Motor unit synchronous firing as revealed by determinism of surface myoelectric signal.

Information on motor strategies can be extracted from the surface electromyogram (EMG) by non-linear methods. The percentage of determinism (%DET) obtained from recurrence quantification analysis (RQA) may be a sensitive variable to detect synchronous motor unit behaviour. The purpose of the present study was to validate this methodology by comparing it with an established technique estimating the degree of synchronization of pairs of voluntary activated motor units from the correlation of their firing in the time-domain. Single motor unit activity was recorded in extensor carpi radialis (ECR) muscle by pairs of tungsten microelectrodes inserted into the muscle belly. Cross-correlation analysis was performed in order to determine synchronization peak area by computing synchronous impulse probability. Surface EMG activity was recorded in parallel by electrodes placed over the skin of the same muscle and %DET was used as a measure of synchronous activity. The %DET appeared to be a valid measure of synchronization yielding results comparable to those obtained with cross-correlation analysis. Increases in %DET (t = 64.59, P < 0.0001) highly correlated (r2 = 0.70, P = 0.0013) with pharmacologically induced increases in the synchronization activity of pairs of ECR motor units (t = 8.71, P < 0.0001). RQA may be used as an alternative methodology for testing synchronous motor unit behaviour from surface EMG under physiological and pathological conditions.

Eye, head and skeletal muscle spindle feedback in the elaboration of body references.

Evidence is presented to support the notion that the sensory feedback originating in muscles is of major importance in the central elaboration of motor representation. The muscle spindle messages during movement and postural performance may be processed in order to elaborate continuously updated static and dynamic body references. These may then form the basis for the interpretation of retinal information in terms of spatial coordinates. The main arguments supporting this view are as follows. Experimental manipulation of muscle spindle proprioceptive feedback by tendon vibration induced segmental or postural kinaesthetic illusory movements in the direction of stretch of the vibrated muscles. By modifying the spatial distribution (agonists and/or antagonists), the frequency and the duration of the vibratory stimuli it is possible to induce simple or complex kinaesthetic illusions the parameters of which may be predicted. Microneurographic recordings confirmed that vibration rather selectively excited spindle Ia afferents, eliciting 1:1 driving up to 80-100 Hz. Moreover painless vibration, applied at increasing frequency (from 10 to 80 Hz) to either the medial, lateral, superior or inferior rectus of a subject's eye, was found to induce directional perceptual and motor effects which were closely related to the postural context. Likewise, the subjects reported illusory directional shifts of a visually fixed target in darkness during extraocular muscle vibration. These data suggest that extraocular proprioception contributes to the coding of eye, head and body position in relation to postural and environmental conditions. As with eye muscle stimulation, directional visual and postural effects were induced by vibration of neck and/or ankle postural muscles. These effects were found to summate when vibrations were applied simultaneously to the eye, neck and ankle muscles. The likely involvement of extraocular proprioception in interrelating body space with extrapersonal space in oriented behaviour is discussed.

Ia presynaptic inhibition in human wrist extensor muscles: effects of motor task and cutaneous afferent activity.

The task-dependence of the presynaptic inhibition of the muscle spindle primary afferents in human forearm muscles was studied, focusing in particular on the modulation associated with the co-contraction of antagonist muscles and the activation of cutaneous afferents. The changes known to affect the motoneuron proprioceptive assistance during antagonist muscle co-activation in human leg and arm muscles were compared. The evidence available so far that these changes might reflect changes in the presynaptic inhibition of the muscle spindle afferent is briefly reviewed. The possible reasons for changes in presynaptic inhibition during the antagonist muscle co-contraction are discussed. Some new experiments on the wrist extensor muscles are briefly described. The results showed that the changes in the Ia presynaptic inhibition occurring during the co-contraction of the wrist flexor and extensor muscles while the hand cutaneous receptors were being activated (the subject's hand was clenched around a manipulandum) could be mimicked by contracting the wrist extensor muscles alone while applying extraneous stimulation to the hand cutaneous receptors. It is concluded that besides the possible contribution of inputs generated by the co-contraction of antagonist muscles and by supraspinal pathways, cutaneous inputs may play a major role in modulating the proprioceptive assistance during manipulatory movements.

Effects of tonic vibration reflex on motor unit recruitment in human wrist extensor muscles.

Tonic vibration reflex was used to investigate the effects of muscle spindle Ia afferent activation on motor unit (MU) recruitment in human wrist extensor muscles. The MU force recruitment threshold recorded in the extensor carpi radialis muscles were quantitatively compared under two experimental situations: (1) during tonic isometric reflex contractions induced by mechanical tendon vibration and during voluntary contractions performed at the same velocity; (2) during two voluntary imposed ramp contractions (0.25 N.s-1) performed the one immediately before, and the other immediately after a tonic vibration reflex. In the first situation, it was observed that the Ia afferents activated by tendon vibration exerted a strong homonymous facilitatory action on their bearing muscles (extensor carpi radialis longus and brevis), while their heteronymous action on the synergistic muscle (extensor carpi ulnaris) was very weak. The MU recruitment thresholds in the extensor carpi radialis muscles were therefore significantly lower during the tonic reflex contraction than during the voluntary contraction. In the second situation, the tonic vibration reflex induced a facilitatory after-effect which decreased the MU recruitment thresholds during the subsequent voluntary imposed ramp contraction. It is suggested that this post-vibratory effect may have been due either to a postsynaptic potentiation of the motoneurones or to a reflex sensitization of the muscle spindles increasing their response to voluntary isometric contraction and consequently, increasing their facilitatory reflex action on the motoneurone pool.

Electromechanical coupling and synchronous firing of single wrist extensor motor units in sporadic amyotrophic lateral sclerosis.

Electrical and contractile properties of motor units (MU) were studied in the extensor carpi radialis muscles during voluntary contraction. The discharge of 234 single MUs was recorded in 11 patients with sporadic amyotrophic lateral sclerosis (ALS) and compared with that of the 260 MUs recorded in 12 healthy control subjects. Characteristics of the MU twitches and of the macro-potentials, the electromechanical coupling and the synchronization of the motor neurone discharges, were compared. In 5 patients (population ALS1), the twitch contraction force and macro-MUP area values were much larger than those of the controls. In the 6 other patients (population ALS2), the twitch force was considerably depressed, whereas the macro-MUP area was slightly, but significantly, increased. In ALS1, as well as in ALS2, the electromechanical coupling was much weaker than in the controls, and the fast-contracting MUs were more severely affected than the slowly contracting MUs. The motoneuronal synchronization was assessed by performing cross-correlation analysis on MUs discharges, and was used as an index to the strength of the common motoneuronal inputs. The rate of occurrence of synchronous firing was conspicuously lower in both populations of patients than in the control group. This might reflect the loss of corticospinal projections that occurs in ALS. The data are discussed in terms of the time course of motor neurone axonal sprouting, and in terms of the neuronal and muscular dysfunction possibly involved in ALS disease.

Task dependence of Ia presynaptic inhibition in human wrist extensor muscles: a single motor unit study.

OBJECTIVE: Task-dependent changes in the Ia presynaptic inhibition generated by flexor group I afferents were investigated in 25 identified motor units (MUs) located in human extensor carpi radialis (ECR) muscles. METHODS: Seven subjects had to voluntarily contract their ECR muscles either alone during isometric wrist extension or concurrently with their wrist and finger flexor muscles while clenching their hand around a manipulandum. The MU reflex responses to the radial nerve stimulation (test stimulation) yielded narrow peaks in the post-stimulus time histograms (PSTH). The Ia presynaptic inhibition induced while stimulating the median nerve (conditioning stimulation) 20 and 40 ms before the radial nerve was assessed from the changes in the contents of the first 0.5 ms in the peaks. RESULTS: With both stimulation intervals, the Ia presynaptic inhibition, as assessed from the first 0.5 ms of the PSTH peaks, was consistently weaker during hand clenching. With both motor tasks, the Ia presynaptic inhibition was strongest at the 20 ms interval, in which it showed a downward gradient, working from slow to fast contracting MUs. With both intervals, the presynaptic inhibition was consistently weaker during hand clenching. The decrease in the Ia presynaptic inhibition observed at the 40 ms conditioning-test interval was less pronounced during wrist extension. CONCLUSION: It is suggested that the reason why Ia presynaptic inhibition was weaker during hand clenching may have been that this task involved numerous cutaneous inputs originating from the palm and finger tips. During gripping tasks, these cutaneous inputs may therefore contribute to adjusting the wrist stiffness by relieving the presynaptic inhibition.

Encoding of the angular position of the two distal joints of the spiny lobster antennae by a single mechanoreceptor.

In the spiny lobster antennae, the two distal joints (J2, J3) rotate in approximately the same plane, their combined movements extending and flexing the flagellum forwards and backwards from the cephalothorax. A single mechanoreceptor (the chordotonal organ CO J2.J3) provides information about movement and angular position of these two joints. Four categories of static sensory neurones were identified. The specific neurones are selectively sensitive to either the J2 position or the J3 position. Homo-directional neurones code J2- and J3-flexed positions or J2- and J3-extended positions. Hetero-directional neurones code the position of each of the two joints in opposite ways, increasing and then decreasing their activity when J3 and J2 are successively flexed. The origin of these different kinds of static response is discussed with regard to the morphology and mechanical properties of the chordotonal organ and to the distribution of the bipolar sensory neurones on the elastic sheet which forms this proprioceptor. Its functional properties are briefly discussed with regard to the contribution of the CO J2.J3 to reflex actions. The sensitivity of this receptor is compared to that of joint receptors in other species.

Response to pressure and vibration of slowly adapting cutaneous mechanoreceptors in the human foot.

Unit activity from slowly adapting cutaneous mechanoreceptors of the foot and of the tibial part of the leg was recorded in man using the microneurography method. All the receptors increased their discharge frequency when pressure was applied to the zone of maximum sensitivity of their receptive field. They were also very sensitive to mechanical vibrations of small amplitude (0.2-0.5 mm) applied precisely to their zone of maximum sensitivity. Some of them are able to respond one-to-one to vibration at frequencies up to 200 Hz. The conditions of activation of the slowly adapting receptors seem to exclude a major contribution to kinaesthetic effects induced by vibrations applied to the tendon of tibialis anterior and extensor digitorum longus muscles [12].

Fluctuations in motor unit recruitment threshold during slow isometric contractions of wrist extensor muscles in man.

Motor unit activity was recorded in the two extensor carpi radialis muscles with metal microelectrodes during isometric contractions. When tested at different 'free' increasing contraction velocities the recruitment thresholds (rt) systematically decreased, whereas when tested at different 'imposed' increasing velocities the rt exhibited great variability, with no observable overall tendency. Moreover, when measured at a given test velocity imposed periodically during MU investigation, rt also displayed great variability. These results suggest that the notion of rt tends to be oversimplified. The motoneurone pool excitability is highly dependent on both the experimental situation and the complexity of the motor task.

Vibration sensitivity of slowly and rapidly adapting cutaneous mechanoreceptors in the human foot and leg.

The activities of 30 rapidly adapting cutaneous receptors (FA) and 23 slowly adapting cutaneous receptors (SA) were recorded from the lateral peroneal nerve using the microneurographic method. Their sensitivity to mechanical vibrations with constant amplitude applied at various frequencies to the center of the receptive field was studied. These two populations of cutaneous receptors were found to be very sensitive to this stimulus: they could be driven in a one to one manner up to between 100 and 200 Hz. The difference lay in the response observed when the vibration frequency was increased to above this critical value: the FA receptors sharply stopped firing, whereas the SA receptors became progressively unlinked from the stimulus. The effects of vibration on the physiological messages were also studied. The results showed that the messages coding the properties of tactile stimuli were either completely or partly masked by the receptor response to vibration. These vibration-induced modifications of cutaneous sensory messages might be at least partly responsible for the sensorimotor alterations observed when subjects are exposed to vibration.

Is the human masticatory system devoid of recurrent inhibition?

The aim of the present study was to investigate the existence or otherwise of a functional recurrent inhibitory system (Renshaw cell system) in the motoneurons that innervate human masticatory muscles. In a previous study, L: -acetylcarnitine (L: -Ac), a substance known to potentiate recurrent inhibition in humans was found to alter, in a specific way, the discharge variability, and the synchronous activity of motor units depending on the presence or absence of recurrent inhibition in the corresponding motoneuron pool. Using a similar paradigm, we have recorded the tonic discharge activity of motor unit pairs from the masseter muscle during voluntary isometric contraction while subjects were undergoing continuous intravenous saline (SAL, NaCl 0.9%) perfusion. Following a brief baseline-recording period, the subjects were given a test injection of either L: -Ac or isotonic saline (SAL) in a double blind manner. The variability, synchronization, and coherence between the motor unit discharges were analysed during three successive periods: pre-injection, during injection, and post-injection, each lasting 2-3 min. Neither L: -Ac nor SAL injection induced a significant change in the inter-spike interval (ISI) or the coefficient of variation of the ISIs in the motor units tested. There were also no significant changes in the pattern of synchronous activity or in the coherence, which reflects the common frequency content of the unit discharges. Reminiscent of what had been observed previously with motoneurons without recurrent inhibition in the Abductor Digitorum Minimi muscle, the lack of effects of L: -Ac injection on the firing behaviour of masseter motoneurons may suggest that classical Renshaw cell inhibition is lacking in this motoneuron pool.

Morphology and physiology of a hair plate sensory organ located on the antenna of the rock lobster Palinurus vulgaris.

At the level of the J1 joint of each antenna of the rock lobster Palinurus vulgaris a hair plate sensory organ (hp) similar to those described in insects has been observed. The hp is located on the internal side of the S2 segment of the antenna, close to the soft articulating membrane of the J1 joint. It is formed by a triangular surface of cuticle about 3mm2 in area, covered with numerous hairs of different lengths (Figs. 1 and 2). Details of the hp were studied by scanning electron microscopy (Fig. 2). Physiological stimulation of the hp hairs occurs during medial movement of the J1 joint. Under this condition the soft articulating membrane rolls over the hairs and bends them progressively back onto the cuticle. Flexion of all the hairs corresponds to a medial movement of the J1 through 40 degrees. During this type of movement, the number of successively flexed hairs increases linearly (Fig. 3). Electrophysiological recordings of the hp sensory nerve correlated with selective mechanical stimulation of individual hairs demonstrated that each hair is innervated by a single sensory fiber. This sensory neurone responds phasically when the hair is flexed back onto the cuticle (as during an S2 medial movement) and when it returns to its resting position (as during an S2 lateral movement). Most of the sensory neurones are sensitive to the movement velocity of the hairs (Figs. 4 and 5). When the hair is maintained flexed its sensory neurone discharges tonically (Fig. 4). Electrical stimulation of the hp sensory nerve induced reflex actions in the external and internal rotator muscles of the segment S1. These effects were found to selectively activate the tonic motor command of these muscles (Fig. 6).

[Effects of caudate nucleus stimulations on the primary and secondary endings activity of soleus muscle spindle]. / Effets des stimulations du noyau caudé sur l'activité des terminaisons fusoriales primaires et secondaires du muscle soléaire.

Effects of repetitive stimulation of the contralateral caudate nucleus on the static discharge and dynamic sensitivity of soleus muscle spindle primary and secondary endings were studied in cats anaesthetized with Halothane (Fluothane). By progressive increasing the depth of anaesthesia two different fusimotor effects could be observed. 1. A static facilitatory effect; under light anaesthesia the static discharge of primary and secondary endings were strongly increased. The response of the primary endings to phasic stretch of the muscle was generaly decreased. 2. A depressant effect; under deep anaesthesia the static discharge of primary and secondary endings were decreased. The dynamic sensitivity of the primary endings was lightly increased or remained unaltered. The ventro-lateral part of the caudate nucleus is mainly responsible for these effects; Caudate nucleus fusimotor effects were compared with those induced by gamma dynamic and gamma static fibers stimulations.

Kinaesthetic role of muscle afferents in man, studied by tendon vibration and microneurography.

The characteristics of vibration-induced illusory joint movements were studied in healthy human subjects. Unseen by the subject, constant frequency vibration trains applied to the distal tendon of the Triceps or Biceps induced an almost constant velocity illusory movement of the elbow whose direction corresponded to that of a joint rotation stretching the vibrated muscle. Vibration trains of the same duration and frequency applied alternatively to the Biceps and Triceps evoked alternating flexion-extension illusory movements. During successive application of vibration trains at frequencies from 10 to 120 Hz, the perceived velocity of the illustory movements increased progressively from 10 to 70-80 Hz, then decreased from 80 to 120 Hz. The maximal perceived velocity was three times higher during alternating vibration of the Biceps and Triceps than during single muscle stimulation. Unit activity from 15 muscle spindle primary endings and five secondary endings located in Tibialis anterior and Extensor digitorum longus muscles were recorded using microneurography in order to study their responses to tendon vibration and passive and active movements of the ankle. Primary endings were all activated by low amplitude tendon vibration (0.2-0.5 mm) previously used to induce illusory movements of the elbow. The discharge of some was phase-locked with the vibration cycle up to 120 Hz, while others responded one-to-one to the vibration cycle up to 30-50 Hz, then fired in a sub-harmonic manner at higher frequencies. Secondary endings were much less sensitive to low amplitude tendon vibration. Primary and secondary ending responses to ramp and sinusoidal movements of the ankle joint were compared. During the movement, the primary ending discharge frequency was almost constant, while the secondary ending activity progressively increased. During ankle movements the primary ending discharge appeared mainly related to velocity, while some secondary activities seemed related to both movement velocity and joint angle position. Muscle spindle sensory ending responses to active and passive ankle movements stretching the receptor-bearing muscle (plantar flexion) were qualitatively and quantitatively similar. During passive reverse movements (dorsiflexion) most of the sensory endings stopped firing when their muscle shortened. Active muscle shortening (isotonic contraction) modulated differently the muscle spindle sensory ending discharge, which could stop completely, decrease or some times increase during active ankle dorsiflexion. During isometric contraction most of the muscle spindle sensory endings were activated. The characteristics of the vibration-induced illusory movements and the muscle spindle responses to tendon vibration and to active and passive joint movements strengthened the possibility of the contribution of primary endings to kinesthesia, as suggested by several previous works...

[Modulation of primary and secondary endings activity of tibialis anterior muscle spindles during contraction induced by caudate nucleus stimulations]. / Modulation de l'activité des terminaisons fusoriales primaires et secondaires du muscle tibial antérieur au cours de contractions induites par stimulations du noyau caudé.

1. Effects of repetitive stimulations of caudate nucleus were studied on static and dynamic sensitivities of contralateral flexor (tibialis anterior) muscle spindle primary and secondary endings in cats anaesthetized with Halothane (Fluothane). Fusimotor effects are interpreted in term of gamma dynamic and gamma static activation. 2. Threshold stimulation induce a gamma dynamic effect on the primary endings sensitivity. Secondary endings activity and muscle tension remainded unaltered. 3. A slight increase of the stimulus strength elicit a gamma dynamic-gamma static effect on the primary endings sensitivity. Simultaneously secondary endings activity was increased and muscle contractions induced. 4. Using graded voltage stimulation permit to increase primary and secondary endings activity without muscle contraction. 5. Two stimulations applied at sufficiently short time interval produce different fusimotor effects. The first stimulation induce the general motor effect previously obtained: gamma dynamic-gamma static modulation of primary endings sensitivity, increase of secondary endings activity, muscular contraction. The second stimulation elicit a gamma dynamic effect on primary endings sensitivity and a depressant effect on secondary endings activity during muscular contractions. 6. Tendon organs discharge during caudate nucleus stimulation was studied comparatively with the primary and secondary endings activity.