Motor-unit activation patterns in lengthening and isometric contractions of hindlimb extensor muscles in the decerebrate cat

JNK activation in TA and EDL muscle is load-dependent in rats receiving identical excitation patterns

As the excitation-contraction coupling is inseparable during voluntary exercise, the relative contribution of the mechanical and neural input on hypertrophy-related molecular signalling is still poorly understood. Herein, we use a rat in-vivo strength exercise model with an electrically-induced standardized excitation pattern, previously shown to induce a load-dependent increase in myonuclear number and hypertrophy, to study acute effects of load on molecular signalling. We assessed protein abundance and specific phosphorylation of the four protein kinases FAK, mTOR, p70S6K and JNK after 2, 10 and 28 min of a low- or high-load contraction, in order to assess the effects of load, exercise duration and muscle-type on their response to exercise. Specific phosphorylation of mTOR, p70S6K and JNK was increased after 28 min of exercise under the low- and high-load protocol. Elevated phosphorylation of mTOR and JNK was detectable already after 2 and 10 min of exercise, respectively, but greatest after 28 min of exercise, and JNK phosphorylation was highly load-dependent. The abundance of all four kinases was higher in TA compared to EDL muscle, p70S6K abundance was increased after exercise in a load-independent manner, and FAK and JNK abundance was reduced after 28 min of exercise in both the exercised and control muscles. In conclusion, the current study shows that JNK activation after a single resistance exercise is load-specific, resembling the previously reported degree of myonuclear accrual and muscle hypertrophy with repetition of the exercise stimulus.

Increased hypertrophic response with increased mechanical load in skeletal muscles receiving identical activity patterns

It is often assumed that mechanical factors are important for effects of exercise on muscle, but during voluntary training and most experimental conditions the effects could solely be attributed to differences in electrical activity, and direct evidence for a mechanosensory pathway has been scarce. We here show that, in rat muscles stimulated in vivo under deep anesthesia with identical electrical activity patterns, isometric contractions induced twofold more hypertrophy than contractions with 50-60% of the isometric force. The number of myonuclei and the RNA levels of myogenin and myogenic regulatory factor 4 were increased with high load, suggesting that activation of satellite cells is mechano dependent. On the other hand, training induced a major shift in fiber type distribution from type 2b to 2x that was load independent, indicating that the electrical signaling rather than mechanosignaling controls fiber type. RAC-α serine/threonine-protein kinase (Akt) and ribosomal protein S6 kinase β-1 (S6K1) were not significantly differentially activated by load, suggesting that the differences in mechanical factors were not important for activating the Akt/mammalian target of rapamycin/S6K1 pathway. The transmembrane molecule syndecan-4 implied in overload hypertrophy in cardiac muscle was not load dependent, suggesting that mechanosignaling in skeletal muscle is different.

Current injection and receptor-mediated excitation produce similar maximal firing rates in hypoglossal motoneurons

The maximum firing rates of motoneurons (MNs), activated in response to synaptic drive, appear to be much lower than that elicited by current injection. It could be that the decrease in input resistance associated with increased synaptic activity (but not current injection) might blunt overall changes in membrane depolarization and thereby limit spike-frequency output. To test this idea, we recorded, in the same cells, maximal firing responses to current injection and to synaptic activation. We prepared 300 μm medullary slices in neonatal rats that contained hypoglossal MNs and used whole-cell patch-clamp electrophysiology to record their maximum firing rates in response to triangular-ramp current injections and to glutamate receptor-mediated excitation. Brief pressure pulses of high-concentration glutamate led to significant depolarization, high firing rates, and temporary cessation of spiking due to spike inactivation. In the same cells, we applied current clamp protocols that approximated the time course of membrane potential change associated with glutamate application and with peak current levels large enough to cause spike inactivation. Means (SD) of maximum firing rates obtained in response to glutamate application were nearly identical to those obtained in response to ramp current injection [glutamate 47.1 ± 12.0 impulses (imp)/s, current injection 47.5 ± 11.2 imp/s], even though input resistance was 40% less during glutamate application compared with current injection. Therefore, these data suggest that the reduction in input resistance associated with receptor-mediated excitation does not, by itself, limit the maximal firing rate responses in MNs.

Forward stair descent with hybrid neuroprosthesis after paralysis: Single case study demonstrating feasibility

The ability to negotiate stairs is important for community access and independent mobility but requires more effort and strength than level walking. For this reason, previous attempts to utilize functional neuromuscular stimulation (FNS) to restore stair navigation after spinal cord injury (SCI) have had limited success and are not readily generalizable. Stair descent is particularly challenging because it requires energy absorption via eccentric muscle contractions, a task not easily accomplished with FNS. This article presents the design and initial testing of a hybrid neuroprosthesis with a variable impedance knee mechanism (VIKM-HNP) for stair descent. Using a 16-channel percutaneous FNS system, a muscle activation pattern was synthesized to descend stairs with the VIKM-HNP in a step-by-step fashion. A finite state control system was implemented to deactivate knee extensor stimulation and utilize the VIKM-HNP to absorb energy and regulate descent speed. Feasibility testing was performed on one individual with complete thoracic-level SCI. Stair descent was achieved with maximum upper-limb forces of less than 45% body weight compared with previously reported value of 70% with FNS only. The experiments also provided insight into design requirements for future hybrid systems for stair navigation, the implications of which are discussed.

Distinguishing intrinsic from extrinsic factors underlying firing rate saturation in human motor units

During voluntary contraction, firing rates of individual motor units (MUs) increase modestly over a narrow force range beyond which little additional increase in firing rate is seen. Such saturation of MU discharge may be a consequence of extrinsic factors that limit net synaptic excitation acting on motor neurons (MNs) or may be due to intrinsic properties of the MNs. Two sets of experiments involving recording of human biceps brachii MUs were carried out to evaluate saturation. In the first set, the extent of saturation was quantified for 136 low-threshold MUs during isometric ramp contractions. Firing rate-force data were best fit by a saturating function for 90% of MUs recorded with a maximum rate of 14.8 ± 2.0 impulses/s. In the second set of experiments, to distinguish extrinsic from intrinsic factors underlying saturation, we artificially augmented descending excitatory drive to biceps MNs by activation of muscle spindle afferents through tendon vibration. We examined the change in firing rate caused by tendon vibration in 96 MUs that were voluntarily activated at rates below and at saturation. Vibration had little effect on the discharge of MUs that were firing at saturation frequencies but strongly increased firing rates of the same units when active at lower frequencies. These results indicate that saturation is likely caused by intrinsic mechanisms that prevent further increases in firing rate in the presence of increasing synaptic excitation. Possible intrinsic cellular mechanisms that limit firing rates of motor units during voluntary effort are discussed.

A numeric study of the noise-induced tremor in a mathematical model of the stretch reflex

A mathematical model of the stretch reflex for the cat soleus muscle is presented. The time-delay differential equations of the model are solved using the fourth-order Runge-Kutta algorithm, introducing a Gaussian-noise term to simulate the environmental noise. The muscle response dynamics are then studied under various levels of average muscle activation. Finally, the feasibility of explaining the so-called physiological tremor from the properties of the stretch reflex mechanisms is discussed by comparing our results with reported experimental evidence.

Hill muscle model errors during movement are greatest within the physiologically relevant range of motor unit firing rates

This study evaluated the accuracy of Hill-type muscle models during movement. Hill-type models are ubiquitous in biomechanical simulations. They are attractive because of their computational simplicity and close relation to commonly measured experimental variables, but there have been surprisingly few experimental validations of these models during functionally relevant conditions. Our hypothesis was that model errors during movement are largest at the low motor unit firing rates most relevant to normal movement conditions. This hypothesis was evaluated in the cat soleus muscle activated either by electrical stimulation at physiological rates or via the crossed-extension reflex (CXR) thereby obtaining normal patterns of motor unit recruitment and rate modulation. These activation paradigms were applied during continuous movements approximately matched to locomotor length changes. The resulting muscle force was modeled using a common Hill model incorporating independent activation, tetanic length-tension and tetanic force-velocity properties. Errors for this model were greatest for stimulation rates between approximately 10-20Hz. Errors were especially large for muscles activated via the CXR, where most motor units appear to fire within this range. For large muscle excursions, such as those seen during normal locomotion, the errors for naturally activated muscle typically exceeded 50%, supporting our hypothesis and indicating that the Hill model is not appropriate for these conditions. Subsequent analysis suggested that model errors were due to the common Hill model's inability to account for the coupling between muscle activation and force-velocity properties that is most prevalent at the low motor unit firing rates relevant to normal activation.

Effects of excitatory modulation on intrinsic properties of turtle motoneurons

The purpose of this study was to quantify the effects of excitatory modulation on the intrinsic properties of motoneurons (MNs) in slices of spinal cord taken from the adult turtle. Responses were noted following application of an excitatory modulator: serotonin (5-HT), muscarine, trans-1-amino-1,3-cyclopentane dicarboxylic acid (tACPD), or all three combined. A sample of 44 MNs was divided into 2 groups, on the basis of whether MNs did (28/44) or did not (16/44) demonstrate a nifedipine-sensitive acceleration of discharge during a 2-s, intracellularly injected stimulus pulse. Such acceleration indicates the development of a plateau potential (PP). Excitatory modulation lowered the MNs' resting potential, increased input resistance, decreased rheobase, reduced several afterhyperpolarization values, and shifted the conventional, one-phase stimulus current-spike frequency (I-f) relation to the left. For both MN groups, the relative efficacy of excitatory modulation on both non-PP and PP MNs was generally in the following order: combined application > 5-HT approximately muscarine > tACPD. In many instances, the effects of modulation differed significantly for non-PP versus PP MNs, the most pronounced being in their I-f relation. To describe this difference, it was necessary to measure a two-phase relation. In PP MNs, excitatory modulation considerably increased the slope of the first (initial) phase and flattened the second (later) phase of this relation. The latter result bore similarities to that obtained in a previous study, which addressed MN firing behavior during fictive locomotion of the high-decerebrate cat.

Input-output functions of mammalian motoneurons

Our intent in this review was to consider the relationship between the biophysical properties of motoneurons and the mechanisms by which they transduce the synaptic inputs they receive into changes in their firing rates. Our emphasis has been on experimental results obtained over the past twenty years, which have shown that motoneurons are just as complex and interesting as other central neurons. This work has shown that motoneurons are endowed with a rich complement of active dendritic conductances, and flexible control of both somatic and dendritic channels by endogenous neuromodulators. Although this new information requires some revision of the simple view of motoneuron input-output properties that was prevalent in the early 1980's (see sections 2.3 and 2.10), the basic aspects of synaptic transduction by motoneurons can still be captured by a relatively simple input-output model (see section 2.3, equations 1-3). It remains valid to describe motoneuron recruitment as a product of the total synaptic current delivered to the soma, the effective input resistance of the motoneuron and the somatic voltage threshold for spike initiation (equations 1 and 2). However, because of the presence of active channels activated in the subthreshold range, both the delivery of synaptic current and the effective input resistance depend upon membrane potential. In addition, activation of metabotropic receptors by achetylcholine, glutamate, noradrenaline, serotonin, substance P and thyrotropin releasing factor (TRH) can alter the properties of various voltage- and calcium-sensitive channels and thereby affect synaptic current delivery and input resistance. Once motoneurons are activated, their steady-state rate of repetitive discharge is linearly related to the amount of injected or synaptic current reaching the soma (equation 3). However, the slope of this relation, the minimum discharge rate and the threshold current for repetitive discharge are all subject to neuromodulatory control. There are still a number of unresolved issues concerning the control of motoneuron discharge by synaptic inputs. Under dynamic conditions, when synaptic input is rapidly changing, time- and activity-dependent changes in the state of ionic channels will alter both synaptic current delivery to the spike-generating conductances and the relation between synaptic current and discharge rate. There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions. Even under steady-state conditions, the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood, due to the paucity of direct recordings from motoneuron dendrites. It seems likely that resolving these important issues will keep motoneuron afficiandoes well occupied during the next twenty years.

Mechanomyography and electromyography force relationships during concentric, isometric and eccentric contractions

The purpose of this study was to investigate systematically if complementary knowledge could be obtained from the recordings of electromyography (EMG) and mechanomyography (MMG) signals. EMG and MMG activities were recorded from the first dorsal interosseous muscle during slow concentric, isometric, and eccentric contraction at 0, 25, 50, 75 and 100% of the maximal voluntary contraction (MVC). The combination of the EMG and MMG recordings during voluntary concentric-isometric-eccentric contraction showed significant different non-linear EMG/force and MMG/force relationships (P<0.001). The EMG root mean square (rms) values increased significantly from 0 to 50% MVC during concentric and isometric contraction and up to 75% MVC during eccentric contraction (P<0.05). The MMG rms values increased significantly from 0 to 50% MVC during concentric contraction (P<0.05). The non-linear relationships depended mainly on the type and the level of contraction together with the angular velocity. Furthermore, the type of contraction, the contraction level, and the angular velocity influenced the electromechanical efficiency evaluated as the MMG to EMG ratio (P<0.05). These results highlight that EMG and MMG provide complementary information about the electrical and mechanical activity of the muscle. Different activation strategies seem to be used during graded isometric and anisometric contraction.

Whole muscle length-tension properties vary with recruitment and rate modulation in areflexive cat soleus

The length-tension relationship is a fundamental property of muscle. In its classic form, which is used in muscle models incorporated into studies of motor control, the length-tension relationship is measured during maximal activation via tetanic electrical stimulation in whole muscles or during high intracellular calcium levels in single muscle fibers. In this study, we measured the length-tension relationship of the cat soleus muscle during different levels of natural activation consisting of recruitment and rate modulation of motor units generated by the crossed extension reflex. The ipsilateral dorsal roots were cut to eliminate sensory feedback from the soleus. Length-tension was measured by large shortening steps that transiently allowed force to drop to zero. Force then recovered to a new steady value as the shorter length was maintained for several seconds. The effects of various levels of crossed extension activation on length-tension were compared with direct electrical stimulation of the muscle at 5, 10, 20, and 100 Hz. At all levels of crossed extension, the slope of the length-tension function was much steeper than the slope for tetanic stimulation at 100 Hz. Most slopes for crossed extension fell between the slopes seen with electrical stimulation at 10 and 20 Hz. There was a modest overall tendency for slope to decrease with the level of crossed extension activation. Because much of the normal movement repertoire requires submaximal activation, muscle models based on the tetanic length tension relationship will greatly underestimate the contribution of this relationship to force modulation at different muscle lengths.

Stability of motor-unit force thresholds in the decerebrate cat

To further test the hypothesis that some fixed property of motoneurons determines their recruitment order, we quantified the variation in force threshold (FT) for motoneurons recruited in muscle stretch reflexes in the decerebrate cat. Motor axons supplying the medial gastrocnemius (MG) muscle were penetrated with micropipettes and physiological properties of the motoneuron and its muscle fibers, i.e., the motor unit, were measured. FT, defined as the amount of MG force produced when the isolated motor unit was recruited, was measured from 20 to 93 consecutive stretch trials for 29 motor units. Trials were selected for limited variation in base force and rate of rise of force, which have been shown to covary with FT, and in peak stretch force, which gives some index of motor-pool excitability. Under these restricted conditions, large variation in FT would have been inconsistent with the hypothesis. Analysis of the variation in FT employed the coefficient of variation (CV), because of the tendency for FT variance and mean to increase together. We found that CV was distributed with a median value of 10% and with only 2 of 29 units exceeding 36%. Some of this variation was associated with measurement error and with intertrial fluctuations in base, peak, and the rate of change of muscle force. CV was not significantly correlated with motor-unit axonal conduction velocity, contraction time, or force. In three cases FT was measured simultaneously from two motor units in the same stretch trials. Changes in recruitment order were rarely observed (5 of 121 stretch trials), even when FT ranges for units in a pair overlapped. We suggest that the large variation in recruitment threshold observed in some earlier studies resulted not from wide variation in the recruitment ranking of motoneurons within one muscle, but rather from variation in the relative activity of different pools of motoneurons. Our findings are consistent with the hypothesis that recruitment order is determined by some fixed property of alpha-motoneurons and/or by some unvarying combination of presynaptic inputs that fluctuate in parallel.

Isovelocity investigation of the lengthening behaviour of the erector spinae muscles

The purpose of this study was to investigate the force-velocity (F/v) relationship for the erector spinae muscles in submaximal activation movements, with particular attention to their response during lengthening movements and at lower shortening contraction velocities. Dynamic models that predict lower back muscle forces require reasonable representations of the modulating effect of instantaneous velocity. Ten males were observed performing trunk flexion and extension in the sagittal plane under constant load. Contraction velocities were measured as the first derivative from a devise sensitive to changes in spine curvature, and controlled by a visual feedback system while a constant load was applied through a chest harness. The erector spinae exhibited a yielding phenomenon which causes an abrupt drop in force during constant velocity stretching under constant, submaximal, stimulation. The findings were consistent with previous isovelocity muscle lengthening experiments. Yielding appeared dependent on the level of load/activation supporting the theory of a "state-variable" F/v relationship. The eccentric behaviour of the lower erectors (L3) seemed independent of velocity and length, while that of the upper erectors (T9) showed a dependence on length. At lower concentric velocities, concavity in torque-velocity curves was noted after a "threshold" velocity. The findings of this study strongly reinforce the notion that the F/v length relationship is not a continuous hyperbolic relationship during muscle shortening and that the commonly modelled force augmentation effect of lengthening is incorrect, at least for submaximal activation of the extensors of the lower back.

Electromyographic basis of inaccurate movement; its dependence upon the mode of muscle contraction

The electromyographic basis of inaccurate performance was investigated in two rapid precision-grip skills controlled by concentric and eccentric muscle contractions respectively. Surface electromyograms, recorded from the first dorsal interosseous (DI), adductor pollicis (AP) and abductor pollicis brevis, were utilised to identify changes in the timing and intensity of muscle activation which may be responsible for inaccurate performance. The results showed that when fast precision-grip skills were controlled by concentric DI and AP muscle contraction, variations in the intensity of muscle contraction were responsible for inaccurate performance. However, when these skills were controlled by eccentric DI and AP muscle contractions, inaccurate performance resulted from variations in the timing of muscle activation. It was concluded that the nature of the deficiency in the patterns of muscle activation resulting in inaccurate performance was dependent upon the type of muscle contraction used in the skill.

Force and EMG signal patterns during repeated bouts of concentric or eccentric muscle actions

Healthy males (n = 14) performed three bouts of 32 unilateral, maximal voluntary concentric (CON) or eccentric (ECC) quadriceps muscle actions on separate days. Surface electromyography (EMG) of the m. vastus lateralis (VL) and m. rectus femoris (RF) and torque were measured. Integrated EMG (IEMG), mean (MPF) and median power frequencies and torque were averaged for seven separate blocks of four consecutive muscle actions. Torque was greater (P less than 0.05) for ECC than for CON muscle actions at the start of exercise. It did not decline throughout ECC exercise, but decreased (P less than 0.05) markedly for each bout and over bouts of CON exercise. Thus, torque overall was substantially greater (P less than 0.05) for ECC than for CON exercise. At the start of exercise IEMG of VL or RF was greater (P less than 0.05) for CON than for ECC muscle actions. This was also true for overall IEMG activity during exercise. The IEMG increased (P less than 0.05) modestly for both muscles during each bout of CON or ECC muscle actions, but did not change for the VL over bouts. The IEMG of RF decreased (P less than 0.05) modestly over CON but not ECC exercise bouts. At the beginning of the first bout of exercise the IEMG/torque ratio was twofold greater (P less than 0.05) for CON than ECC muscle actions. The ratio of IEMG/torque increased (P less than 0.05) markedly during CON but did not change during ECC exercise. Thus, by the end of the third bout there was a fivefold difference (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Bistability of alpha-motoneurones in the decerebrate cat and in the acute spinal cat after intravenous 5-hydroxytryptophan

1. In the preceding paper (Crone, Hultborn, Kiehn, Mazieres & Wigström, 1988) it was shown that a short-lasting synaptic excitation ('on' stimulus) of extensor motoneurones (primarily triceps surae) in the decerebrate cat often resulted in a maintained excitability increase, which could be reset by a short-lasting inhibitory stimulus train ('off' stimulus). In the present experiments intracellular recording from triceps surae motoneurones and the electroneurogram (ENG activity) from triceps surae nerve branches were performed in parallel. 2. Sustained firing of individual triceps surae motoneurones was most often recorded in parallel with the maintained ENG activity following a synaptic 'on' stimulus. When the motoneurone was silenced, by a hyperpolarizing current through the microelectrode, there was no sign of on-going synaptic excitation during the maintained ENG activity following an 'on' stimulus. It was therefore suggested that voltage-dependent intrinsic properties of the motoneurones themselves could be responsible for the maintained firing. 3. In confirmation of this hypothesis it was found that short-lasting depolarizing current pulses through the recording microelectrode could trigger a self-sustained firing in the motoneurone provided that the bias current (i.e. the holding potential) was kept within certain limits. Hyperpolarizing current pulses terminated the firing. When the spike-generating mechanism was inactivated (by long-lasting excessive depolarization) similar depolarizing and hyperpolarizing current pulses could initiate and terminate plateau potentials in the motoneurones. By grading the depolarizing current pulses it was found that the plateau potentials were of all-or-none character, typically around 10 mV in amplitude. The two levels of excitability which can be triggered by short-lasting excitation and inhibition of the motoneurones is referred to as 'bistable' behaviour of the motoneurones. 4. After an acute spinal transection, in the unanaesthetized cat, the bistable behaviour of the motoneurones disappeared. However, it reappears following intravenous injection of the serotonin precursor 5-hydroxytryptophan (50-120 mg/kg). 5. Individual triceps surae motor units were recorded by selective EMG electrodes during tonic stretch reflexes in the decerebrate preparations. Based on an analysis of their firing pattern during lengthening and shortening (or vibration) of the muscle it is suggested that plateau potentials in motoneurones are recruited during the tonic stretch reflex. Furthermore, it is argued that a quantitatively important part of the depolarization of motoneurones during the tonic stretch reflex indeed originates from these plateau potentials.(ABSTRACT TRUNCATED AT 400 WORDS)