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Biomedical subjects

A Fourment

Publications and source records attributed to A Fourment.

At least 19 recordsLinked to original sources
PubMed2002

Effects of postural muscle fatigue on the relation between segmental posture and movement.

The purpose of this study was to examine whether fatigue of postural muscles might influence the coordination between segmental posture and movement. Seven healthy adults performed series of fifteen fast wrist flexions and extensions while being instructed to keep a dominant upper limb posture as constant as possible. These series of voluntary movements were performed before and after a fatiguing submaximal isometric elbow flexion, and also with or without the help of an elbow support. Surface EMG from muscles Deltoïdeus anterior, Biceps brachii, Triceps brachii, Flexor carpi ulnaris, Extensor carpi radialis were recorded simultaneously with wrist, elbow and shoulder accelerations and wrist and elbow displacements. Fatigue was evidenced by a shift of the elbow and shoulder muscles EMG spectra towards low frequencies. Kinematics of wrist movements and corresponding activations of wrist prime-movers, as well as the background of postural muscle activation before wrist movement were not modified. There were only slight changes in timing of postural muscle activations. These data indicate that postural fatigue induced by a low-level isometric contraction has no effect on voluntary movement and requires no dramatic adaptation in postural control.

Adult↗
PubMed2001

Electromyographic and biomechanical characteristics of segmental postural adjustments associated with voluntary wrist movements. Influence of an elbow support.

This study re-investigates the characteristics of segmental postural adjustments associated with rapid mono-articular movements and analyses their dependence on initial postural conditions. Subjects performed rapid voluntary wrist flexions and extensions while maintaining their upper limb posture as stable as possible, with or without an elbow support. Surface electromyographic activity (EMG) was recorded from Flexor carpi ulnaris, Extensor carpi radialis, Biceps brachii, Triceps brachii and Deltoideus anterior. The kinematics of the three joints and kinetics in the support condition were also recorded. A planar mechanical model was used to determine the muscle torque required to keep the upper limb posture constant while performing wrist movements. All subjects showed anticipatory postural adjustments (APA) which, unlike those described for whole-body postural control, could not counteract in advance the perturbing inter-segmental forces created by the movement. Postural muscles were activated before the wrist movement with a chronology specific to the direction of the wrist movement. Some postural muscular activities anticipated that of the prime-movers in accordance with muscle torque, which had to be applied to the joints to keep the upper limb posture constant. These results reveal that the central nervous system (CNS) uses the same organization of the motor command for the control of both segmental and whole-body posture: APA and corrective postural adjustments (CPA), which are based on well-organized anticipatory postural muscle activities (APMA), except that APA can be non-efficient in segmental postural control. The presence or absence of an elbow support influenced the level of activation of postural muscle but not their chronology. This result suggests that the CNS uses a sequence of APMA: a postural muscle synergy which is predetermined as a function of the intended direction of the movements and modulates the gain towards certain muscles, in accordance with the gravitational effects, and supports reaction changes.

Action Potentials↗
PubMed1999

Chronology of upper limb anticipatory postural adjustments associated with voluntary wrist flexions and extensions in humans.

Six sitting healthy subjects were instructed to keep a constant upper limb posture while performing wrist flexions and extensions. Acceleration of the wrist, elbow and shoulder joints, and surface electromyograms (EMGs) of the upper limb's main flexors and extensors were studied. Results indicated the existence of anticipatory (APA) and corrective postural adjustments. The APAs were based on a reproducible directional chronology of postural muscle activations. As shown by a simple mechanical model, this chronology was in accordance with the muscular torque which should be applied to the joints to keep the upper limb posture constant. All these data indicate that APA are involved in segmental posture, where their general organization is similar to those of APA associated with whole-body movements. The use of constant directional postural synergies well agrees with a simplification of the motor control according to Bernstein's theory.

Arm↗
PubMed1998

Summation of elementary phonomyograms during isometric twitches in humans.

To study its summation principle, the phonomyogram (PMG) from the first interosseus dorsalis muscle was recorded in five subjects during single twitches evoked by electrical stimulation over the motor point. By increasing the current pulse from threshold to maximal intensity, PMG amplitude increased linearly with motor unit recruitment. The twitch amplitude-intensity relationship was also linear. The PMG amplitude was therefore linearly related to the external force. For all these relationships highly significant correlation coefficients were found. These relationships were interpreted as being a consequence of an orderly recruitment, although, contrary to what happens during voluntary contraction, the largest and strongest motor units were recruited before the smallest and weakest ones during axon electrical stimulation. The PMG onset always preceded twitch onsets as indicated by latency measurements [mean 3.2 (SD 1.3) ms versus 11.5 (SD 3.9) ms, respectively]. Moreover, PMG and twitch latencies may have been significantly reduced by recruitment, suggesting that orderly recruitment influenced both PMG amplitude and occurrence. These results were interpreted as being the result of the summation of elementary PMG from every contracting motor unit and the stiffness change of the muscle medium occurring with recruitment.

Acoustics↗
PubMed1996

Responses of motor cortical cells to short trains of vibration.

The response discharges of precentral motor cortical cells to brief trains of vibration applied to the tendon of biceps brachii were analyzed in two alert but passive monkeys. The activity of 20 phasic-tonic and 6 tonic cells was analyzed. All had functional linkages with flexor muscles during a preceding flexion task and responded to passive movement of the elbow. Taking as a reference the stereotyped reflex response in the stretched muscle, the effect of changes in the amplitude of a constant frequency vibration (4 vibrations at 58 Hz) was quantified statistically in peristimulus histograms of the cortical cell discharges. All cells were transiently influenced by low vibration amplitudes. Most responses (71%) were excitatory and occurred at a mean latency of 24 ms, which is consistent with cells activated by input from stretch receptors. Excitatory, reproducible responses to the lowest vibration amplitudes were more frequent in phasic-tonic than in pure tonic cells. Large-amplitude vibrations always excited the motor cortical cells. The sign of the responses to vibration matched that to passive elbow movements for most cells. These findings show that elbow-related motor cortical cells are very sensitive to proprioceptive input from primary spindle afferents.

Animals↗
PubMed1996

Adaptation of the precentral cortical command to elbow muscle fatigue.

The control exerted by individual motor cortical cells on their fatigued target muscles was assessed by analyzing the discharge patterns and electromyographic (EMG) postspike effects of cortical cells in monkeys making repeated forceful, but submaximal, isometric flexions of the elbow to produce fatigue. Two monkeys were trained to perform self-paced isometric contractions (for longer than 2 s) at forces greater than 35% maximal contraction, with three sets of 20 consecutive contractions; the first and last sets were at the same force level. Pairs of EMG electrodes were implanted in the biceps brachii, brachioradialis, and triceps brachii. The cortical cell discharges were modulated with the active and passive movements of the elbow and produced consistent EMG postspike effects during isometric contraction. Muscle fatigue was assessed as a statistically significant (P < 0.05) drop in the mean power frequency of the EMG power spectrum in one or both flexors in the last set of contractions. Clear signs of muscular fatigue occurred in 20 different experimental sessions. Before fatigue, cortical cells were classified as phasic-tonic (18), phasi-cramp (three), or tonic (five). Twenty cells briskly fired to passive elbow extension, and 9 also responded to passive flexion. Only 6 cells showed a decreased discharge to passive extension. A 22-30% increase in the contraction force produced a higher discharge frequency in 13 cells, and a lower frequency in 5 cells. All cells exerted EMG postspike effects in their target muscles: 20 cells facilitated the flexors, and some of these also inhibited (3 cells) or cofacilitated (5 cells) the extensor; the other 6 cells had mixed effects: 5 of them inhibited at least one flexor, and 1 cell only facilitated the extensor. Most cells (24/26) still produced EMG postspike effects in their target muscles during fatigue, and the number of facilitated muscles increased: 21 cells facilitated the flexors, and 12 of them cofacilitated the extensor. Only 3 cells still inhibited the flexors and were tonic cells. The cortical cell firing frequency increased during fatigue in 13 cells and decreased in 8 cells. Increases involved 10 cells excited by passive elbow extension. Fourteen cells showed parallel changes in firing frequency with fatigue and force, and 9 of these cells facilitated both extensors and flexors in fatigue. Increases were found in 8 cells, decreases in 5 cells and no change in 1 cell. As muscle afferents provide substantial information to cortical cells, which in turn establish functional linkages with their target muscles before and during fatigue, the changes in cell firing frequencies during fatigue demonstrate the active participation of the motor cortex in the control of compensation for the peripheral adjustments concomitant with muscle fatigue.

Adaptation, Physiological↗
PubMed1995

Functional linkages between motor cortical cells and elbow flexor muscles. Evidence for and characteristics of postspike facilitation.

1. Two monkeys (Macaca fascicularis) making high-level but submaximal isometric flexions of the elbow were investigated for the output effect of motor cortical cells on the electromyogram (EMG) activity of two main elbow flexors using the method of spike-triggered averaging of rectified EMGs (STAs). 2. Monkeys were trained to perform individual isometric contractions for > 2 s, and two series of > or = 20 contractions, the second series being at a greater force. EMG electrodes pairs were implanted in the biceps brachii and brachioradialis. A total of 257 cortical cells were found that discharged with the active and passive movements of the elbow. We examined the EMG postspike facilitations (PSFs) produced in either one or the two flexors for only those cells that discharged during the isometric contraction, and provoked PSFs in the two series of contractions. 3. The main characteristics of the EMG isometric contractions in the agonists were analyzed. Spectral analysis showed that the increases in the EMG median frequency with force stabilized at the force levels performed by monkeys. Cross correlation methods showed no cross talk between agonists. 4. The 26 selected cortical cells had a regular discharge frequency. Ten cells did not change frequency with a 22-30% force increase, 14 cells discharged at a higher frequency, and 2 cells discharged at a lower frequency. For single-cell frequencies of 5-65 Hz, interspike intervals < 10 ms were rare: the median and modal intervals were 20-30 ms. 5. The significance of PSFs with respect to the EMG background noise was estimated statistically. STAs from successive epochs under identical load conditions, and STAs performed at a distance from the trigger, showed that PSFs were authentic postspike effects and not sudden EMG changes synchronized by chance with the triggering cell. The features distinguishing PSF from secondary postspike EMG changes or coactivation and task-related effects were studied in simultaneous STAs of flexors and autocorrelogram of cortical spikes. 6. The magnitude of the PSF was expressed as the percent peak amplitude above the mean EMG baseline. The mean percent amplitude of the 90 PSFs produced in both muscles and series was 4.0 +/- 2.4% (mean +/- SD). There was no difference in the average amplitude of PSFs in the two flexors, although the baseline voltages in the biceps brachii were higher. Neither was there any significant change with force while the baseline level increased by 29 +/- 10%, indicating that the absolute PSF amplitude increased in the same proportion as baseline.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Cortical post-spike facilitations in elbow muscles during isometric contraction.

Averaged responses of elbow muscles to action potentials of 37 single motor cortex cells were analyzed in two Macaca fascicularis monkeys during performance of a submaximal but high level isometric contraction. Post-spike facilitations (PSFs) of the rectified electromyographic activity (EMG) were frequently (72%) observed. They consisted of PSFs largely distributed in agonistic (biceps brachii and brachioradialis) and antagonistic (triceps brachii) muscles. Some post-spike inhibitions (7%) were also noted. As compared with distal muscles PSFs, most of the proximal PSFs were of lower amplitude, probably because of the higher level of the contractions. These PSFs were also of longer latency. The data indicate functional connectivity between motor cortical cells and proximal limb muscles. Cortical cells with oligosynaptic and even, for a few of them, with monosynaptic connections to the alpha-motoneurones may contribute to recruitment of motor units (MUs) involved in proximal limb muscle contraction.

Action Potentials↗
PubMed1988

Reticular control of thalamic transmission during behavioral states: a study in dorsal lateral geniculate nucleus relay neurons of the cat.

In behaving cats adapted to head restraint we examined the changes of spontaneous activity of dorsal lateral geniculate relay neurons provoked by weak stimulation of mesencephalic reticular formation during different states of sleep and alertness. The neurons were classified as fast- and slow-conducting neurons depending on their optic tract latency. When the reticular stimulation was delivered during slow-wave sleep, it increased the firing rate principally in slow neurons and suppressed the burst pattern. The increases occurred on a long depolarization in intracellular recordings (n = 9). The effect resembled one that was simultaneous with pontogeniculooccipital waves. Only slow neurons showed a correlation between the relative increases and the prestimulus firing rate. The excitatory effect was reduced and less frequent during wakefulness and paradoxical sleep in both latency groups. When reticular stimulation did not influence the spontaneous activity of the activated states, it also did not modify the optic tract-elicited hyperpolarization. The data tend to demonstrate that the efficiency of the reticular stimulation depends principally on the rate and pattern of the spontaneous activity which, itself, varies largely according to the behavioral state and moderately according to the optic tract latency.

Animals↗
PubMed1985

Oscillations of the spontaneous slow-wave sleep rhythm in lateral geniculate nucleus relay neurons of behaving cats.

Intracellular recordings of 31 lateral geniculate nucleus relay neurons were performed in darkness in behaving cats in order to analyse electrical postsynaptic events which appeared during slow-wave sleep. A specific pattern characterized slow-wave sleep: a rapid depolarizing potential arising from baseline initiated a slow depolarization lasting for 40-60 ms which in turn most often elicited delayed fast spikes. This pattern recurred at a frequency of 6-12/s. The slow depolarizations were voltage dependent, usually not separated by any obvious phasic hyperpolarization and showed refractoriness. Other rapid depolarizing potentials occurring during the time course or at the end of a slow depolarization could have generated spike(s) but were followed by a rapid decay. Slow depolarizations were not observed during arousal or paradoxical sleep when the neurons tonically depolarized and displayed either rapid depolarizing potentials with a fast decay or repetitive firing and long high frequency bursts. In five of the studied neurons, decreases in frequency of the spontaneous rapid depolarizing potentials occurred during slow-wave sleep for 3-30 s oscillatory periods without any change in the behavioural state. During these periods all of the few remaining rapid depolarizing potentials arose from a flat baseline, had a higher amplitude and initiated a slow depolarization which always elicited a spike or burst of spikes after a brief delay. The slow-wave sleep rhythm decreased to 1-5/s. Simultaneously the baseline membrane potential hyperpolarized by a few millivolts and reached a level for reversal of inhibitory postsynaptic potentials. Imposed hyperpolarization of the membrane during wakefulness did not reveal any slow depolarization. But strong synaptic excitatory inputs and direct excitation (a break of the current pulse) from a hyperpolarized membrane did evoke the slow depolarization and eventually the fast spike(s) in both control and oscillatory neurons. A rhythm similar to that of slow-wave sleep was elicited during wakefulness by optic tract stimulation and was enhanced by membrane hyperpolarization. But under these conditions the rhythm was initiated by a phasic hyperpolarization and was composed of an alternating hyperpolarization-depolarization. Spontaneously and synaptically evoked rapid depolarizing potentials arising from baseline had a similar rising slope. The spontaneous ones initiated a slow depolarization leading to fast spike(s) during slow-wave sleep and could directly generate fast spike(s) during wakefulness.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Modulation of postsynaptic activities of thalamic lateral geniculate neurons by spontaneous changes in number of retinal inputs in chronic cats. 1. Input-output relations.

The experiments were designed to explore the role of retinal inputs compared with that of the behavioral state in the modulation of the output of thalamic lateral geniculate neurons during sleep and wakefulness in cats with intact visual pathways. We made the following assumptions: the retinal dark discharge, while showing spontaneous pauses in activity, does not vary with the behavioral state; the optic tract inputs postsynaptically elicit subthreshold activities called S-potentials which in turn generate spikes, the degree of transformation being dependent on the level of alertness. On the basis of these assumptions, it could be expected that changes in retinal input frequency would modify the rate of the S-potentials. Therefore the effect of spontaneous decreases in frequency of S-potentials on the spike rate and pattern was examined in juxta- and intracellular recordings from chronically implanted cats during natural sleep and wakefulness. During quiet wakefulness and light slow-wave sleep, lateral geniculate relay neurons normally displayed numerous S-potentials associated with a moderate firing rate. Many neurons occasionally showed transient reductions in frequency of the S-potentials and an oversimplification of the discharges which combined a decreased rate with a prevalent rhythmical burst pattern. Antidromic responsiveness remained unchanged. The oscillatory periods recurred two to six times without any alteration in the control state level. They were not observed throughout wakefulness and paradoxical sleep, during which neuronal activity combined a high spike rate with a low S-potential rate. The modifications were confirmed by computation of the mean rates and of the inter-event intervals. The transfer ratio (spikes/S-potentials + spikes) significantly increased both during the oscillatory periods poor in S-potentials of quiet wakefulness and during active wakefulness. But the correlation between the transfer ratio and the spike frequency, which was high throughout the control behavioral states, faded during the periods poor in S-potentials. Thus the transient falls in the frequency of S-potentials which occurred spontaneously during quiet wakefulness caused burst discharges in lateral geniculate relay neurons, which resembled a sleep deepening, but also paralleled the effect of experimental deafferentation. The data indicate that the iterative spikes grouped in well spaced bursts which persisted during decreases in subthreshold postsynaptic activities result in an enhanced signal-to-noise ratio.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1983

The effect of midbrain reticular stimulation upon perigeniculate neurons activity during different states of the sleep-waking cycle in the cat.

Electrical stimulation of the mesencephalic reticular formation in chronic cats induced state-dependent effects on spontaneous firing of perigeniculate neurons. Perigeniculate neurons fired at lower rates during slow wave sleep than during wakefulness of paradoxical sleep. The stimulation caused a firing decrease in slow wave sleep; an effect which faded during wakefulness and paradoxical sleep and was superseded by a firing increase during periods of eye movements in 30% of the neurons. The responsiveness of perigeniculate neurons to optic tract and visual cortex stimulation either remained unchanged or was enhanced during the reticular induced firing changes.

Animals↗
PubMed1983

Sleep-related variations of membrane potential in the lateral geniculate body relay neurons of the cat.

Membrane potential of lateral geniculate body relay neurons was monitored in chronic cats during the sleep-waking cycle. Neurons were tonically depolarized throughout paradoxical (P) sleep and the maximal level of polarization occurred during slow (S) sleep (mean difference of membrane potential between S and P sleep: + 10.2 +/- 1.3 mV, n = 6, range: 8-12 mV). Some features of the spontaneous activity of S and P sleep are briefly discussed in relation to the level of membrane potential. In particular it is suggested that the phasic depolarizations underlying the bursts of action potentials during S sleep, and which are reproduced retinal cell axons impinging upon the hyperpolarized membrane.

Animals↗
PubMed1981

[Effects of a cortical after-discharge on lateral geniculate neurones. Electrophysiological study in the chronic cat (author's transl)].

(1) A chronic preparation is described in which it is possible to record juxtacellularly from identified thalamo-cortical relay (TCR) neurones in lateral geniculate body (LGB) of normally respiring, nonanaesthetized cats during natural sleep. (2) Cellular events were analyzed during and after focal electrical stimulation of homotopic visual cortex. (3) Projection of cortical epileptiform after-discharge (EAD) was strictly limited to functionally related areas of LGB. Of the 47 neurones tested only 30% were activated. (4) During the 2 sec, 50 c/sec tetanus the cortically evoked IPSPs in TCR cells were replaced by facilitation of cortico-thalamic transmission as demonstrated by the occurrence of a burst of action potentials (AP) following the antidromic AP. (5) During the ensuing EAD, high frequency AP discharges occurred in the cortico-thalamic axons during each cortical wave. This period was accompanied by prolonged (50-300 msec) low amplitude rhythmic depolarisations leading to temporary spike inactivation of TCR neurones. Comparable inactivating responses were recorded during paradoxical sleep. (6) Between paroxysmal bursts facilitation of synaptic transmission to the optic tract stimulation was observed. (7) Persistence of a positive collision test after a spontaneous AP indicates that AP are orthodromically propagated during the EAD.

Action Potentials↗
PubMed1980

Synaptic potentials in cat's lateral geniculate neurons during natural sleep with special reference to paradoxical sleep.

Juxtacellular DC recordings from lateral geniculate body (LGB) relay cells were performed in completely undrugged cats during natural sleep. Paradoxical sleep (P) was characterized by a decrease in synaptic (S) potentials and an increase in spike discharge as compared to slow wave sleep. During the P grouped discharges simultaneous with eye movements and cortical waves, a further decrease in S potentials accompanied by an increased cell excitability as indicated by the higher probability to elicit an antidromic spike occurred. The results suggest that the decrease of S potentials in P is not the result of presynaptic inhibition but that most of them reach firing level during the P grouped discharges.

Action Potentials↗