Activity of renshaw cells during fictive scratch reflex in the cat

Summary The experiments were performed on decerebrate curarized cats with a hindlimb either completely deafferented or partly deefferented. Through tactile stimulation of the pinna, a fictive scratch reflex was evoked and activity in muscle efferents was then observed, similar to that in actual scratching. The duration of the cycle was about 250 ms, with extensor activity during a short period of about 50 ms (S phase) and flexor activity during a much longer one (about 200 ms; L phase). Appearance of rhythmic bursts of discharges was preceded by tonic flexor activity (tonic phase of scratching). Discharges of Renshaw cells were recorded extracellularly during these three phases, in parallel with discharges in the gastrochemius-soleus nerve. During the tonic phase of the scratch reflex, some Renshaw cells with input from flexors decreased their activity while others increased. No change in Renshaw cell activity with input from extensors was then observed. During the rhythmic phases of the scratch reflex a majority of Renshaw cells was phasically active. They usually responded once per cycle, with a burst of 1–30 impulses of 50–100 ms duration, most often occurring at the end of the L phase and during the S phase. Bursts of Renshaw cells with input from flexors and of extensors, respectively, overlapped to a high degree. However, maximal firing of extensor-coupled Renshaw cells occurred somewhat later than that of flexor-coupled cells. Flexor-coupled Renshaw cells discharged mainly at the end of the L phase and during the S phase, i.e. when the flexor moto-neurones terminated their activity. Extensor-coupled Renshaw cells reached maximal activity during the S phase, i.e. when the extensor motoneurones were recruited. After spinal transection at C1 level, Renshaw cells responded with an increased number of spikes but without change in timing of the discharges during the scratch cycle. Most of the contralaterally located Renshaw cells studied were also phasically active during the scratch reflex. The role of motoneurones and spinal interneurones in determining the timing of Renshaw cell activity and the role of the latter in control of posture and rhythmic movements are discussed.     

On the function of recurrent inhibition in the spinal cord

Summary Recurrent inhibition of -motoneurons, via motor axon collaterals and Renshaw cells, obviously reduces the response (output) from a motor nucleus to a given synaptic input. It is proposed that the supraspinal convergence on Renshaw cells allows recurrent inhibition to serve as a variable gain regulator at motoneuronal level. This would allow for an optimal resolution in the force control during weak as well as strong contractions. Renshaw cells are not only inhibiting -motoneurons but also -motoneurons and Ia inhibitory interneurons. It is argued that this distribution is meaningful since all these receptive neurons act together as a functional unit, forming an output stage of the motor system.     

The effect of muscle vibration on human position sense during movements controlled by lengthening muscle contraction

Summary Muscle vibration studies suggest that during voluntary movement limb position is coded by muscle spindle information derived from the lengthening, antagonist muscle. However, these investigations have been limited to movements controlled by shortening contractions. This study further examined this property of kinesthesia during movements controlled by lengthening contraction. Subjects performed a horizontal flexion of the right forearm to a mechanical stop randomly positioned at 30, 50 and 70° from the starting position. The movement was performed against a flexor load (1 kg) requiring contraction of the triceps muscle. Vision was occluded and movements were performed under three conditions: no vibration, vibration of the right biceps and vibration of the right triceps. The perceived position of the right forearm was assessed by instructing subjects to simultaneously match the right limb position with the left limb. Vibration of the shortening biceps muscle had no effect on limb matching accuracy. However, triceps vibration resulted in significant overestimation of the vibrated limb position (10–13°). The variability in movement distance was uninfluenced by muscle vibration. During movements controlled by lengthening contraction, there is a concurrent gamma dynamic fusimotor input that would enhance primary afferent discharge. Despite this additional regulating input to the muscle spindle, it appears that muscle spindle information from the lengthening muscle is important for the accurate perception of limb movement and/or position.     

Pattern of heteronymous recurrent inhibition in the human lower limb

Changes in the firing probability of motor units belonging to leg and thigh muscles were used to describe the pattern of distribution of recurrent inhibition evoked by motor discharges from various motor nuclei in the human lower limb. Discharges of units in soleus, gastrocnemius medialis, peroneus brevis, tibialis anterior, quadriceps and biceps femoris were investigated following a conditioning stimulation which evoked either a monosynaptic reflex in quadriceps, triceps surae or peroneal motor neurones, or an antidromic motor volley in one of the following nerves: inferior soleus, gastrocnemius medialis, superficial peroneal, deep peroneal, or femoral nerve. In many motor unit-nerve combinations a trough in the post-stimulus time histogram, indicating an inhibition, appeared immediately after the heteronymous Ia excitation. This inhibition is thought to be Renshaw in origin, because it appeared and increased with the conditioning motor discharge, was independent of the conditioning stimulus intensity per se and had a long duration. These recurrent connections were widely distributed with a pattern very similar to that described for heteronymous monosynaptic Ia excitation. In particular Renshaw coupling between muscles operating at different joints seems to be the rule in the human lower limb.     

The sensitivity of rat spinal interneurones and Renshaw cells to L-glutamate and L-aspartate

Summary L-Glutamate and L-aspartate were administered electrophoretically near spinal interneurones and Renshaw cells of pentobarbitone-anaesthetized rats. Other spinal interneurones were consistently more sensitive to L-glutamate than to L-aspartate. Renshaw cells, however, showed no consistent difference in their sensitivity to these two amino acids. The results, which are compared with those reported previously in the cat, support the hypothesis that L-glutamate could be a transmitter at spinal primary afferent terminals.Wellcome Trust Research Training Scholar.Animal Health Trust/Wellcome Trust Research Fellow.     

Diffusion of myoglobin in skeletal muscle cells — dependence on fibre type,contraction and temperature

We measured the diffusion coefficient of myoglobin (D _Mb) inside mammalian skeletal muscle cells with a microinjection technique. A small bolus of horse Mb was injected into a single muscle fibre and the subsequent time-dependent changes of the Mb profiles along the fibre axis were measured with a microscope-photometer. For fibres of the rat soleus muscle at 22° C, a D _Mb of 1.3·10^–7 cm²/s was found, confirming a result obtained previously by us for rat diaphragm muscle with a photo-oxidation technique. In the extensor digitorum longus muscle of the rat, a higher value of 1.9 · 10^–7 cm²/s was measured. Auxotonic muscle contractions did not change the apparent D _Mb. For the temperature range between 22 ° C and 37 ° C, a temperature coefficient, Q ₁₀, of 1.5 was calculated. The implication of this result for the role of Mb in the facilitation of oxygen transport was examined. Model calculations show that with this relatively low D _Mb value, the intracellular oxygen supply can be improved only slightly.     

Autogenetic effects of muscle contraction on extensor gamma motoneurones in the cat

Summary The effect of isometric twitch contractions of hind limb muscles on the discharge of gamma motoneurones has been studied in decerebrated cats with thoracic spinal cord section. Contractions of flexor digitorum longus (FDL) or gastrocnemiussoleus (GS) strongly inhibited the background discharge of their homonymous gamma motoneurones. In contrast, contraction of FDL generally caused less inhibition, or did not affect the discharge, of the synergist GS gamma motoneurones. Both the autogenetic and synergist inhibition were considerably weaker in decerebrated cats with intact spinal cords.The inhibition lasted 20 to 50 ms and occurred principally during the rising phase of contraction. It could be followed or terminated by a weaker period of facilitation during relaxation of the muscle. Shortening the muscle so that no active tension developed during contraction either abolished or substantially curtailed the inhibition.We discuss the probability that the inhibition is a spinal segmental reflex brought about by impulses generated in Ib afferents of tendon organs. The inhibition is under tonic inhibitory control in the decerebrated cat when the spinal cord is intact. Unlike the Ib inhibition of alpha motoneurones that of gamma motoneurones has a particularly strong autogenetic component.MRC Scholar     

The origin of activity in the biceps brachii muscle during voluntary contractions of the contralateral elbow flexor muscles

During strong voluntary contractions, activity is not restricted to the target muscles. Other muscles, including contralateral muscles, often contract. We used transcranial magnetic stimulation (TMS) to analyse the origin of these unintended contralateral contractions (termed “associated” contractions). Subjects (n = 9) performed maximal voluntary contractions (MVCs) with their right elbow-flexor muscles followed by submaximal contractions with their left elbow flexors. Electromyographic activity (EMG) during the submaximal contractions was matched to the associated EMG in the left biceps brachii during the right MVC. During contractions, TMS was delivered to the motor cortex of the right or left hemisphere and excitatory motor evoked potentials (MEPs) and inhibitory (silent period) responses recorded from left biceps. Changes at a spinal level were investigated using cervicomedullary stimulation to activate corticospinal paths (n = 5). Stimulation of the right hemisphere produced silent periods of comparable duration in associated and voluntary contractions (218 vs 217 ms, respectively), whereas left hemisphere stimulation caused a depression of EMG but no EMG silence in either contraction. Despite matched EMG, MEPs elicited by right hemisphere stimulation were ∼1.5–2.5 times larger during associated compared to voluntary contractions (P < 0.005). Similar inhibition of the associated and matched voluntary activity during the silent period suggests that associated activity comes from the contralateral hemisphere and that motor areas in this (right) hemisphere are activated concomitantly with the motor areas in the left hemisphere. Comparison of the MEPs and subcortically evoked potentials implies that cortical excitability was greater in associated contractions than in the matched voluntary efforts.     

Disinhibition of upper limb motor area by voluntary contraction of the lower limb muscle

It is well known that monosynaptic spinal reflexes and motor evoked potentials following transcranial magnetic stimulation (TMS) are reinforced during phasic and intensive voluntary contraction in the remote segment (remote effect). However, the remote effect on the cortical silent period (CSP) is less known. The purpose of the present study is to determine to what extent the CSP in the intrinsic hand muscle following TMS is modified by voluntary ankle dorsiflexion and to elucidate the origin of the modulation of CSP by the remote effect. CSP was recorded in the right first dorsal interosseous while subjects performed phasic dorsiflexion in the ipsilateral side under self-paced and reaction-time conditions. Modulation of the peripherally-induced silent period (PSP) induced by electrical stimulation of the ulnar nerve was also investigated under the same conditions. In addition, modulation of the CSP was investigated during ischemic nerve block of the lower limb and during application of vibration to the tibialis anterior tendon. The duration of CSP was significantly shortened by phasic dorsiflexion, and the extent of shortening was proportional to dorsiflexion force. Shortening of the CSP duration was also observed during tonic dorsiflexion. In contrast, the PSP duration following ulnar nerve stimulation was not altered during phasic dorsiflexion. Furthermore, the remote effect on the CSP duration was seen during ischemic nerve block of the lower limb and the pre-movement period in the reaction-time paradigm, but shortening of the CSP was not observed during tendon vibration. These findings suggest that phasic muscle contraction in the remote segment results in a decrease in intracortical inhibitory pathways to the corticospinal tract innervating the muscle involved in reflex testing and that the remote effect on the CSP is predominantly cortical in origin.     

Limitation of muscle deoxygenation in the triceps during incremental arm cranking in women

The present study investigated the difference in oxygen kinetics in the exercising muscle between arm cranking and leg cycling in women. Twenty-seven females completed incremental arm cranking and leg cycling tests on separate days. During each exercise, spatially resolved near-infrared spectroscopy was used to measure changes in the tissue oxygen saturation (SO₂), oxygenated (oxy-) hemoglobin and/or myoglobin (Hb/Mb), deoxygenated (deoxy-) Hb/Mb, and total Hb/Mb in the triceps during arm cranking and in the vastus lateralis during leg cycling. During arm cranking, there was a rapid increase in the respiratory exchange ratio and a lower ventilatory threshold compared to leg cycling, which confirmed accelerated anaerobic glycolysis in this mode of exercise. During leg cycling, SO₂ remained decreased near to or until approaching peak oxygen uptake (O_2peak). During arm cranking, however, the decrease in oxy-Hb/Mb and increase in deoxy-Hb/Mb stopped at the middle of O_2peak (mean 51.4%), consequently resulting in a leveling off in the SO₂ decrease, although total Hb/Mb continued to increase. These results might suggest that the oxygen demand in the triceps attained the maximum at that intensity, despite an adequate oxygen supply during arm cranking.     

The “fastness” of rat motoneurones: time-course of afterhyperpolarization in relation to axonal conduction velocity and muscle unit contractile speed

In normal adult rats, intracellular recordings were obtained from motoneurones of the medial gastrocnemius (MG) and other tibial-nerve muscle branches. Among the whole population of tibial motoneurones, there was a significant negative correlation between the duration of afterhyperpolarization (AHP) and axonal conduction velocity (CV). However, this AHP vs CV relationship differed from that previously demonstrated in cats, in that for a given axonal CV, the AHPs were briefer in the rat than in the cat. For MG cells, twitches were also recorded from their muscle units. As has previously been observed in cats, there was a significant correlation between the time-course of the AHP of a motoneurone and the time-course of its muscle unit twitch. This relationship was, in principle, similar between the two species, but the AHPs and the twitches were briefer in the rat. The present results provide the first demonstration, for the rat model, of the presence of a functionally relevant “speed-match” between the intrinsic properties of α-motoneurones and those of their muscle units. Worked on the experiments while on leave of absence during subbatical year from Département d'éducation physique, Université de Montréal, Montréal, Québec, Canada H3C 3J7     

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

Summary 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 contractions, 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.     

Movement of the upper body and muscle activity patterns following a rapidly applied load: the influence of pre-load alterations

Sudden loading of the spine is not only considered a risk factor for the development of low-back pain but also enables an evaluation of the stability of the spine when conducted under laboratory conditions. In the present study the upper spine was pulled in the anterior direction and the stiffness as well as activity in the erector spinae muscle was measured with different pre-tension in the erector spinae. The results showed that increased activity in the erector spinae prior to loading led to increased stiffness (stiffness coefficients from 297 Nm rad^–1 to 438 Nm rad^–1) and a decrease in the extra neural signal input to the muscles to maintain the stability. It is therefore clear that increased tension in the erector spinae muscle will create a larger stability of the spine to anterior perturbations. However, contracting the muscles around the spine increases the load on the spinal structures. In 34% of the experiments a silent period in the electromyographic signal was present after loading in the period when the torso was moving in the anterior direction. This phenomenon is discussed.     

Frequency peaks of tremor, muscle vibration and electromyographic activity at 10 Hz, 20 Hz and 40 Hz during human finger muscle contraction may reflect rhythmicities of central neural firing

 The output from the central nervous system to muscles may be rhythmic in nature. Previous recordings investigating peripheral manifestations of such rhythmic activity are conflicting. This study attempts to resolve these conflicts by employing a novel arrangement to measure and correlate rhythms in tremor, electromyographic (EMG) activity and muscle vibration sounds during steady index finger abduction. An elastic attachment of the index finger to a strain gauge allowed a strong but relatively unfixed abducting contraction of the first dorsal interosseous (1DI). An accelerometer attached to the end of the finger recorded tremor, surface electrodes over 1DI recorded EMG signals and a heart-sounds monitor placed over 1DI recorded vibration. This arrangement enabled maintenance of a constant overall muscle contraction strength while still allowing measurement of the occurrence of tremulous movements of the finger. Ten normal subjects were studied with the index finger first extended at rest and then contracting 1DI to abduct the index finger against three different steady forces up to 50% of maximal voluntary contraction (MVC). Power spectral analysis of tremor, EMG activity and muscle vibration signals each revealed three frequency peaks occurring together at around 10 Hz, 20 Hz and 40 Hz. Coherence analysis showed that the same three peaks were present in the three signals. Phase analysis indicated a fixed time lag of tremor behind EMG of around 6.5 ms. This is compared with previous measurements of electromechanical delay. Other experiments indicated that the three peaks were of central nervous origin. Introducing mechanical perturbations or extra loading to the finger and making recordings under partial anaesthesia of the hand and forearm demonstrated preservation of all the peaks, suggesting that they did not originate from mechanical resonances or peripheral feedback loop resonances. It is concluded that, at least for a small hand muscle, there exist not one but a number of separate peak frequencies of oscillation during active contraction, and that these oscillations reflect synchronization of motor units at frequencies determined within the central nervous system. It is proposed that the multiple oscillations may be a means of frequency coding of motor commands. Received: 23 April 1996 / Accepted: 8 October 1996     

The impact of sarcopenia and exercise training on skeletal muscle satellite cells

It has been well-established that the age-related loss of muscle mass and strength, or sarcopenia, impairs skeletal muscle function and reduces functional performance at a more advanced age. Skeletal muscle satellite cells (SC), as precursors of new myonuclei, have been suggested to be involved in the development of sarcopenia. In accordance with the type II muscle fiber atrophy observed in the elderly, recent studies report a concomitant fiber type specific reduction in SC content. Resistance type exercise interventions have proven effective to augment skeletal muscle mass and improve muscle function in the elderly. In accordance, recent work shows that resistance type exercise training can augment type II muscle fiber size and reverse the age-related decline in SC content. The latter is supported by an increase in SC activation and proliferation factors that generally appear following exercise training. Present findings strongly suggest that the skeletal muscle SC control myogenesis and have an important, but yet unresolved, function in the loss of muscle mass with aging. This review discusses the contribution of skeletal muscle SC in the age-related loss of muscle mass and the efficacy of exercise training as a means to attenuate and/or reverse this process.     

The movement of muscle precursor cells between adjacent regenerating muscles in the mouse

Summary Regeneration of mature skeletal muscle fibers involves the formation of new multinucleate muscle fibres by the fusion together of mononucleate muscle precursor cells. Such precursor cells appear to be largely or entirely derived from satellite cells, located between the basement membrane and the sarcolemma of the muscle fibre. We have previously presented evidence that precursor cells which contribute to regenerating muscle in a region of muscle damage are not all locally derived but that some migrate in from exogenous sources. The present study examines the possibility that a regenerating muscle might receive muscle precursor cells from neighbouring muscles. To do this we have made whole muscle allografts in the mouse and used the two murine isoenzyme allotypes of the dimeric enzyme Glucose-6-Phosphate Isomerase (GPI) as markers to demonstrate whether there is movement of muscle precursor cells between these allografts and adjacent host muscles. In host muscles adjacent to some allografts, a hybrid form of GPI was detected, each molecule consiting of one donor and one host GPI subunit. Such heterodimers can form only where host and donor nuclei share a common cytoplasm: in muscles this means that mosaic host/donor muscle fibres are present. The presence of such fibres implies that muscle precursor cells must have migrated into the host muscle from the neighbouring allograft.     

Periodic increases in force during sustained contraction reduce fatigue and facilitate spatial redistribution of trapezius muscle activity

This study compared fatigue and the spatial distribution of upper trapezius electromyographic (EMG) amplitude during a 6-min constant force shoulder elevation task at 20% of the maximal voluntary contraction force (MVC) (constant force) and during the same task interrupted by brief (2 s) periodic increases in force to 25% MVC every 30 s (variable force). Surface EMG signals were recorded with a 13 × 5 grid of electrodes from the upper trapezius muscle of nine healthy subjects. The centroid (center of activity) of the EMG root mean square map was computed to assess changes over time in the spatial distribution of EMG amplitude. MVC force decreased by (mean ± SD) 9.0 ± 3.9% after the constant force task (P < 0.05) but was unchanged following the variable force contraction. The centroid of EMG amplitude shifted in the cranial direction across the duration of the variable force contraction (P < 0.05) but not during the constant force contraction (shift of 2.9 ± 2.3 mm and 1.4 ± 1.1 mm, respectively). The results demonstrate that periodic increases in force during a sustained contraction enhance the modifications in spatial distribution of upper trapezius EMG amplitude and reduce fatigue compared to a constant force contraction performed at a lower average load. The change in spatial distribution of EMG amplitude over time during a sustained contraction may reflect a mechanism to counteract fatigue during prolonged muscle activity.     

The dependence of cardiac contraction on depolarization and slow inward current

Summary The membrane potential of the guinea-pig's papillary muscle was controlled by a voltage clamp technique which employs one sucrose gap and one intracellular microelectrode. Both the membrane potential and the membrane current were recorded together with the contraction. The steady-state contractions were obtained after 6–8 contractions. The difference between the steady-state contractions and the succeeding test contractions was considered to be due to the difference in the trigger action between the conditioning and the test pulses. The following results were obtained: 1. when the test pulses were appropriately shortened so that a large tail of inward current flowed after the repolarization step, the contraction was augmented. This paradoxical increase of the contraction can be explained by assuming a direct action of the slow inward current on contraction. 2. The peak amplitude of the contraction increased with the size of the depolarizing step in the range of the membrane potential at which the calcium current should be small or zero. This suggests that in addition to the calcium current another effect of depolarization controls the contraction. 3. The rate of rise of contraction seemed to be determined by the calcium current which was maximum at a depolarization to +10 mV. At this level the slow inward current also reached the maximum. With further increase of the depolarization the rise of the contraction and the slow inward current were decreased. 4. During a series of the pulses the positive inotropic effect (staircase) was the more pronounced the larger the depolarization. This holds true for depolarization up to +70 mV. The potentiation occurred in spite of decrease of the slow inward current. It seems necessary to postulate a potentiating process which is dependent on the amplitude of the depolarization.This work was supported by the Deutsche Forschungsgemeinschaft.     

Effect of low frequency fatigue on human muscle strength and fatigability during subsequent stimulated activity

Summary Fatiguing contractions of the adductor pollicis muscle were produced by intermittent supramaximal stimulation of the ulnar nerve in a set frequency pattern, in six normal subjects. At the end of an initial fatiguing contraction series, low frequency fatigue (LFF) had been induced and persisted at 15 min of recovery. Stimulated fatiguing activity was then repeated in an identical fashion to the initial series. At high frequencies, declines in force were similar for both series. At low frequencies, declines in force were greater during the second series despite similar changes in compound muscle action potential amplitude. This confirmation that LFF persists during subsequent stimulated activity, and reduces low but not high frequency fatigue resistance, suggests that the impaired endurance of fatigued muscle during voluntary activity primarily results from peripheral changes at low frequency. These findings also have implications for therapeutic electrical stimulation of muscle.     

The distribution of activity among the muscles of a single group during isometric contraction

Summary The study of the relations between the electrical activity of a group of synergistic muscles and the biomechanical parameters characterizing the resultant movement is generally reduced to one of the muscles of this group, this muscle being considered as equivalent. This is in particular the case for the biceps brachii in the group of elbow flexor muscles. The validity of this concept in the case of isometric, isotonic contraction can be tested by the simultaneous utilization of surface-electrode and wire-electrode detection techniques.It has been shown 1. that the quadratic relation between the global integrated EMG of the biceps brachii and the external force resulting from the action of the group of flexors can be extended to each of the muscles of this group (brachialis, brachioradialis, pronator teres); 2. that the slope of the relation between the integrated EMG of the biceps brachii and the force differs according to whether the hand is supine or prone. An explanation of a mechanical nature has been proposed for this phenomenon; 3. that in a given situation the relations between the different integrated EMG's remain constant whatever the value of the force.It follows that 1. each value of the integrated EMG of a muscle equals a coefficient times the value of the force exerted by this muscle; 2. the notion of muscle equivalent is justified for a given situation.