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J E Desmedt

Publications and source records attributed to J E Desmedt.

At least 19 recordsLinked to original sources
PubMed1979

Voluntary motor commands in human ballistic movements.

The integrated electromyogram of the first dorsal interosseous muscle in normal humans presents a brief early burst in voluntary ballistic abduction of the index finger. Unexpected decrease in torque load does not change the burst. By contrast, in fast ramp abduction with a time-to-peak of 0.5 second, the electromyographic activity is fairly continuous up to the peak displacement, and it is silenced with a delay of 50 msec by unexpected unloading. Thus fast ramp, but not ballistic, voluntary contractions are under current long loop control from proprioceptive input. In a second set of experiments, the analysis of single motor unit potentials with highly selective electrodes in interosseous muscle indicated that ballistic thresholds are smaller than ramp thresholds under either isotonic or isometric conditions. However, the recruitment rank of the motor units was identical in finger abductions carried out in either ramp or ballistic modes.

Adult↗
PubMed1978

Ballistic contractions in fast or slow human muscles: discharge patterns of single motor units.

1. Single motor units were recorded from the masseter, soleus and first dorsal interosseous muscles of normal adult man. An analysis of discharge patterns was carried out either during slow ramp voluntary contractions, or during self-initiated isolated ballistic voluntary contractions. The isometric myogram was simultaneously recorded.2. Each motor unit was only recruited when the peak force of a brisk contraction exceeded a certain value and a ;ballistic force threshold' (in kg) was estimated for the unit from a large series of brisk contractions of different strengths. For each muscle, the ranking order for recruitment of different motor units recorded from one electrode position was virtually identical in slow ramp versus brisk ballistic contractions of different force (Kendall rank correlation coefficient = 0.91-1.0). There was no evidence for any consistent selective activation of fast twitch motor units in ballistic contractions.3. The ballistic force threshold is considerably reduced with respect to the slow ramp force threshold for the motor units of the soleus muscle. This drop is also marked for the units of the first interosseous and tibialis anterior muscles, whereas it is only small for the units of the masseter muscle. These data have been validated after consideration of the complicating factor related to the possible differential involvement of synergic muscles in ramp or ballistic contractions.4. In the masseter and first interosseous muscles, the time to peak is about 80 msec in small ballistic voluntary contractions and it increases to about 150 msec in strong contractions. This effect appears related to repetitive discharges of single motor units when their force threshold is exceeded. By contrast, in the soleus muscle, the time to peak remains at about 150 msec both in small and in strong ballistic contractions and most soleus motor units fire only one spike in the ballistic burst.5. Brisk ballistic contractions are graded in force by the recruitment of additional motor units according to their usual rank order. The importance of rate gradation through the repetitive firing of motor units varies in the different human muscles investigated, being quite significant in isometric brisk contractions of the masseter and first interosseous muscles but much less so in the soleus muscle in which little repetitive firing of single motor units was observed over the range of ballistic forces studied.

Action Potentials↗
PubMed1978

Mechanism of the vibration paradox: excitatory and inhibitory effects of tendon vibration on single soleus muscle motor units in man.

1. The parameters of presynaptic inhibition of the Ia spindle afferents from soleus muscle by vibration have been investigated. The inhibitory effects increase with the amplitude of vibration, but decrease when the vibration frequency is increased.2. The monosynaptic reflex threshold of twenty-one single soleus motor units activated in the H (Hoffmann) reflex by a single electrical stimulus to the posterior tibial nerve was estimated quantitatively and expressed in relation to the size of the simultaneously recorded H reflex.3. A parametric study of the effects of various Achilles tendon vibrations on the reflex threshold of the single soleus motor units indicated that their order of derecruitment is concordant with their rank order for activation in the phasic reflexes of the soleus. The last recruited motoneurones are the most susceptible to being silenced by steady vibration.4. Muscle vibration progressively recruits single motor units according to the motoneurone size principle through polysynaptic proprioceptive pathways. However the presynaptic inhibition of Ia spindle afferents simultaneously induced by the vibration works in reverse on the same rank order of motoneurones of the soleus spinal pool, thereby limiting the polysynaptic recruitment of units in the tonic vibration reflex while depressing the autogenic phasic proprioceptive reflexes. These mechanisms elucidate the so-called vibration paradox and extend the size principle of Henneman to presynaptic inhibitory effects.

Achilles Tendon↗
PubMed1978

Muscular dystrophy contrasted with denervation: different mechanisms underlying spontaneous fibrillations.

The data reviewed in this paper indicate that spontaneous fibrillations do not involve a cholinergic mechanism since non-depolarizing anticholinesterase drugs such as Mestinon fail to increase spontaneous fibrillations in denervated muscle. Fibrillation potentials are related to the changes in electrical properties of the membrane of denervated muscle fibres which lead to the appearance of spontaneous subthreshold depolarizations, sometimes triggering a propagated potential. Fibrillations seem to appear in cycles and this may depend on the depression of spontaneous depolarization by muscle activity itself. Fibrillations are also an important feature of Duchenne muscular dystrophy and polymyositis, but they have not been found in Landouzy-Dejerine muscular dystrophy. These "myopathic" fibrillations probably arise from subthreshold depolarizations in the membrane of muscle fibre segments which have been functionally or anatomically isolated from the end-plate by a pathological lesion (Fig. 4). Experimental demonstration of spontaneous fibrillations in baboon biceps muscles after extrajunctional myotomies indicates that such an isolated muscle fibre segment can indeed develop and sustain spontaneous fibrillation activities. Studies of motor unit potentials in myopathies by "coherent" electromyography disclose linked potentials after the main potential in Duchenne dystrophy, but not in Landouzy-Dejerine muscular dystrophy. The linked potentials are signs of collateral innervation by sprouts of the motor axons. The fact that linked potentials occur in Duchenne dystrophy, including in obviously dystrophic motor units (Fig. 5), shows that such motor axons are quite healthy and able to sprout efficiently. The muscle fibres thus innervated collaterally are probably the ones which fibrillated and were deprived of trophic motor control as a result of myopathic lesions of the type considered in Fig.4. This correlation receives support from the finding that both spontaneous fibrillations and linked potentials are lacking in Landouzy-Dejerine muscular dystrophy, which obviously presents a different type of muscle lesion.

Acetylcholine↗
PubMed1977

Active touch exploration of extrapersonal space elicits specific electrogenesis in the right cerebral hemisphere of intact right-handed man.

Language and analytic processing are currently thought to be represented in the left hemisphere, whereas spatial and holistic processing would involve primarily the right hemisphere in man. An experimental paradigm for engaging the nonlanguage hemisphere (generally the right) is described. This involves active touch exploration with the index finger to identify the orientation of a ridge with respect to the subject's body. The task is compatible with the electronic averaging of transient event-related cerebral potentials recorded from the intact scalp. A consistent positive electrogenesis of 1-5 muV and about 0.5-1.5 sec in duration was recorded over the nonlanguage hemisphere, regardless of whether the left or the right index finger performed the tactile scanning. The lateralized specific electrogenesis did not extend to the midline, and it is to be differentiated from the decision P300 component. These findings provide a new procedure for analyzing, in intact man, measurable focal potentials associated with unique processor subsystems during cognitive behavior. The method will make it possible to investigate the dynamic distribution of processing tasks between the two hemispheres in normal man in whom the commissural integration is normal, thereby adding to the data collected on patients with unilateral brain lesions or with surgical transection of the corpus callosum.

Adult↗