Inducing any virtual two-dimensional movement in humans by applying muscle tendon vibration.

In humans, tendon vibration evokes illusory sensation of movement. We developed a model mimicking the muscle afferent patterns corresponding to any two-dimensional movement and checked its validity by inducing writing illusory movements through specific sets of muscle vibrators. Three kinds of illusory movements were compared. The first was induced by vibration patterns copying the responses of muscle spindle afferents previously recorded by microneurography during imposed ankle movements. The two others were generated by the model. Sixteen different vibratory patterns were applied to 20 motionless volunteers in the absence of vision. After each vibration sequence, the participants were asked to name the corresponding graphic symbol and then to reproduce the illusory movement perceived. Results showed that the afferent patterns generated by the model were very similar to those recorded microneurographically during actual ankle movements (r=0.82). The model was also very efficient for generating afferent response patterns at the wrist level, if the preferred sensory directions of the wrist muscle groups were first specified. Using recorded and modeled proprioceptive patterns to pilot sets of vibrators placed at the ankle or wrist levels evoked similar illusory movements, which were correctly identified by the participants in three quarters of the trials. Our proprioceptive model, based on neurosensory data recorded in behaving humans, should then be a useful tool in fields of research such as sensorimotor learning, rehabilitation, and virtual reality.

Delayed and prolonged effects of a near threshold EPSP on the firing time of human alpha-motoneurones.

In order to investigate the effects of near-threshold excitatory inputs on the precise timing of the action potentials during the tonic discharge of human motoneurones, the activity of single motor units was recorded in the extensor carpi radialis muscles while tendon taps (indentation, 0.1 mm; duration, 1 ms) were being delivered irregularly at a mean rate of 0.8 s(-1). New methods of analysis, such as the phase response function, were used to study the relative changes in the interspike interval (ISI1) during which the stimulus was being delivered and in the three subsequent intervals (ISI2, ISI3, ISI4) as a percentage of the pre-stimulus interspike interval (ISI0). The consistency of the effects of the actual stimulus as regards the spontaneous variability was assessed by comparing the data with those obtained with virtual stimulation. When the stimulus occurred at the end of ISI1, and triggered a spike, ISI1 and ISI3 were generally shortened, whereas ISI2 was lengthened, probably due to the negative correlation induced by the summation of the after-hyperpolarisations (AHPs). When the stimulus occurred in the middle of ISI1 without triggering a spike, ISI1, ISI2 and more rarely ISI3 were shortened. Lastly, when the stimulus occurred during the AHP scoop in ISI1, ISI2 was shortened although ISI1 remained unchanged. ISI4 was not consistently affected in any of these cases. The present results show that the tendon tap-induced inputs (probably from muscle spindle primary endings) mediated delayed and prolonged shortening effects of the ISIs on most of the alpha-motoneurones tested (n = 16). These effects undetected in classic peri-stimulus histogram analysis may involve long-lasting conductance changes although the contribution of polysynaptic pathways cannot be excluded. The changes in ISI were quite moderate (< 15% of ISI) but highly consistent. Their functional involvement in the synchronisation or desynchronisation processes and/or the mechanisms of optimisation of muscle contraction still remains to be explored.

Recurrent inhibition of wrist extensor motoneurones: a single unit study on a deafferented patient.

In order to document the effects of recurrent inhibition on the firing times of human alpha-motoneurones during natural motor behaviour, a case study was performed on a deafferented patient. The fact that this subject had completely lost the large-diameter sensory afferents provided us with a unique opportunity of selectively stimulating the motor axons in the nerves. The tonic activity of single motor units (n = 21) was recorded in the extensor carpi radialis muscles while applying randomly timed antidromic electrical stimuli to the radial nerve. The peristimulus time histogram analysis showed the presence of biphasic inhibitory effects, including an early, short-lasting component followed by a longer-lasting component occurring 20-40 ms later. The interspike interval (ISI) during which the stimulation occurred was generally lengthened as compared to the previous ISIs. The stimulation was most effective when delivered early (20-30 ms) after a spike. It was also effective, although less so, when delivered at the end of the ISI (70-100 ms after a spike). The lengthening effect sometimes extended over one or two of the subsequent ISIs. The lengthening effect of the motor axon stimulation was followed by an excitatory-like effect, which took the form of a shortening that affected up to five ISIs after the stimulation. The biphasic inhibitory effects and the subsequent facilitatory effects are discussed in terms of the dual nature of the synaptic processes involved in the recurrent inhibitory network, the postactivation facilitation/depression processes and the mutual inhibition occurring between Renshaw cells.

Pharmacologically induced enhancement of recurrent inhibition in humans: effects on motoneurone discharge patterns.

The aim of the present study was to investigate the effects of spinal recurrent inhibition on human motoneurone discharge patterns. The tonic discharge activity of motor unit pairs was recorded in the extensor carpi radialis (ECR) and abductor digiti minimi (ADM) muscles during voluntary isometric contraction. While undergoing continuous intravenous saline (NaCl 0.9 %) perfusion, the subjects were given a short lasting injection of L-acetylcarnitine (L-Ac), which has been found to potentiate recurrent inhibition in humans. The variability, synchronization and coherence of the motor unit discharges were analysed during four successive test periods (lasting 2-3 min each). A significant decrease in the inter-spike interval (ISI) coefficient of variation was observed in the discharge patterns of the motor units tested in the ECR and not in the ADM, which were not accompanied by any consistent changes in the mean ISIs of the motor unit activity in either muscle. The L-Ac injection also led to a significant increase in the synchronization in half of the motor unit pairs tested in the ECR muscle (n = 29), whereas no consistent changes were observed with the ADM motor units (n = 25). However, coherence analysis failed to reveal any consistent differences in the incidence of significant values of coherence spectrum between the pre-injection and injection periods among the motor unit pairs tested with either saline or L-Ac injections, in either the ECR or ADM muscles. The contrasting effects on the variability and the synchronization of the motor unit discharges observed with ECR motoneurones known to undergo recurrent inhibition and with ADM motoneurones known to lack recurrent inhibition suggest that the drug may have specific effects which are mediated by an enhancement of the Renshaw cell activity. The decrease in the ISI variability is in line with the hypothesis that recurrent inhibition may contribute along with the post-spike after-hyperpolarization to limiting the influence of the synaptic noise on the firing times of steadily discharging motoneurones. The present data, which suggest that recurrent inhibition plays a synchronizing rather than a desynchronizing role, are in keeping with the fact that the Renshaw cells may provide an important source of common inhibitory inputs.