pathological conditions.

Human spinal cord injury: motor unit properties and behaviour

Spinal cord injury (SCI) results in widespread variation in muscle function. Review of motor unit data shows that changes in the amount and balance of excitatory and inhibitory inputs after SCI alter management of motoneurons. Not only are units recruited up to higher than usual relative forces when SCI leaves few units under voluntary control, the force contribution from recruitment increases due to elevation of twitch/tetanic force ratios. Force gradation and precision are also coarser with reduced unit numbers. Maximal unit firing rates are low in hand muscles, limiting voluntary strength, but are low, normal or high in limb muscles. Unit firing rates during spasms can exceed voluntary rates, emphasizing that deficits in descending drive limit force production. SCI also changes muscle properties. Motor unit weakness and fatigability seem universal across muscles and species, increasing the muscle weakness that arises from paralysis of units, motoneuron death and sensory impairment. Motor axon conduction velocity decreases after human SCI. Muscle contractile speed is also reduced, which lowers the stimulation frequencies needed to grade force when paralysed muscles are activated with patterned electrical stimulation. This slowing does not necessarily occur in hind limb muscles after cord transection in cats and rats. The nature, duration and level of SCI underlie some of these species differences, as do variations in muscle function, daily usage, tract control and fibre-type composition. Exploring this diversity is important to promote recovery of the hand, bowel, bladder and locomotor function most wanted by people with SCI.

High-versus low-frequency stimulation effects on fine motor control in chronic hemiplegia: a pilot study

BACKGROUND

The optimal parameters of neuromuscular electrical stimulation (NMES) for recovery of hand function after stroke are not known. This clinical pilot study examined whether higher or lower frequencies are more effective for improving fine motor control of the hand in a chronic poststroke population.

METHODS

A 1-month, 4 times per week, in-home regimen of either a high-frequency (40 Hz) or low-frequency (20 Hz) NMES program was applied to the hemiplegic thenar muscles of 16 persons with chronic stroke. Participants were identified a priori as having a low level of function (LF) or a high level of function (HF). Outcome measures of strength, dexterity, and endurance were measured before and after participation in the regimen.

RESULTS

LF subjects showed no significant changes with either the high- or the low-frequency NMES regimen. HF subjects showed significant changes in strength, dexterity, and endurance. Within this group, higher frequencies of stimulation yielded strength gains and increased motor activation; lower frequencies affected dexterity and endurance.

CONCLUSIONS

The results suggest that higher frequencies of stimulation could be more effective in improving strength and motor activation properties and that lower frequencies may affect coordination and endurance changes. Results also indicate that persons with a higher functional level of recovery may respond more favorably to NMES regimens, but further study with larger patient groups is warranted.

Firing patterns of spontaneously active motor units in spinal cord-injured subjects

Involuntary motor unit activity at low rates is common in hand muscles paralysed by spinal cord injury. Our aim was to describe these patterns of motor unit behaviour in relation to motoneurone and motor unit properties. Intramuscular electromyographic activity (EMG), surface EMG and force were recorded for 30 min from thenar muscles of nine men with chronic cervical SCI. Motor units fired for sustained periods (>10 min) at regular (coefficient of variation ≤ 0.15, CV, n =19 units) or irregular intervals (CV>0.15, n =14). Regularly firing units started and stopped firing independently suggesting that intrinsic motoneurone properties were important for recruitment and derecruitment. Recruitment (3.6 Hz, SD 1.2), maximal (10.2 Hz, SD 2.3, range: 7.5-15.4 Hz) and derecruitment frequencies were low (3.3 Hz, SD 1.6), as were firing rate increases after recruitment (~20 intervals in 3 s). Once active, firing often covaried, promoting the idea that units received common inputs.Half of the regularly firing units showed a very slow decline (>40 s) in discharge before derecruitment and had interspike intervals longer than their estimated after hyperpolarisation potential (AHP) duration (estimated by death rate and breakpoint analyses). The other units were derecruited more abruptly and had shorter estimated AHP durations. Overall, regularly firing units had longer estimated AHP durations and were weaker than irregularly firing units, suggesting they were lower threshold units. Sustained firing of units at regular rates may reflect activation of persistent inward currents, visible here in the absence of voluntary drive, whereas irregularly firing units may only respond to synaptic noise.

Effect of electromyostimulation on apoptosis-related factors in denervation and reinnervation of rat skeletal muscles

Electromyostimulation (EMS) has been utilized to reduce muscle atrophy, but its effect on denervated muscles is controversial. This study was performed to determine the effect of EMS on intramuscular changes and apoptosis during denervation and reinnervation following nerve damage. Rat sciatic nerves were denervated completely (CD) or partially (PD), and EMS was applied for 2 weeks. The same numbers of cases were followed without EMS. Nerve conduction studies, muscle weights, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay to measure apoptotic changes, and Western blot were done 4, 8, and 12 weeks after injury. TUNEL-positive nuclei of CD muscles (18.6 +/- 5.5%) were more prevalent than those of PD muscles (7.5 +/- 3.3%). The EMS group showed greater muscle weight, fewer positive nuclei (4.7 +/- 1.9%), and lower BAX and Bcl-2 expression levels compared with the non-EMS group at 4 weeks after PD but not after CD. Denervated muscle atrophy delayed by EMS may be linked with enhanced anti-apoptosis under the control of apoptosis-related factors.

Effects of baclofen on motor units paralysed by chronic cervical spinal cord injury

Baclofen, a gamma-aminobutyric acid receptor(B) agonist, is used to reduce symptoms of spasticity (hyperreflexia, increases in muscle tone, involuntary muscle activity), but the long-term effects of sustained baclofen use on skeletal muscle properties are unclear. The aim of our study was to evaluate whether baclofen use and paralysis due to cervical spinal cord injury change the contractile properties of human thenar motor units more than paralysis alone. Evoked electromyographic activity and force were recorded in response to intraneural stimulation of single motor axons to thenar motor units. Data from three groups of motor units were compared: 23 paralysed units from spinal cord injured subjects who take baclofen and have done so for a median of 7 years, 25 paralysed units from spinal cord injured subjects who do not take baclofen (median: 10 years) and 45 units from uninjured control subjects. Paralysed motor unit properties were independent of injury duration and level. With paralysis and baclofen, the median motor unit tetanic forces were significantly weaker, twitch half-relaxation times longer and half maximal forces reached at lower frequencies than for units from uninjured subjects. The median values for these same parameters after paralysis alone were comparable to control data. Axon conduction velocities differed across groups and were slowest for paralysed units from subjects who were not taking baclofen and fastest for units from the uninjured. Greater motor unit weakness with long-term baclofen use and paralysis will make the whole muscle weaker and more fatigable. Significantly more paralysed motor units need to be excited during patterned electrical stimulation to produce any given force over time. The short-term benefits of baclofen on spasticity (e.g. management of muscle spasms that may otherwise hinder movement or social interactions) therefore have to be considered in relation to its possible long-term effects on muscle rehabilitation. Restoring the strength and speed of paralysed muscles to pre-injury levels may require more extensive therapy when baclofen is used chronically.

Transplanted mouse embryonic stem-cell-derived motoneurons form functional motor units and reduce muscle atrophy

Prolonged muscle denervation resulting from motor neuron (MN) damage leads to atrophy and degeneration of neuromuscular junctions (NMJs), which can impart irreversible damage. In this study, we ask whether transplanted embryonic stem (ES) cells differentiated into MNs can form functional synapses with host muscle, and if so what effects do they have on the muscle. After transplantation into transected tibial nerves of adult mice, ES-cell-derived MNs formed functional synapses with denervated host muscle, which resulted in the ability to produce average tetanic forces of 44% of nonlesioned controls. ES-cell-derived motor units (MUs) had mean force values and ranges similar to control muscles. The number of type I fibers and fatigue resistance of the MUs were increased, and denervation-associated muscle atrophy was significantly reduced. These results demonstrate the capacity for ES-cell-derived MNs not only to incorporate into the adult host tissue, but also to exert changes in the target tissue. By providing the signals normally active during embryonic development and placing the cells in an environment with their target tissue, ES cells differentiate into MNs that give rise to functional MU output which resembles the MU output of endogenous MNs. This suggests that these signals combined with those present in the graft environment, lead to the activation of a program intended to produce a normal range of MU forces.

Muscle and bone plasticity after spinal cord injury: review of adaptations to disuse and to electrical muscle stimulation

The paralyzed musculoskeletal system retains a remarkable degree of plasticity after spinal cord injury (SCI). In response to reduced activity, muscle atrophies and shifts toward a fast-fatigable phenotype arising from numerous changes in histochemistry and metabolic enzymes. The loss of routine gravitational and muscular loads removes a critical stimulus for maintenance of bone mineral density (BMD), precipitating neurogenic osteoporosis in paralyzed limbs. The primary adaptations of bone to reduced use are demineralization of epiphyses and thinning of the diaphyseal cortical wall. Electrical stimulation of paralyzed muscle markedly reduces deleterious post-SCI adaptations. Recent studies demonstrate that physiological levels of electrically induced muscular loading hold promise for preventing post-SCI BMD decline. Rehabilitation specialists will be challenged to develop strategies to prevent or reverse musculoskeletal deterioration in anticipation of a future cure for SCI. Quantifying the precise dose of stress needed to efficiently induce a therapeutic effect on bone will be paramount to the advancement of rehabilitation strategies.

Deficiency of the E3 ubiquitin ligase TRIM32 in mice leads to a myopathy with a neurogenic component

Limb-girdle muscular dystrophy type 2H (LGMD2H) and sarcotubular myopathy are hereditary skeletal muscle disorders caused by mutations in TRIM32. We previously identified TRIM32 as an E3 ubiquitin ligase that binds to myosin and ubiquitinates actin. To date four TRIM32 mutations have been linked to LGMD2H, all of which occur in the C-terminal NHL domains. Unexpectedly, a fifth mutation in the B-box of TRIM32 causes a completely different, multisystemic disorder, Bardet-Biedl syndrome type 11. It is not understood how allelic mutations in TRIM32 can create such diverse phenotypic outcomes. To generate a tool for elucidating the complex in vivo functions of TRIM32, we created the first murine Trim32 knock-out model (T32KO). Histological analysis of T32KO skeletal muscles revealed mild myopathic changes. Electron microscopy showed areas with Z-line streaming and a dilated sarcotubular system with vacuoles -- the latter being a prominent feature of sarcotubular myopathy. Therefore, our model replicates phenotypes of LGMD2H and sarcotubular myopathy. The level of Trim32 expression in normal mouse brain exceeds that observed in skeletal muscle by more than 100 times, as we demonstrated by real-time PCR. Intriguingly, analysis of T32KO neural tissue revealed a decreased concentration of neurofilaments and a reduction in myelinated motoraxon diameters. The axonal changes suggest a shift toward a slower motor unit type. Not surprisingly, T32KO soleus muscle expressed an elevated type I slow myosin isotype with a concomitant reduction in the type II fast myosin. These data suggest that muscular dystrophy due to TRIM32 mutations involves both neurogenic and myogenic characteristics.

The effects of frequency-dependent dynamic muscle stimulation on inhibition of trabecular bone loss in a disuse model

Clinical electrical muscle stimulation has been shown to alleviate muscle atrophy resulting from functional disuse, yet little is known about its effect on the skeleton. The objective of this study is to evaluate the potential of dynamic muscle stimulation on disused trabecular bone, and to investigate the importance of optimized stimulation frequency in the loading regimen. Fifty-six skeletally mature Sprague-Dawley rats were divided into seven groups for the 4-week experiment: baseline control, age-matched control, hindlimb suspended (HLS), and HLS with muscle stimulation at 1 Hz, 20 Hz, 50 Hz, and 100 Hz. Muscle stimulation was carried out for 10 min per day for 5 days per week, total of 4 weeks. The metaphyseal and epiphyseal trabecular regions of the distal femurs were analyzed with microcomputed tomography and histomorphometry methods. HLS alone for 4-week resulted in a significant amount of trabecular bone loss and structural deterioration. Muscle contraction at 1 Hz was not sufficient to inhibit trabecular bone loss and resulted in similar amount of loss to that of HLS alone. Bone quantity and structure were significantly improved by applying muscle stimulation at mid-frequency (20 Hz and 50 Hz). Dynamic stimulation at 50 Hz demonstrated the greatest preventive effect on the skeleton against functional disused alone animals (up to +147% in bone volume fraction, +38% in trabecular number and -36% in trabecular separation). Histomorphometric analysis showed that the stimulation, regardless of its frequency, did not have an effect on the bone formation indices, such as mineral apposition rate and bone formation rate. Overall, the data demonstrated the potentials of frequency-dependent dynamic muscle contraction in regulating skeletal adaptive responses under disuse conditions. Dynamic muscle stimulation, with a specific regimen, may be beneficial to future orthopedic research in developing a countermeasure for disuse osteopenia and osteoporosis.

Preferential motor unit loss in the SOD1 G93A transgenic mouse model of amyotrophic lateral sclerosis

The present study investigated motor unit (MU) loss in a murine model of familial amyotrophic lateral sclerosis (ALS). The fast-twitch tibialis anterior (TA) and medial gastrocnemius (MG) muscles of transgenic SOD1(G93A) and SOD1(WT) mice were studied during the presymptomatic phase of disease progression at 60 days of age. Whole muscle maximum isometric twitch and tetanic forces were 80% lower (P < 0.01) in the TA muscles of SOD1(G93A) compared to SOD1(WT) mice. Enumeration of total MU numbers within TA muscles showed a 60% reduction (P < 0.01) within SOD1(G93A) mice (38 +/- 7) compared with SOD1(WT) controls (95 +/- 12); this was attributed to a lower proportion of the most forceful fast-fatigable (FF) MU in SOD1(G93A) mice, as seen by a significant (P < 0.01) leftward shift in the cumulative frequency histogram of single MU forces. Similar patterns of MU loss and corresponding decreases in isometric twitch force were observed in the MG. Immunocytochemical analyses of the entire cross-sectional area (CSA) of serial sections of TA muscles stained with anti-neural cell adhesion molecule (NCAM) and various monoclonal antibodies for myosin heavy chain (MHC) isoforms showed respective 65% (P < 0.01) and 28% (P < 0.05) decreases in the number of innervated IIB and IID/X muscle fibres in SOD1(G93A), which paralleled the 60% decrease (P < 0.01) in the force generating capacity of individual fibres. The loss of fast MUs was partially compensated by activity-dependent fast-to-slower fibre type transitions, as determined by increases (P < 0.04) in the CSA and proportion of IIA fibres (from 4% to 14%) and IID/X fibres (from 31% to 39%), and decreases (P < 0.001) in the CSA and proportion of type IIB fibres (from 65% to 44%). We conclude that preferential loss of IIB fibres is incomplete at 60 days of age, and is consistent with a selective albeit gradual loss of FF MUs that is not fully compensated by sprouting of the remaining motoneurons that innervate type IIA or IID/X muscle fibres. Our findings indicate that disease progression in fast-twitch muscles of SOD1(G93A) mice involves parallel processes: (1) gradual selective motor axon die-back of the FF motor units that contain large type IIB muscle fibres, and of fatigue-intermediate motor units that innervate type IID/X muscle fibres, and (2) activity-dependent conversion of motor units to those innervated by smaller motor axons innervating type IIA fatigue-resistant muscle fibres.

Variability of the twitch parameters of the rat medial gastrocnemius motor units—experimental and modeling study

In the present study a previously proposed model of a twitch based on an analytical function with four-parameters (lead, contraction and half-relaxation times and maximum force of the twitch) was validated on 115 motor units (MUs), divided into slow (S), fast-fatigue resistant (FR) and fast fatigable (FF) types. The original records were collected from electrophysiological experiments performed on MUs from the medial gastrocnemius muscle of five rats. Besides the easy calculation of the twitch parameters and their variability, the usefulness of the model was confirmed by eliminating artifacts and noise in the original twitch records, as well as by calculations of the velocity of force increase and decrease, the area under force records, and by normalization of all twitches with respect to the maximal force and contraction time. It was concluded that: (1) the four-parameter twitch model describes precisely the individual contractions of various MUs; (2) all physiological twitch parameters are distributed continuously and located within overlapping intervals for different MU types; this distribution is not linear, but exponential; (3) S MUs can be distinguished from fast ones on the basis of some twitch parameters (contraction and half-relaxation times, velocity of contraction), but the same cannot be applied for FF and FR MUs; (4) the analysis of the normalized twitches reveals the differences in shapes for different types of MUs, which shows that twitches of different MUs cannot be obtained from one standard pattern scaled in time and force. These results may have functional implications for studying effectiveness of twitch summation during tetanic contractions and the work performed by various types of MUs.

Eletroestimulação muscular: alternativa de tratamento coadjuvante para pacientes com doença arterial obstrutiva periférica

A doença arterial periférica faz parte de um grupo de patologias vasculares que evolui de forma lenta e progressiva. A proposta deste artigo foi avaliar, por meio de revisão bibliográfica, os possíveis benefícios da eletroestimulação crônica como tratamento coadjuvante para pacientes arteriopatas. De acordo com a literatura analisada, concluímos que a eletroestimulação é capaz de provocar alterações importantes no perfil metabólico das fibras musculares, convertendo-as do tipo II para o tipo I, o que induz o crescimento capilar, a densidade capilar e o suprimento de oxigênio. Desta forma, este recurso terapêutico aumenta a capacidade aeróbica oxidativa e a resistência à fadiga dos músculos isquêmicos. Assim, a eletroestimulação é mais um recurso terapêutico capaz de melhorar a habilidade para caminhar destes pacientes, diminuindo gastos com cirurgias de revascularização e complicações maiores.

Twitch and Tetanic Properties of Human Thenar Motor Units Paralyzed by Chronic Spinal Cord Injury

Little is known about how human motor units respond to chronic paralysis. Our aim was to record surface electromyographic (EMG) signals, twitch forces, and tetanic forces from paralyzed motor units in the thenar muscles of individuals ( n = 12) with chronic (1.5–19 yr) cervical spinal cord injury (SCI). Each motor unit was activated by intraneural stimulation of its motor axon using single pulses and trains of pulses at frequencies between 5 and 100 Hz. Paralyzed motor units ( n = 48) had small EMGs and weak tetanic forces ( n = 32 units) but strong twitch forces, resulting in half-maximal force being achieved at a median of only 8 Hz. The distributions for cumulative twitch and tetanic forces also separated less for paralyzed units than for control units, indicating that increases in stimulation frequency made a smaller relative contribution to the total force output in paralyzed muscles. Paralysis also induced slowing of conduction velocities, twitch contraction times and EMG durations. However, the elevated ratios between the twitch and the tetanic forces, but not contractile speed, correlated significantly with the extent to which unit force summated in response to different frequencies of stimulation. Despite changes in the absolute values of many electrical and mechanical properties of paralyzed motor units, most of the distributions shifted uniformly relative to those of thenar units obtained from control subjects. Thus human thenar muscles paralyzed by SCI retain a population of motor units with heterogeneous contractile properties because chronic paralysis influenced all of the motor units similarly.

Effects of long-term creatine feeding and running on isometric functional measures and myosin heavy chain content of rat skeletal muscles

The purpose of this study was to investigate whether creatine (Cr) supplementation during 12 weeks of phasic high-frequency voluntary wheel running would result in a faster myosin heavy chain (MHC) isoform profile in the rat mixed fast-twitch plantaris and alter its corresponding isometric contractile properties. The fast-twitch extensor digitorum longus and medial gastrocnemius and slow-twitch soleus were also studied. Forty weanling Sprague-Dawley male rats were assigned to one of four groups: creatine-sedentary (Cre-Sed); creatine-voluntary running (Cre-Run); control-sedentary (Con-Sed); control-voluntary running (Con-Run). Daily running distance was similar between Cre-Run and Con-Run. Average daily Cr ingestion was also similar being 2.4+/-0.17 and 3.0+/-0.14 g/kg in Cre-Sed and Cre-Run, respectively. Total creatine (TCr) content was elevated (P<0.03) in the plantaris of Cre-Run [211.4+/-16.9 mmol/kg dry weight (dw)], compared with Con-Run (175.1+/-5.69). In the plantaris, MHCIIb was 13% greater (P<0.00001) in Cre-Run compared with Con-Run, while MHCIId/x and MHCIIa were lower in Cre-Run by 7 and 6% (P<0.0002), respectively. No differences were observed in twitch force, time-to-peak tension, half-rise time or half-fall time. Greater tetanic force production (P<0.05) in Cre-Sed compared with Con-Sed corresponded to a 12% increase in MHCIId/x (P<0.0001) and a 12% decrease in MHCIIb (P<0.0006). The fatigue index of the plantaris at 10 s (FI(10s)) was reduced only after running (Cre-Run vs Con-Run), while in all other muscles the FI(10s) was lower only in the Cre-Sed group. In conclusion, Cr supplementation had differential effects on MHC isoform content and fatigability that depended on the level of contractile activity. Cr feeding combined with running exercise resulted in a faster MHC-based phenotype in the rat plantaris but the impact on associated isometric contractile properties was minimal.

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.

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

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

Reliability of isolated isometric function measures in rat muscles composed of different fibre types

The present study investigated the absolute reliability (R(Ab)) of isometric measures of time-to-peak tension (TTP), half-rise time ((1/2)RT), half-fall time ((1/2)FT), twitch force (TW(f)) tetanic force (TET(f)) and the sag ratio as applied to the slow soleus (SOL) and the fast-twitch extensor digitorum longus (EDL) and medial gastrocnemius (MG) muscles of the rat hindlimb. In addition, the relationship of each individual isometric measure was examined with regard to the pattern of myosin heavy chain (MHC) isoform expression. Measures of TTP, (1/2)RT, (1/2)FT and sag ratio were negatively correlated with MHCIId(x) and MHCIIb (P < 0.0001), and positively correlated with MHCI (P < 0.0001). TW(f) and TET(f) were negatively correlated with MHCI content (P < 0.0001) and positively with MHCIId(x) (P < 0.0001) and MHCIIb (P < 0.001). Comparisons of isometric measures using a paired Student's t test revealed that they were not different between the right and left legs; all measures displayed high correlations between the left and right legs (r= 0.71-0.85, P < 0.0001). In contrast to standard tests of statistical significance, these functional measures exhibited a considerable range of R(Ab) when individual muscles were studied in only one hindlimb. When averaged across all muscles, however, the (1/2)FT, (1/2)RT, TW(f) and TTP measures possessed high overall reliability; measures of TET(f) and sag ratio were moderately reliable. The results of this study show that the isometric measures studied possess significant predictive value with regard to MHC isoform content; the left and right legs are interchangeable but display a considerable range of reliability when only one hindlimb is studied.

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

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

Prevention of muscle disuse atrophy by low-frequency electrical stimulation in rats

When muscles lose neural drive, they atrophy rapidly. Neuromuscular electrical stimulation (NMS) has been used in attempts to prevent or reverse the atrophy, but optimal stimulation programs and parameters are not well defined. In this study, we investigated the effects of four different stimulation patterns on disuse atrophy produced in the tibialis anterior, lateral gastrocnemius, and soleus muscles of rats paralyzed with tetrodotoxin for seven days. Stimulation paradigms differed from one another by their stimulation frequency (2 or 10 pulses/s) and by their stimulation period (2 or 10 h a day). Results showed that stimulation with 2 pulses/s, paradigms were more effective at preventing disuse muscle atrophy than higher-frequency stimulation. The most marked difference was in the slow soleus muscle, which had only 10% mean atrophy when stimulated at 2 pulses/s for 10 h, compared to 26% atrophy when stimulated at 10 pulses/s for either 2 or 10 h and 32% atrophy in unstimulated, paralyzed controls. The level of atrophic change was not correlated with the levels of serum creatine kinase, used as an index of muscle damage. Results suggest that remediation of disuse atrophy may be accomplished using unphysiologically low rates of motor-unit activation despite the relatively low force produced by such unfused contractions. This may have significant implications for the design of therapies for muscle paralysis consequent to upper-motoneuron lesions.

Functional alterations of cat abducens neurons after peripheral tetanus neurotoxin injection

Tetanus neurotoxin (TeNT) cleaves synaptobrevin, a protein involved in synaptic vesicle docking and fusion, thereby preventing neurotransmitter release and causing a functional deafferentation. We injected TeNT into the lateral rectus muscle of adult cats at 0.5 or 5 ng/kg (low and high dose, respectively). In the periphery, TeNT slightly slowed motor axon conduction velocity, and at high doses, partially blocked neuromuscular transmission. TeNT peripheral actions displayed time courses different to the more profound and longer-lasting central actions. Central effects were first observed 2 days postinjection and reversed after 1 mo. The low dose induce depression of inhibitory inputs, whereas the high dose produce depression of both inhibitory and excitatory inputs. Simultaneous recordings of eye movement and neuronal firing revealed that low-dose injections specifically reduced inhibition of firing during off-directed saccadic movements, while high-dose injections of TeNT affected both inhibitory and excitatory driven firing patterns. Motoneurons and abducens interneurons were both affected in a similar way. These alterations resulted in modifications in all discharge characteristic analyzed such as background firing, threshold for recruitment, and firing sensitivities to both eye position and velocity during spontaneous movements or vestibulo-ocular reflexes. Removal of inhibition after low-dose injections also altered firing patterns, and although firing activity increased, it did not result in muscle tetanic contractions. Removal of inhibition and excitation by high-dose injections resulted in a decrease in firing modulation with eye movements. Our findings suggest that the distinct behavior of oculomotor and spinal motor output following TeNT intoxication could be explained by their different interneuronal and proprioceptive control.

The effect of chronic skeletal muscle stimulation on capillary growth in the rat: are sensory nerve fibres involved?

Indirect chronic electrical stimulation of skeletal muscle activates not only efferent but also afferent nerve fibres. To investigate effects specific to this on capillary growth, one of the earliest changes, cell proliferation and capillary ultrastructure were studied in ankle flexors of rats with and without deafferentation of the stimulated side. Two weeks after preganglionic section of dorsal roots L4-L6, the peroneal nerve was stimulated (10 Hz, 8 h day(-1)) for 2 or 7 days. Proliferating nuclei labelled by bromodeoxyuridine or proliferating cell nuclear antigen staining were colocalized to alkaline phosphatase-stained capillaries (Lc) or other interstitial nuclei (Li) in frozen sections of extensor digitorum longus. Capillary fine structure was examined in extensor hallucis proprius by transmission electron microscopy. The stimulation-induced increase in capillary and interstitial proliferation (Lc 9.9 +/- 1.9 %, Li 8.8 +/- 2.1 % vs. Lc 2.6 +/- 0.4 %, Li 1.9 +/- 0.3 % in controls, P < 0.05) was depressed at 2 days by dorsal root section (Lc 4.8 +/- 0.7 %, Li 3.2 +/- 0.9 %, P < 0.05), an effect likely to be mainly on fibroblasts; no depression was seen at 7 days. Dorsal root section reduced stimulation-induced capillary endothelial swelling at both time points. In contralateral muscles of intact rats, stimulation increased interstitial cell proliferation and capillary swelling, both effects being eliminated by dorsal root section. Capillary growth induced by stimulation (24 % increase in capillary : fibre ratio at 7 days) was unaffected by deafferentation. The reduction in capillary ultrastructural changes and interstitial proliferation in both stimulated and contralateral muscles implies that stimulation of afferent fibres leads directly to release of humoral factors and/or activation via dorsal roots of fibres that release humoral substances. Contralateral muscles are an inadequate control for the effects of chronic stimulation in the intact animal.

Endurance training alters the biophysical properties of hindlimb motoneurons in rats

The purpose of the study was to determine the effect of daily endurance treadmill training (2 h/day, 30 m/min) on motoneuron biophysical properties. Electrophysiological properties of tibial motoneurons were measured in situ in anesthetized (ketamine/xylazine) control and trained rats using sharp glass microelectrodes. Motoneurons from trained rats had significantly hyperpolarized resting membrane potentials and spike trigger levels, and faster antidromic spike rise-times. "Fast" motoneurons (after-hyperpolarization half-decay time <20 ms) in trained rats also had a significantly larger mean cell capacitance than those in control rats, suggesting that they were larger, although this had no effect on indices of excitability (rheobase, cell input resistance). Motoneurons are thus targets for activity-induced adaptations, which may have clinical significance for the role of physical activity as a therapeutic modality in cases of neurological deficit. The specific adaptations noted, which reflect alterations in ionic conductances, may serve to offset decreases in membrane excitability that occur during sustained excitation.

Hierarchical genetic algorithm versus static optimization-investigation of elbow flexion and extension movements

The applicability of static optimization (and, respectively, frequently used objective functions) for prediction of individual muscle forces for dynamic conditions has often been discussed. Some of the problems are whether time-independent objective functions are suitable, and how to incorporate muscle physiology in models. The present paper deals with a twofold task: (1) implementation of hierarchical genetic algorithm (HGA) based on the properties of the motor units (MUs) twitches, and using multi-objective, time-dependent optimization functions; and (2) comparison of the results of the HGA application with those obtained through static optimization with a criterion "minimum of a weighted sum of the muscle forces raised to the power of n". HGA and its software implementation are presented. The moments of neural stimulation of all MUs are design variables coding the problem in the terms of HGA. The main idea is in using genetic operations to find these moments, so that the sum of MUs twitches satisfies the imposed goals (required joint moments, minimal sum of muscle forces, etc.). Elbow flexion and extension movements with different velocities are considered as proper illustration. It is supposed that they are performed by two extensor muscles and three flexor muscles. The results show that HGA is a suitable means for precise investigation of motor control. Many experimentally observed phenomena (such as antagonistic co-contraction, three-phasic behavior of the muscles during fast movements) can find their explanation by the properties of the MUs twitches. Static optimization is also able to predict three-phasic behavior and could be used as practicable and computationally inexpensive method for total estimation of the muscle forces.

Effects of daily spontaneous running on the electrophysiological properties of hindlimb motoneurones in rats

No evidence currently exists that motoneurone adaptations in electrophysiological properties can result from changes in the chronic level of neuromuscular activity. We examined, in anaesthetized (ketamine/xylazine) rats, the properties of motoneurones with axons in the tibial nerve, from rats performing daily spontaneous running exercise for 12 weeks in exercise wheels ('runners') and from rats confined to plastic cages ('controls'). Motoneurones innervating the hindlimb via the tibial nerve were impaled with sharp glass microelectrodes, and the properties of resting membrane potential, spike threshold, rheobase, input resistance, and the amplitude and time-course of the afterhyperpolarization (AHP) were measured. AHP half-decay time was used to separate motoneurones into 'fast' (AHP half-decay time < 20 ms) and 'slow' (AHP half-decay time >/= 20 ms), the proportions of which were not significantly different between controls (58 % fast) and runners (65 % fast). Two-way ANOVA and ANCOVA revealed differences between motoneurones of runners and controls which were confined to the 'slow' motoneurones. Specifically, runners had slow motoneurones with more negative resting membrane potentials and spike thresholds, larger rheobasic spike amplitudes, and larger amplitude AHPs compared to slow motoneurones of controls. These adaptations were not evident in comparing fast motoneurones from runners and controls. This is the first demonstration that physiological modifications in neuromuscular activity can influence basic motoneurone biophysical properties. The results suggest that adaptations occur in the density, localization, and/or modulation of ionic membrane channels that control these properties. These changes might help offset the depolarization of spike threshold that occurs during rhythmic firing.

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

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

Size of myelinated nerve fibres is not increased by expansion of the peripheral field in cats

This study tests the hypothesis that target size regulates the size of myelinated sensory and motor fibres in peripheral nerves. Cat medial gastrocnemius (MG) muscles were partially denervated and the size of the remaining nerve fibres that sprouted was examined 6.4 +/- 0.9 months later to determine whether nerve fibre size increased with target size. Electrophysiological and morphometric analyses were used to quantify myelinated nerve fibre size. Charge measurements from dorsal and ventral roots were used to electrophysiologically quantify the relative number of cut nerve fibres and the average size of the remaining intact sensory and motor nerve fibres. Medial gastrocnemius muscle and motor unit forces provided indirect measurements of the increase in target size. Conduction velocities and amplitude of unitary action potentials of motor nerve fibres innervating single motor units were also measured after partial denervation. Electrophysiological measurements of nerve fibre size and morphometric measurements of outer fibre perimeters and fibre areas concurred and demonstrated that myelinated nerve fibres supplying partially denervated MG muscles did not increase in size in parallel with the increase in the target size. Thus, unlike non-myelinated nerve fibres, the size of myelinated nerve fibres does not increase as target size increases. Retrograde control of size in non-myelinated but not in myelinated nerve fibres demonstrates differences in plasticity of neurons in the somatic and autonomic nervous systems.

Stability of the input-output properties of chronically implanted multiple contact nerve cuff stimulating electrodes

The objective of this investigation was to measure the input-output (I-O) properties of chronically implanted nerve cuff electrodes. Silicone rubber spiral nerve cuff electrodes, containing 12 individual platinum electrode contacts, were implanted on the sciatic nerve of seven adult cats for 28-34 weeks. Measurements of the torque generated at the ankle joint by electrical stimulation of the sciatic nerve were made every 1-2 weeks for the first 6 weeks post-implant and every 3-5 weeks between 6 weeks and 32 weeks post-implant. In three implants the percutaneous lead cable was irreparably damaged by the animal within 4 weeks after implant and further testing was not possible. One additional lead cable was irreparably damaged by the animal at 17 weeks post-implant. The three remaining implants functioned for 28, 31, and 32 weeks. Input-output curves of ankle joint torque as a function of stimulus current amplitude were repeatable within an experimental session, but there were changes in I-O curves between sessions. The degree of variability in I-O properties differed between implants and between different contacts within the same implant. After 8 weeks, the session to session changes in the stimulus amplitude required to generate 50% of the maximum torque (I50) were smaller (15+/-19%, mean +/- s.d.) than the changes in I50 measured between 1 week and 8 weeks post-implant (34+/-42%). Furthermore, the I-O properties were more stable across changes in limb position in the late post-implant period than in acutely implanted cuff electrodes. These results suggest that tissue encapsulation acted to stabilize chronically implanted cuff electrodes. Electrode movement relative to the nerve, de- and regeneration of nerve fibers, and the inability to precisely reproduce limb position in the measurement apparatus all may have contributed to the variability in I-O properties.

Selective fasciculation and divergent pathfinding decisions of embryonic chick motor axons projecting to fast and slow muscle regions

Proper motor function requires the precise matching of motoneuron and muscle fiber properties. The lack of distinguishing markers for early motoneurons has made it difficult to determine whether this matching is established by selective innervation during development or later via motoneuron-muscle fiber interactions. To examine whether chick motoneurons selectively innervate regions of their target containing either fast or slow muscle fibers, we backlabeled neurons from each of these regions with lipophilic dyes. We found that motor axons projecting to fast and slow muscle regions sorted into separate but adjacent fascicles proximally in the limb, long before they reached the muscle. More distally, these fascicles made divergent pathfinding decisions to course directly to the appropriate muscle fiber region. In contrast, axons projecting to different areas of an all-fast muscle did not fasciculate separately and became more intermingled as they coursed through the limb. Selective fasciculation of fast- and slow-projecting motoneurons was similar both before and after motoneuron cell death, suggesting that motoneurons specifically recognized and fasciculated with axons growing to muscle regions containing the appropriate muscle fiber type. Taken together, these results strongly support the hypothesis that "fast" and "slow" motoneurons are molecularly distinct before target innervation and that they use these differences to selectively fasciculate, pathfind to, and branch within the correct muscle fiber region from the outset of neuromuscular development.

Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. II. Motoneuron properties

Chronic stimulation (for 2-3 mo) of the medial gastrocnemius (MG) muscle nerve by indwelling electrodes renders the normally heterogeneous MG muscle mechanically and histochemically slow (type SO). We tested the hypothesis that motoneurons of MG muscle thus made type SO by chronic stimulation would also convert to slow phenotype. Properties of all single muscle units became homogeneously type SO (slowly contracting, nonfatiguing, nonsagging contraction during tetanic activation). Motoneuron electrical properties were also modified in the direction of type S, fatigue-resistant motor units. Two separate populations were identified (on the basis of afterhyperpolarization, rheobase, and input resistance) that likely correspond to motoneurons that had been fast (type F) or type S before stimulation. Type F motoneurons, although modified by chronic stimulation, were not converted to the type S phenotype, despite apparent complete conversion of their muscle units to the slow oxidative type (type SO). Muscle units of the former type F motor units were faster and/or more powerful than those of the former type S motor units, indicating some intrinsic regulation of motor unit properties. Experiments in which chronic stimulation was applied to the MG nerve cross-regenerated into skin yielded changes in motoneuron properties similar to those above, suggesting that muscle was not essential for the effects observed. Modulation of group Ia excitatory postsynaptic potential (EPSP) amplitude during high-frequency trains, which in normal MG motoneurons can be either positive or negative, was negative in 48 of 49 chronically stimulated motoneurons. Negative modulation is characteristic of EPSPs in motoneurons of most fatigue-resistant motor units. The general hypothesis of a periphery-to-motoneuron retrograde mechanism was supported, although the degree of control exerted by the periphery may vary: natural type SO muscle appears especially competent to modify motoneuron properties. We speculate that activity-dependent regulation of the neurotrophin-(NT) 4/5 in muscle plays an important role in controlling muscle and motoneuron properties.