[COMPARATIVE ANALYSIS OF ELECTROMYOGRAPHIC MUSCLE ACTIVITY OF THE HUMAN HAND DURING CYCLIC TURNS OF ISOMETRIC EFFORT VECTOR OF WRIST IN OPPOSITE DIRECTIONS].

The features of the formation of targeted tonic isometric effort created by muscles of the shoulder girdle and shoulder were compared for two opposite changes of the directions of arbitrary force vector-clockwise and counterclockwise. The magnitude of the effort vector remained constant. The intensity of the central motor commands (CMC) was evaluated according to the area of founded rectified, averaged and approximated electromyographic (EMG) recorded from the investigated muscles. The effort development was carried out in determined direction in operation space of the horizontal plane under angles of 75 degrees in the shoulder joint and 90 degrees in the elbow one. The synergic interaction of individual functional groups of muscles under certain reaction force was found. In the case of space, it was determined the main activity areas of the investigated muscles within the operating space. The differences were shown in the levels of EMG amplitude in opposite directions bypass of effort vector. Coactivation of flexor muscles was found, while the extensors showed their activity only within their functional sectors. Half of the investigated muscles showed an increase of the EMG amplitude for the counter-clockwise direction in relation to EMG amplitude for the clockwise direction. This can be attributed to more complex patterns of motor commands organization which consistently addressed to the muscles of the shoulder and forearm caused by atypical sequence of activation of antagonist muscles under making hand movements in cyclic mode during tonic isometric contraction.

Subthreshold activation of spinal motoneurones in the stretch reflex: experimental data and modeling.

Responses of gastrocnemius-soleus motoneurones to stretches of the homonymous muscles were recorded intrasomatically in decerebrate cats; changes of membrane potential (MP) were evoked by smoothed trapezoid stretches of the muscles. Amplitudes of separate excitatory postsynaptic potentials (EPSPs) were defined via differences between values of MP at the end and beginning of the positive derivative waves, which were also used as basic elements in the model of the excitatory postsynaptic currents (EPSCs). EPSCs were assumed to be transformed into EPSPs by low-pass filtering properties of the somatic membrane; parameters of the filtering were firstly defined from analysis of Ia EPSP in the same cell and then were applied in model P ( m0). The model showed unsatisfactory quality in tracking slow components of MP; to overcome the disadvantage there was proposed model P ( m1) based on addition to P ( m0) the difference between two low-pass filtered signals MP and P ( m0) (the cutoff frequency 10 or 20 Hz). An overestimation of EPSPs' amplitudes was corrected in model P ( m2). The mismatch in tracking slow changes of MP was assumed to be connected with summation of a great number of low-amplitude EPSPs generated at distal dendrites; information about waveform of separate EPSPs could disappear in this process. One can speculate that slow components of membrane depolarization at least partly are linked with the persistent inward currents in dendrites; variable and, sometimes, too fast decays in EPSPs seem to reflect inhibitory synaptic influences.

Distinctive pattern of c-fos expression in the feline cervico-lumbar spinal cord after stimulation of vanilloid receptors in dorsal neck muscles.

In the present study, c-fos expression in the spinal cord has been used as a marker of neuronal activation induced by capsaicin-sensitive sensory afferents from the dorsal neck muscles in cats (n = 6). The number of Fos-immunoreactive neurons, which were revealed using the avidin-biotin-peroxidase method, was significantly increased in the cervical and lumbar spinal cord. In contrast to the control group (n = 3), 2 h after intramuscular capsaicin injection, c-fos expression was more extensive ipsilaterally to the injected side in the C3-C6 segments, and bilaterally in the L4-L6 segments. Most labeled neurons in the cervical spinal cord were small and giant cells, predominantly located in the middle and lateral parts of lamina I and, additionally, at the neck of the dorsal horn (lamina V), i.e., within the zones of termination of high-threshold muscle afferents. The widespread distribution of labeled cells throughout the cervical cord within the intermediate zone (lamina VII) coincided with the sites of last-order premotor interneurons and cells of origin of long crossed and uncrossed descending propriospinal pathways to the lumbar spinal cord. These findings suggest possible mechanisms for spreading of nociceptive signals between cervical and lumbar regions.

Spreading of fatigue-related effects from active to inactive parts in the medial gastrocnemius muscle of the cat.

In the medial gastrocnemius muscle of the decerebrate cat, the spatial spread of fatigue between active and inactive muscle parts was studied. Conditioning fatiguing stimulation (CFS) was applied to a part of the muscle to test whether it had an effect on the contraction efficiency in an unstimulated part. To exclude somato-sympathetic reflexes during CFS, a full rhizotomy of the lumbo-sacral spinal cord was performed. The same ipsilateral ventral root, either L7 or S1, was divided into seven filaments, one of which was used for the test stimulation, and four or five for CFS. The CFS consisted of 12 s sessions of distributed stimulation of five (or four) filaments at a rate of 40 s(-1), the sessions were repeated, every 40 s, 15 or more times. The test consisted of 12 s of regular stimulation at a rate of 10 s(-1), preceded and followed by a single stimulus. The tests applied just after CFS showed a strong decline of both tension and electromyogram (EMG), amounting to only [mean (SD)] 0.45 (0.18) and 0.51 (0.19) (n = 15), respectively, of the corresponding values in the tests before CFS. It thus turned out that depressive fatigue-related effects could spread within the muscle. At the same time, control reactions recorded in the lateral gastrocnemius during stimulation of its cut nerve did not change. Subsequent repetitions of the tests usually revealed a tendency towards restoration. The EMG reactions recovered more quickly than tension. The depression of EMG after CFS was accompanied by a slowing of the constituent M-waves; their latencies decreased during restoration. Distinct changes in the systemic blood pressure were observed during CFS. These changes were usually correlated well with muscle tension changes. The factors possibly underlying the observed effects may include diffusion of metabolites from active to inactive muscle fibres, lowering of the efficiency of neuro-muscular transmission due to squeezing of efferent motor terminals and changes in outer metabolite content, as well as local hypoxia due to increases in intramuscular pressure.

c-fos Expression and NADPH-d reactivity in spinal neurons after fatiguing stimulation of hindlimb muscles in the rat.

The distribution of Fos-immunoreactive (Fos-ir) and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-reactive neurons in the rat lumbar spinal cord was examined following muscle fatigue caused by intermittent high-rate (100 s(-1)) electrical stimulation of the triceps surae muscle or the ventral root L5 (VRL5) for 30 min. Following both types of stimulation, the fatigue-related c-fos gene expression was more extensive in the L2-L5 segments on the stimulated side, and the majority of Fos-ir neurons were concentrated in the dorsal horn. After direct muscle stimulation, the highest number of Fos-ir neurons were detected in two regions: layer 5, and superficial layers (1 and 2(o)), although many labeled cells were also found in layers 3, 4, 6, and 7. In response to VRL5 stimulation, the maximal density of Fos-ir neurons was detected in the middle and lateral parts of layers 1 and 2(o), the zone of termination of high-threshold muscle afferents(.) Statistically significant prevalence of Fos-ir cell number was also found in layers 5 and 7 on the stimulated side. A few Fos-ir neurons were detected in the ventral horn (layer 8 and area 10) on both sides. The lamellar distribution of NADPH-d-reactive neurons was similar over all experimental groups of animals. In the L3-L6 segments, such reactive cells were arranged in two distinct regions: dorsal horn (layers 2(i), 3, and 5) and area 10; in the L1 and L2 segments, an additional cluster of NADPH-d positive cells was found in the intermediolateral cell column (IML). Double-labeled cells were not detected. We suggest that c-fos expression in response to muscle fatigue reveals activity of functionally different types of spinal neurons which could operate together with NOS-containing cells in pre-motoneuronal networks to modulate the motoneuron output.

Fatigue effects in the cat gastrocnemius during frequency-modulated efferent stimulation.

Effects of low- and high-frequency fatigue were studied on muscle dynamics in isometric conditions of the cat gastrocnemius. Fatiguing sessions consisted of 25-28 repetitions of the standard tests that included an 18-s interval of continuous frequency-modulated stimulation preceded and followed by single stimuli evoking twitch contractions. The rate of the continuous part was changed in accordance with a symmetrical double-trapezoidal signal, including three successive phases of constant rate at 10, 40 and 10s(-1); between these phases, each lasting for 4s, the rate changed linearly within a 2-s interval. The following modes of muscle activation were applied: (i) stimulation of single filaments constituting approximately one-fifth to one-seventh of the total cross-section of the L(7) and S(1) ventral roots; (ii) the distributed stimulation of five similar filaments; and (iii) direct stimulation of muscle through bipolar wire electrodes. A relative drop in tension, the fatigue index, expressed as the ratio at the end of a fatigue session over its value at the beginning of the test, was used to quantify fatigue effects. The fatigue indices during low-rate stimulation were 0.56+/-0.03 (mean+/-S.D.) at the first phase and 0. 64+/-0.02 at the third phase, while during high-rate stimulation this parameter was only 0.32+/-0.02. The high-rate stimulation noticeably increased the mean tension during low-rate stimulation; the ratio between the reactions at the third and the first phases could be as much as two to three times greater than that at the beginning of the fatigue session. It was demonstrated that the potentiation was connected with after-effects of the rate-tension hysteresis. The hysteresis decreased with fatigue, the fatigue index for the rate-tension loop areas ranging from 0.39 to 0.52 (0.45+/-0. 05, mean+/-S.D.). The fatigue processes developed more quickly and intensively in the previously fatigued muscles: the obtained fatigue indices were 0.73+/-0.05 and 0.70+/-0.10 at the first and third phases, and 0.62+/-0.06 (mean+/-S.D.) at the second phase of stimulation, respectively. In the cases of distributed and direct stimulation applied to muscles in a fresh state, fatigue dynamics did not differ significantly from those observed during single-filament stimulation. In experiments with distributed stimulation applied to previously fatigued muscles, a powerful depression of the high-rate components was registered in several cases, which seemed to be connected with depressive effects at the level of nerve-muscle synaptic transmission. The effects of low- and high-frequency fatigue were studied in isometric conditions of muscle contraction. In addition to the well-known differentiation between low- and high-frequency fatigue effects, the complex pattern of efferent stimulation used allowed us to identify additional fatigue-related changes in the rate-tension hysteresis. This hysteresis seems to be one of the possible mechanisms directed to compensate for low-frequency fatigue in the muscle contraction.

Fatigue-related changes in electomyogram activity of the cat gastrocnemius during frequency-modulated efferent stimulation.

Changes in the compound muscle action potentials of cat gastrocnemius muscle were studied during low- and high-frequency fatigue. Fatiguing session consisted of 25-28 repetitions of the standard single fatigue tests (1.5min interval between the tests) that included the part of continuous frequency-modulated stimulation preceded and followed by single stimuli evoking twitch contractions in the muscle. The rate of the continuous part was changed in accordance with symmetrical double-trapezoidal signal, including three successive phases of constant rate at 10, 40 and 10s(-1); between these phases of 4s duration the rate changed linearly within a 2s interval. During fatigue relative changes in compound muscle action potential waves were usually smaller than changes in tension. Within the same fatigue procedure applied to a fresh muscle, the drop in tension was as much as 35% for high-rate stimulation and 59-71% for low-rate stimulation, whereas the decrease of the peak-to-peak compound muscle action potential waves amplitudes did not exceed 10-20%. Compound muscle action potential waves underwent the most pronounced depression during high-rate stimulation, the decrease proceeding during the following phase of low-rate stimulation. The tension changes during long-lasting activation were different for low- and high-frequency fatigue, with more pronounced depression during low-rate stimulation. As a rule, compound muscle action potential waves changes followed opposite patterns. Compound muscle action potential waves progressively split up, which was probably associated with a continuous slowing of the action potentials in the most fatigable motor units and the subsequent disappearance of the reactions at least in part of the motor units. Hysteresis effects in muscle contraction seem to be able, at least in part, to compensate for some of the depressive effects appearing during conduction of action potentials in muscle fibres. Changes in the compound muscle action potentials were studied during development of the muscle fatigue. These changes showed pronounced dependency on stimulation rate allowing differentiating effects of low- and high-frequency stimulation of the efferents supplying muscle under study. At the same time the fatigue-related changes in the action potentials were noticeably smaller than changes in tension, thus supporting existing concepts in the field arguing that fatigue effects are mainly connected with corresponding activity-dependent changes in muscle contraction machinery.

Analysis of the electromyographic activity of human elbow joint muscles during slow linear flexion movements in isotorque conditions.

Electromyograms were recorded by surface electrodes from the mm. biceps brachii (caput longum et breve), brachioradialis and triceps brachii (caput longum) in 10 healthy human subjects during slow linear movements in the elbow joint against a weak extending torque. The test movements were carried out under visual control through combining on a monitor screen a signal from a joint angle sensor with an appropriate command generated by a computer. The movements were fulfilled against a weak constant extending torque (0.5-2.5 Nm) and the extensor muscles were inactive. Surface electromyograms were full-wave rectified, filtered and averaged within sets of 10 identical tests. For the test movements in the range from 20 degrees to 100 degrees (0 degrees corresponds to a completely extended joint in these designations) the dynamic components of the efferent commands to actively contracted muscles frequently had a well-expressed monotonous increase. The electromyography intensity during movement quite often increased exponentially in all three investigated flexors. At the same time, the averaged electromyograms in one or two muscles could contain non-monotonous oscillations, thus showing a well-expressed decrease in the intensity of the efferent inflow within a middle range of a movement phase. The non-monotonous oscillations could occur in some subjects under minimal loads (0.5-0.75 Nm); they usually appeared initially only in one muscle, whereas the frequency of their occurrence was not high. The probability of finding the non-monotonous oscillations in the electromyograms from the muscles under study increased with heightening the extending load. Under the loads of 2.0-2.5 Nm this type of reaction could be found in almost half of all records (for the three muscles in each of the 10 subjects under testing). The presence of noticeable non-monotonous components in the electromyograms of the elbow flexors during their contraction is probably connected to their biomechanical arrangement within the joint. It is known that the arms of the forces generated by elbow flexors are maximal in the middle range of joint angles and decrease with a change in angle in both directions. Thus, we can suppose that non-monotonous decrement components in the electromyograms of the elbow flexors are presumably connected with an obvious necessity for a subject to decrease the excitatory efferent inflow to the muscles in the middle range of the joint angles. The pattern of electromyograms in the flexors acting around the joint was also dependent on a redistribution of activity between agonists. In the subjects showing stable non-monotonous components on the averaged electromyogram records in two agonists, a redistribution of the activity between these muscles has been demonstrated when phases of the electromyogram diminishing in one of them coincided with the appropriate increments in the other.

Length changes of the cat soleus muscle under frequency-modulated distributed stimulation of efferents in isotony.

The length changes of the cat soleus muscle have been examined in isotony within closed cycles of the stimulation rate change. Successive stimuli were applied in a cycle to five filaments of the preliminary dissected L7-S1 ventral roots (method of distributed stimulation), maximal rate did not exceed 80-90/s (16-18/s per single filament). At the beginning of a slow linear increase in the rate a muscle began shortening rather quickly, the rate changes without muscle reaction consisted of 5.91 +/- 0.28/s (mean +/- S.E.M.). A substantially linear movement was observed during increase of the input rate up to 40-50/s (i.e. 8-10/s per filament), a further rate increment could evoke a somewhat slowing of the shortening velocity with a distinctive infection of the rate-length curve. During a significant part of the rate decrease phase no movement was seen, the rate range with the absence of muscle lengthening was 29.93 +/- 1.57/s. After such a pronounced period of length fixation, a muscle began to elongate, the steady-state velocity at this part of the movement trajectory was invariably higher as compared with shortening velocity at the leading edge of the cycle. The striking feature of a powerful length clamping seen in active muscle at the phase of rate decrease preceded by previous rate increment allows us to suppose that this strongly non-linear behaviour of muscle might be a main reason for the existence of powerful dynamic components in efferent activity every time when muscle should shorten against external load in a ramp-and-hold fashion. We used the following experimental paradigm to check the assumption. Stimulation began with regular rate of 15/s; then, after transition from isometry to isotony and cessation of movement transients, the rate was raised linearly; after reaching a peak value of 50-90/s, it decreased linearly or exponentially, being afterwards fixed at several different levels between the maximal and initial values of the rate. It was demonstrated that such pattern of stimulation could be effective for a linear transition between two equilibrium lengths, provided that corresponding parameters in modulation signal were chosen in an optimal way. Duration and amplitude of the leading edge of the dynamic component seemed to completely define amplitude and velocity of the ramp phase of movement, parameters of decay to the final steady rate were, on the other hand, important for efficacy of the length clamping at hold phase. It was concluded that hysteresis effects of muscle contraction seemed to be an extremely important nonlinear property of muscle dynamics in producing any transition movements between two steady-states. Thus, oversimplified muscle models, like the spring model based on the isometric length-tension dependencies, seem to be incorrect. Possible mechanisms for the muscle hysteresis and its role in motor control are discussed.

Muscle hysteresis and movement control: a theoretical study.

In this study we have tried to elaborate necessary theoretical approaches for the adequate analysis of the central motor commands to a mammalian muscle in the equilibrium states and during transition movements between these states. At present, the equilibrium point hypothesis has obtained a wide distribution in this field. The muscle is considered in the framework of the theory as an executive element of the reflex circuits originating in the muscle proprioceptors and being closed at the level of spinal cord and the supraspinal motor centres. The main parameter defining the muscle state is supposed to be the threshold of the stretch reflex--the minimal length value at which muscle begins to resist to the externally applied force. We have attempted to show that the theory has an essential shortcoming because it does not take into account such important non-linearity in the muscle behaviour as hysteresis. In the framework of the equilibrium point hypothesis, the muscle behaviour within the stretch reflex system does not depend on movement direction. The stretch and unloading reflexes are supposed to have the same length tension dependencies when the muscle is stretching or contracting with a rather slow velocity. However, powerful hysteresis of the stretch reflex system requires taking into account the direction of the current movement, the after-effects of previous movement led to a principal uncertainty in the muscle steady-state. We would like to stress that any process of active muscle shortening should be controlled by dynamic components in efferent inflow. At the same time, the resulting steady-state develops by using effective hysteresis mechanism for its maintenance. The following hypothesis was proposed to explain the length clamping mechanisms in shortening transition movements. A significant decrement of the arrived efferent activity at the phase of the length fixation can evoke an internal elongation of the contractile elements within the muscle and corresponding hysteresis-like enhancement of the contractile effectiveness. Hence, instead of considering the quasi-static and dynamic components of movement commands as in the equilibrium point hypothesis, it is preferable to adopt a model of the shared coding of both the final position and movement velocity. The dynamic component of the efferent discharge seems to be required for a complete definition of the final steady-state, but maintenance of the state is closely associated with energetically advantageous hysteresis mechanisms. It was concluded that the dynamic phase of efferent activity should play an extremely important role in the central coding of the real movements produced, in particular, by contraction of agonists in the absence of antagonist activation.

Movement-related and steady-state electromyographic activity of human elbow flexors in slow transition movements between two equilibrium states.

The electromyograms were recorded in healthy human subjects by surface electrodes from the mm. biceps brachii (caput longum et. brevis), brachioradialis, and triceps brachii (caput longum) during slow transition movements in elbow joint against a weak extending torque. The test movements (flexion transitions between two steady-states) were fulfilled under visual control through combining on a monitor screen a signal from a joint angle sensor with a corresponding command generated by a computer. Movement velocities ranged between 5 and 80 degrees/s, subjects were asked to move forearm without activation of elbow extensors. Surface electromyograms were full-wave rectified, filtered and averaged within sets of 10 identical tests. Amplitudes of dynamic and steady-state components of the electromyograms were determined in dependence on a final value of joint angle, slow and fast movements were compared. An exponential-like increase of dynamic component was observed in electromyograms recorded from m. biceps brachii, the component had been increased with movement velocity and with load increment. In many experiments a statistically significant decrease of static component could be noticed within middle range of joint angles (40-60 degrees) followed by a well expressed increment for larger movements. This pattern of the static component in electromyograms could vary in different experiments even in the same subjects. A steady discharge in m. brachioradialis at ramp phase has usually been recorded only under a notable load. Variable and quite often unpredictable character of the static components of the electromyograms recorded from elbow flexors in the transition movements makes it difficult to use the equilibrium point hypothesis to describe the central processes of movement. It has been assumed that during active muscle shortening the dynamic components in arriving efferent activity should play a predominant role. A simple scheme could be proposed for transition to a steady-state after shortening. Decrease of the efferent inflow can evoke internal lengthening of the contractile elements in muscle and, as a result, hysteresis increase in the muscle contraction efficiency. Effectiveness in maintenance of the steady position seems to also be enhanced due to muscle thixotropy and friction processes in the joint. Hysteresis after-effects in elbow flexors were demonstrated as a difference in steady-state levels of electromyograms with oppositely directed approaches to the same joint position.

Interaction of the movement-dependent, extrafusal and fusimotor after-effects in the firing of the primary spindle endings.

The after-effects of the firing of the primary spindle endings were studied in ankle extensor muscles of cats under Nembutal anesthesia. The activities of 27 primary endings of the muscle spindles from mm. soleus, plantaris, and gastrocnemius have been analysed in various combinations of fusimotor and extrafusal stimulation and application of the mechanostimulation to the spindle bearing muscle. Short-term simulation of static gamma-axons evoked a post-stimulation increase in the spindle ending firing, which can be recorded under both isometric and isotonic conditions on applying a weak extrafusal stimulation or without it. The movement-dependent after-effects were tested with a double-trapezoid pattern of muscle length (or load) changes. The after-effects consisted of the difference of firing rates at the same values of muscle length (or load) with opposite direction of movement to the steady states; these uncertainties were also present during constant stimulation of static gamma-axons. The rate difference showed a tendency to a certain decrease with stimulation rate increment. For diapason of the stimulation rates up to 125 impulses/s a small negative correlation (r = -0.61) between the firing rate differences and the gamma-stimulation rate has been registered in the population of primary endings tested under length servo-control conditions. Using a frequency-modulated intrafusal stimulation, a clockwise hysteresis dependence of the spindle firing rate upon stimulation rate was demonstrated. The pronounced after-effects were shown to exist for steady rates of stimulation: the discharge rates were always higher after stimulation rate increase and lower after its decrease. Fusimotor after-effects were effectively destroyed by both the extrafusal stimulation and the cyclic length (load) changes evoked lengthening-shortening movements of the muscle. The results obtained can be considered as evidence for a hypothesis that history-dependent behavior of muscle spindles is mainly connected with hysteresis of the intrafusal muscle fibers and the whole spindle bearing muscle.

Movement-dependent after-effects in the firing of the spindle endings from the de-efferented muscles of the cat hindlimb.

The muscle spindle reactions evoked by the servo-controlled changes in the muscle length or external load were studied on the de-efferented muscles in experiments on cats under Nembutal anaesthesia. The activity of 39 primary and 10 secondary endings of the muscle spindles from four hindlimb extensor muscles: soleus, plantaris, lateral and medial heads of gastrocnemius has been recorded during servo-controlled changes in muscle length (L-control) or external load (P-control). Slow linear reciprocating (triangular) signals and their modification with fixation of the controlled parameter at the same level at forward and reverse phases were used as commands for the muscle stretcher. The steady firing rates in primary and secondary endings during fixation of muscle length or external load were shown to be strongly dependent on the direction of previous changes in these parameters. The firing rates were always higher after preceding lengthening (loading) and lower after shortening (unloading). These rate uncertainties in the steady firing of spindle endings at the same level of controlled parameter could be as much as 15-20 p.p.s. The revealed dependence of the spindle ending firing on the past history of the muscle movement was denoted as movement-dependent after-effects. Two kinds of movement-dependent after-effects in the firing of the spindle endings can be observed: (1) the movement-dependent ones being studied in the present work, and (2) those connected with the conditioning fusimotor stimulation and described elsewhere. Their common origin is supposed to be connected with the hysterical (thixotropic) properties of the intrafusal muscle fibres. Similarity of the movement-dependent after-effects in L- and P-control conditions was shown to be connected with a resemblance of the length-firing rate [F(L)] and load-firing rate [F(P)] hysterical loops registered during reciprocating movements. Both kinds of loops were in a clock-wise direction, their comparison in normalized form showed that F(L) loops were always broader than F(P) ones. Isotonic and isometric uncertainty vectors were introduced to analyse quantitatively the hysterical effects in the spindle firing and their relation to the muscle hysteresis proper. Uncertainty vectors connect the points of equal load (isotonic uncertainty vectors) and equal length (isometric uncertainty vectors) on F(L) and F(P) loops correspondingly. The projections of both uncertainty vectors onto the Y-axis give the rate uncertainty coinciding in sign for both cases, whereas their projections onto the X-axis differ in sign, being positive for isometric uncertainty vectors (the tension uncertainty) and negative for isotonic uncertainty vectors (the length uncertainty).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of torque disturbances on elbow joint movements evoked in unanesthetized cats by microstimulation of the motor cortex.

Flexion and extension movements were evoked in the elbow joint of unanesthetized cats by intracortical microstimulation (ICMS) applied to deep layers of the motor cortex (areas 4 and 6). Pulse trains with duration up to 3-4 s, current intensities of 15-50 microA and rates of approximately 100/s were used. Cortically evoked movements (CEMs) were tested mechanically by applying servo-controlled torque disturbances to the joint. The disturbances consisted of two reciprocating sinusoidal pulses of torque with fixed frequencies (1.2 or 3.2 Hz). A pronounced torque-angle hysteresis with long-lasting after-effects was revealed in the presence of the torque disturbances that opposed the CEMs and/or assisted them. Two parameters were introduced to describe the mechanical testing of the CEMs quantitatively: (1) the resulting stiffness (RS) defined during the forward and reverse phases of the disturbed movement as a ratio between the amplitudes of torque wave and the overall change of angle at these phases; (2) uncertainty index (UI) defined as the subtraction of forward and reverse angle changes, which was normalized by the first of these two values. RS was shown to be dependent on the immediate past movement history of the joint, it increased with changes in the direction of movement, and its magnitude during such changes could be several times higher than when the disturbance was in the same direction as the movement.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle dynamics: dependence of muscle length on changes in external load.

A phenomenological theory of muscle dynamics has been elaborated on the basis of data obtained in experiments on hind limb extensor muscles of narcotized cat. Functional dependence of muscle length on external load was explored in conditions of a constant frequency of the efferent stimulation. It was shown that the system under study could be presented for a rather wide class of input signals as a system with nonlinear statics and linear dynamics. The nonlinear statics was shown to be determined mainly by the hysteretical effects of muscle contraction, whereas dynamic element was described by the first order linear differential equation corresponding to the traditional three-component mechanical model of the muscle. A hypothesis was proposed to explain the hysteresis in active muscle on the basis of functioning of the troponin-tropomyosin regulatory complex. Elaborated mathematical model of muscle dynamics can be used to predict and evaluate changes in the muscle length evoked by arbitrary changes in the external load.

Probability of neuronal spike initiation as a curve-crossing problem for Gaussian stochastic processes.

Probability of neuronal spike initiation was considered within the framework of a simple stochastic model. The time of spike occurrence was defined as the first time of crossing of a stochastic process and a determined time function. This problem has been investigated in the case of a stationary Gaussian stochastic process and a linear time function. An integral equation obtained for the probability density function of the first time crossing was numerically solved by means of computer calculations. The model was applied to the analysis of temporal pattern of spike activity evoked in the cat spinal motoneurones by depolarizing current injected through the recording microelectrode.