Stimulation Of Motor Units Research Articles

Functional Isolation of Single Motor Units of Rat Medial Gastrocnemius Muscle.

This work outlines functional isolation of motor units (MUs), a standard electrophysiological method for determining characteristics of motor units in hindlimb muscles (such as the medial gastrocnemius, soleus, or plantaris muscle) in experimental rats. A crucial element of the method is the application of electrical stimuli delivered to a motor axon isolated from the ventral root. The stimuli may be delivered at constant or variable inter-pulse intervals. This method is suitable for experiments on animals at varying stages of maturity (young, adult or old). Moreover, this protocol can be used in experiments studying variability and plasticity of motor units evoked by a large spectrum of interventions. The results of these experiments may both augment basic knowledge in muscle physiology and be translated into practical applications. This procedure focuses on the surgical preparation for the recording and stimulation of MUs, with an emphasis on the necessary steps to achieve preparation stability and reproducibility of results.

Counterpoint: Spectral properties of the surface emg do not provide information about motor unit recruitment and muscle fiber type

The surface electromyogram (EMG) comprises the sum of the electrical contributions made by the active motor units to the interference signal detected by electrodes placed on the skin overlying the muscle. Because it provides a global measure of motor unit activity, this signal is a valuable tool for

Acute INDUCTION OF Transcription Factors in Fast-Twitch Muscle DURING CHRONIC SCIATIC NERVE STIMULATION

The molecular signals whereby contractile activity induces physiologic and biochemical adaptations in muscles are not fully understood. PURPOSE: To identify transcription factors affected in fast twitch muscle (extensor digitorum longus; EDL) subjected to extrinsic slow motor unit stimulation. METHODS: Adult male Swiss-Webster mice (n=4) were anesthetized and the sciatic nerve of one leg exposed and stimulated electrically at 10 Hz for up to 4 hours through implanted Pt electrodes. The non-stimulated leg served as a paired control. At the end of each experiment, both muscles were flash-frozen for RNA extraction and microarray analysis (GE: CodeLink Mouse 10k Uniset). Paired comparisons corrected for multiple testing were employed to detect differentially expressed genes which were classified into functional groups using NIAID:DAVID. RESULTS: A 50% decrease in peak force and tension time integral occurred within 10 min of stimulation onset and persisted for the entire time course, however time to peak force and half relaxation times remained constant. Stimulation increased gene expression in 78% of the differentially expressed genes detected in the stimulus vs. contralateral control EDL. Significant changes in transcript levels associated with fast to slow phenotype transformation were present including increased troponin C-slow, laminin-beta 3, and tropomyosin 3-γ and decreased myosin heavy chain 2A fast, creatine kinase, myosin light chain-fast, and troponin I fast 2. Transcripts associated with induction and regulation of transcription were statistically overrepresented (vizXlabs: GeneSifter) above other functional classes with 65 of 70 (625 assayed) increased in the stimulated EDL. Specifically, PPARGC1-α (8.4-fold), PPARGC1-β (2.8-fold), PPAR-γ (2.5-fold) and HIF1-α (3.5-fold) were significantly elevated after stimulation although present in the unstimulated leg. Twenty-nine additional transcription factors detected at or below threshold levels in the unstimulated EDL were induced to significant levels in the stimulated EDL. CONCLUSIONS: Our results suggest that 10 Hz short-term stimulation activates both latent and constitutive transcription factors as part of a broad molecular response associated with phenotype modification.

Variability of successive contractions subtracted from unfused tetanus of fast and slow motor units

Stimulation of motor units (MUs) with repeated pulses evokes tetanic contractions, which consist of overlapping mechanical responses. The summation of these responses into tetanus is a nonlinear process due to the dynamic changes in the amplitudes and time parameters of the successive components. In order to study these changes, two MUs (one fast and one slow) of rat medial gastrocnemius muscle were stimulated with a progressively increasing number of pulses, from one ( i = 1) to sixteen ( i = 16) at a frequency of 15 Hz for the slow MU and 60 Hz for the fast MU. The individual responses were calculated by subtracting the ( i)th from the ( i + 1)th tetanus recording. The contractions obtained following subtraction were modeled using a novel 6-parameter analytical function. The main conclusions of this study are (1) the newly presented analytical function is able to precisely describe the variable shape of all subtracted experimental contractions; (2) the shapes of successive contractions are variable and the subtracted contractions differ from the individual twitches; (3) as the pulse number increases, the parameters of the subtracted contractions change in a different manner for the slow and fast MUs: for the slow MU, the maximal forces and the time parameters increase considerably up to the 4th response, after which they remain nearly constant or show only a slight increase; for the fast MU, the maximal forces and durations also increase, whereas the remaining time parameters initially increase and then maintain a constant level or decrease, which explains the sag phenomenon visible in the unfused tetanus of fast MUs.

Modeling of summation of individual twitches into unfused tetanus for various types of rat motor units

Repeated stimulation of motor units (MUs) causes an increase of the force output that cannot be explained by linear summation of equal twitches evoked by the same stimulation pattern. To explain this phenomenon, an algorithm for reconstructing the individual twitches, that summate into an unfused tetanus is described in the paper. The algorithm is based on an analytical function for the twitch course modeling. The input parameters of this twitch model are lead time, contraction and half-relaxation times and maximal force. The measured individual twitches and unfused tetani at 10, 20, 30 and 40 Hz stimulation frequency of three rat motor units (slow, fast resistant to fatigue and fast fatigable) are processed. It is concluded that: (1) the analytical function describes precisely the course of individual twitches; (2) the summation of equal twitches does not follow the results from the experimentally measured unfused tetani, the differences depend on the type of the MU and are bigger for higher values of stimulation frequency and fusion index; (3) the reconstruction of individual twitches from experimental tetanic records can be successful if the tetanus is feebly fused (fusion index up to 0.7); (4) both the maximal forces and time parameters of individual twitches subtracted from unfused tetani change and influence the course of each tetanus. A discrepancy with respect to the relaxation phase was observed between experimental results and model prediction for tetani with fusion index exceeding 0.7. This phase was predicted longer than the experimental one for better fused tetani. Therefore, a separate series of physiological experiments and then, more complex model are necessary for explanation of this distinction.

The inverse dynamics problem of neuromuscular control.

The myoskeletal inverse dynamics problem and the myocybernetic control inverse problem were investigated with respect to their ill-posedness. The first problem consists of finding from observed experimental motion and reaction force data the resultant muscle moments that generated the observed motion, while the second aims at finding the corresponding neural controls. It is shown that both problems belong to the class of incorrectly posed (ill-posed) problems that, by definition, do not possess unique solutions. To illustrate this point, results of a forward dynamics simulation of a comprehensive neuromusculoskeletal model of the human body are presented. These results demonstrate that fairly chaotic neural control perturbations have very little influence on the resulting motion trajectory, at least in the present example. While a regularization procedure may be applied to solve successfully the myoskeletal inverse dynamics problem, the myocybernetic control inverse problem is unsolvable. The latter fact has the important implication that, based on the somatosensory inputs it receives, the pars intermedia in the cerebellum is not able to control individual motor unit stimulation rates and recruitment patterns but only whole muscles by means of a single compound signal. The latter signal is identified as the "common drive." Presumably at the spinal level, special neural circuits are used to decompose the common drive signal into motor unit recruitment patterns and stimulation rates that are specific for a given mode of contraction and probably obey certain optimality principles.

Detection of fatigue in the isometric electrical activation of paralyzed hand muscles of persons with tetraplegia.

Paralyzed muscle fatigue is the eventual depression of force due to either prolonged or repetitive electrical stimulation of motor units. The robustness and safety of future functional electrical stimulation (FES) systems will rely on their ability to detect the onset of muscle fatigue. The relative degree of muscle activation can be estimated by monitoring the M-wave. The aim of this study was to test a proposed method of quantitative fatigue assessment that detects muscle force output and its corresponding M-wave measured concurrently. The detection of force and M-wave concurrently allows any reduction in muscle force output to be attributed to either changes in the fatigue state of the stimulated muscle or changes in the degree of stimulus activation of that muscle. The fatigue assessment scheme can thereby accommodate the corresponding changes in muscle force caused by an alteration in the stimulation intensity during fatigue. The Extensor Digitorum Communis (EDC), Extensor Pollicis Longus (EPL), and Flexor Pollicis Longus (FPL) muscles of two C5/C6 tetraplegic men were studied. Stimulation recruitment tests over the pulsewidth range from 0 to 200 micros, were performed at intervals during 20 min of maximal stimulation (200 micro/s). Muscle force correlated to the M-wave parameter, second phase area, with mean correlation coefficients of greater than 0.82, when the muscle was in either a nonfatigued or fatiguing state. After the initial force, likely to be primarily due to the fast glycolytic (FG) motor units, had declined the M-wave demonstrated only minor changes throughout the fatigue of muscle force during 20 min of constant maximal stimulation. The second phase area and root-mean-square (rms) of the M-wave [see Fig. 2(a) reflected muscle activation during modulated stimulation and also remained relatively constant during the fatigue-related force decline when the muscle was stimulated at a constant intensity. This detection of M-wave parameters satisfies the defined requirement for a myoelectric parameter that indicates electrical activation, but is relatively invariant to muscular fatigue. Index Terms-Electrical stimulation, electromyography (EMG), functional electrical stimulation (FES), muscle fatigue, spinal cord injury, tetraplegia.

Muscle response to the Twin-block appliance: An electromyographic study of the masseter and anterior temporal muscles

An electromyographic study was performed on 10 young growing girls in the age group of 9 to 12 years with Class II Division 1 malocclusion and retruded mandible, who were under treatment with Twin-block appliances. Bilateral EMG activity of elevator muscles of the mandible (ie, anterior temporalis and masseter) was monitored longitudinally with bipolar surface electrodes to determine changes in postural, swallowing, and maximal voluntary clenching activity during an observation period of 6 months. The changes were noted at the start of treatment (0 month), within 1 month of Twin-block insertion, at the end of 3 months, and at the end of 6 months. The results revealed a significant increase in postural and maximal clenching EMG activity in masseter ( P < .01) and a numeric increase in anterior temporalis activity during the 6 month period of treatment. The increased electromyographic activity can be attributed to an enhanced stretch (myotatic) reflex of the elevator muscles, contributing to isometric contractions. The main force for Twin-block treatment appears to be provided through increased active tension in the stretched muscles (motor unit stimulation) and from initiation of myotatic reflex activity and not through passive tension (viscoelastic properties) of jaw muscles. The results of this study reaffirm the importance of full-time wear for functional appliances to exert their maximum therapeutic effect by way of neuromuscular adaptation. (Am J Orthod Dentofacial Orthop 1999;116:405-14)

Dynamic performance of a load-moving skeletal muscle

The dynamic response of the tibialis anterior muscle of the cat was determined while it was subjected to sinusoidally varying orderly stimulation of motor units and to different isotonic loads in the range of 14-85% of the maximal isometric force. The dynamic response consisted of three major components: the displacement gain, the displacement attenuation, and a pure time delay. The displacement gain was dependent on the passive load applied to the muscle and the active force generated during contraction, which could be determined from the length-tension relationships and the corresponding shortening velocity. In general, the load displacement decreased as the load mass increased from 25 to 85% of the maximal isometric force. For loads less than 25% of the maximal isometric force, slight decrease in displacement was consistently observed. The displacement attenuation was dependent on the contraction frequency but uniform for all the load masses applied to the muscle. A pure time delay of 5 ms was present and accounted for various physiological processes such as conduction time in nerve and muscle, neuromuscular junction transmission, and excitation-contraction coupling. A quantitative equation was developed to describe the muscle's dynamic response under isotonic conditions and for a wide range of loads for use in various applications.

The dynamic response model of nine different skeletal muscles

The frequency response model of nine different skeletal muscles in the hindlimb of the cat was determined with the aide of electrical nerve stimulation which allows orderly stimulation of motor units concurrently with firing rate increase. It was shown that the general model consists of a linear second-order system with double real poles and a pure time delay. The pole values were different for the different muscles, ranging from 1.55 to 2.8 Hz. Similarly, the pure time delay varied from muscle to muscle, ranging from 8 to 17 ms. Statistical analysis demonstrates that under isometric contraction with force oscillations in the range of 10-90% of maximal the model poles are determined, and could be predicted, from the muscles functional and anatomical location in the limb and from its pennation pattern.

Motor-unit stimulation patterns during fatiguing contractions of constant tension.

1. Through computer feedback control, muscle-unit tension was maintained by altering the stimulation rate of a functionally isolated motor axon. The required stimulation patterns and fatigue properties of motor units from the flexor carpi radialis (FCR), flexor digitorum longus (FDL), and medial gastrocnemius (MG) muscles of the cat were studied when tension was maintained or "clamped" at a constant average level (25% of maximum tetanic tension). 2. In each muscle, two distinct stimulation patterns were observed during constant-tension contractions, one associated with slow-twitch units and the other with fast-twitch units. Once target tension was reached, slow-twitch units required fairly constant rates in order to maintain a constant force, whereas fast-twitch units displayed a marked decline in rate during the early phases of the contraction, averaging between 42 and 54% for the three muscles. The decline in rate most likely represented potentiation of the contractile response and slowing of contractile speed. In general, slow-twitch units responded with lower mean rates (approximately 14 pps less), averaged over the course of the contraction, than fast-twitch units. 3. For fast-twitch units of each muscle, resistance to fatigue varied continuously and over a wide range. The duration that tension could be maintained at 25% of maximum, defined as endurance time, ranged between 16 and 2063 s. No categorization of fast-twitch units into groups could be made on the basis of endurance time. Of the 5 slow-twitch units followed beyond 2700 s, only one failed to maintain tension during the observation period. 4. For hindlimb fast-twitch units, endurance was independent of the stimulation rate needed to maintain tension during the contraction. By contrast, there was a significant tendency for an inverse relation between endurance time and mean stimulation rate for FCR fast-twitch units. 5. Recovery of maximum tension was evaluated at 30 s, 1 min, 2 min, and 5 min following a constant-tension contraction. After a 5-min rest, fast-twitch units were able to produce an average of 80-85% of their maximum tetanic tension. By using the median endurance time (approximately 100 s) to divide the fast-twitch population into "low" and "high" endurance groups, recovery of tension was found not to be uniform among the two groups. High endurance units were able to recover a greater percentage of their original maximum tetanic tension. No difference was found between force recovery for low and high endurance units at 30 s.(ABSTRACT TRUNCATED AT 400 WORDS)

The motor-unit population of the cat tenuissimus muscle

1. We studied the organization of motor units in the tenuissimus (TEN) muscle of pentobarbital-anesthetized cats. The cat TEN is a long, delicate straplike muscle that spans hip and knee, which has a very flat length-tension curve through 22 mm of length change. 2. The TEN motor nucleus, labeled by retrograde transport of several forms of horseradish peroxidase, was composed of 8-31 cells in different cats, of which about half were, on average, in the size range of alpha-motoneurons. TEN motoneurons were scattered through the ventrolateral portion of lamina IX, over a rostrocaudal distance of up to 6.5 mm, making it relatively easy to isolate individual TEN motor axons for single motor-unit stimulation. 3. Individual TEN muscle units were classified into four groups [fast-twitch, fatigable (FF), intermediate, fatigue-resistant (Fint), fast-twitch, fatigue-resistant (FR), and slow-twitch, fatigue resistant (S)] on the basis of "sag" and fatigue index mechanical properties, as in other cat hindlimb muscles. There was a relatively large proportion of Fint units (28%) in the TEN sample, and the range of tetanic tension (approximately 19-fold) was much smaller than found in other cat hindlimb muscles. 4. A majority of TEN muscle fibers could be classified into the three major histochemical types (IIB, IIA, and I) found in other cat muscles, but a substantial minority remained "unclassified." A single type Fint muscle unit was successfully depleted of glycogen for histochemical study. It exhibited a typical type IIB histochemical profile. 5. Despite its unusual morphology, the cat TEN contains the same types of motor units found in larger, more "typical" limb muscles.

Muscle spindles, Golgi tendon organs, and the neural control of skeletal muscle

Muscle spindles, Golgi tendon organs, and the neural control of skeletal muscle

The discharge of cat tendon organs during unloading contractions.

Tendon organs respond preferentially to contractions of a select set of motor units. If a tendon organ is given a resting discharge by raising the passive tension in the muscle, other motor units can be identified which on contraction interrupt the discharge, presumably by unloading the passive tension. These experiments investigate the possibility that stimulation of motor units with unloading effects can reduce the response of a tendon organ to a loading contraction. We find that over most of the range of muscle lengths the unloading contraction produces only a transient lowering of the firing rate. Only at short muscle lengths where further shortening is accompanied by a steep fall in tension is the unloading contraction able to induce a sustained reduction of afferent discharge.

A comparison of electromyographic and mechanical fatigue properties in motor units of the cat hindlimb

Single motor units were isolated in medial gastrocnemius (MG) or soleus (Sol) muscles of the cat. Single shocks delivered to the motor axon elicited EMG waveforms which were recorded in the muscle. The amplitude of each individual EMG waveform as well as the area under the full-wave rectified waveform were measured. Mechanical properties of the motor units were then measured so that each unit could be classified as FF, FR or S according to the criteria of Burke. Finally, each unit was stimulated continuously at 80 pulses per second (pps), and changes in electrical and mechanical responses were recorded and compared. IEMG was positively correlated with maximum tetanic tension of motor units in MG. The relationship could be fitted with a parabola showing that ‘large’ motor units produce relatively more electrical activity than do ‘small’ motor units. Two types of electrical changes were seen during continuous stimulation of motor units at 80 pps: (1) a smooth continuous decline in IEMG was attributed to electrical changes occurring at a site or sites distal to the neuromuscular junction; and (2) random, abrupt changes in unit-EMG waveform and hence IEMG amplitude, were attributed to failure of the action potential to propagate past axonal branch points, resulting in the intermittent failure of groups of muscle fibers to respond to stimuli. The rates of decline of electrical and mechanical activity were compared for motor units. It was found that in fast fatiguing motor units, tetanic tension declined more rapidly than did IEMG, while in slowly fatiguing motor units, IEMG declined more rapidly than did force. We conclude that fatigue of motor units induced by continuous 80 pps stimulation can occur at different sites in the motor unit, and that the site of fatigue depends on motor unit properties or motor unit type.

Linear and non-linear summation of α-motoneuron potential changes elicited by contractions of homonymous motor units in cat medial gastrocnemius

An analysis is made of linear and non-linear summation of the motor unit (MU) tension and motoneuron (MN) membrane potential (PSP) trajectories evoked by stimulation of MUs as previously reported. In most cases, summation of both averaged responses was linear. Seldom, PSPs summed non-linearly, then usually coinciding with non-linear summation of the responsible MU twitches. The results suggest that the afferent pathways mediating the mechanical events in muscle to MNs behave rather linearly, at least in the small-signal range of interest here.

Sequential motor unit stimulation through peripheral motor nerves in the cat

An electrode array is described that can be used to stimulate skeletal muscle through the peripheral motor nerves. This electrode array differs from the sleeve electrode that is used conventionally in that the motor units are stimulated sequentially, a mode of stimulation which closely mimics the normal asynchronous firing pattern of motor units that occurs during voluntary activity. Using this stimulation technique, tetanic contractions can be developed by muscle at low stimulation voltages and within physiological stimulation frequencies. Furthermore, unlike synchronous electrical stimulation, during sequential stimulation, submaximal tensions can be maintained for sustained periods of time. The applications of this electrode array to rehabilitation medicine are discussed

Innervation of extrafusal and intrafusal fibres in snake muscle.

1. Intrafusal fibres of snake receive motor supply from branches of axons innervating extrafusal motor units. By intramuscular stimulation of motor units by single shocks, and critical curarization of the muscle, we have identified at least some of the motor units contributing motor supply to individual intrafusal fibres. Intrafusal fibre activation was observed by visual examination of the contracting intrafusal fibre, and by recording the resulting spindle afferent discharge.2. The main finding is that in some cases the motor supply to one intrafusal fibre comes from more than one motor unit. The contributing motor units may be either dissimilar twitch units, or twitch and tonic units. Thus some of the intrafusal fibres studied showed polyneuronal motor innervation of heterogeneous origin.3. In critically curarized muscle, the time course of a spindle afferent discharge, following single-shock stimulation of a motor unit contributing motor supply to the intrafusal fibre, showed little variation with the type of motor unit being stimulated.4. The response of each spindle to a standard stretch was recorded. There was no correlation between dynamic index and type of motor unit or units contributing motor supply. However, the method limits the value of negative findings, and this is discussed.5. The contraction times and tensions of a sample of motor unit isometric twitches are described.