Jean-Marc Aimonetti

Changes in Human Muscle Spindle Sensitivity during a Proprioceptive Attention Task

The aim of the present study was to test whether fusimotor control of human muscle spindle sensitivity changed when attention was selectively directed to the recognition of an imposed two-dimensional movement in the form of a written symbol. The unitary activities of 32 muscle spindle afferents (26 Ia, 6 II) were recorded by microneurography at the level of the common peroneal nerve. The patterns of firing rate in response to passive movements of the ankle, forming different letters or numbers, were compared in two conditions: control and recognition. No visual cues were given in either condition, but subjects had to recognize and name the character in one condition compared with not paying attention in the control condition. The results showed that 58% of the tested Ia afferents presented modified responses to movements when these had to be recognized. Changes in Ia afferent responses included decreased depth of modulation, increased variability of discharge, and changes in spontaneous activity. Not all changes were evident in the same afferent. Furthermore, the percentage of correctly recognized movements amounted to 63% when changes were observed, but it was only 48% when the primary ending sensitivity was unaltered. The responses of group II afferents were only weakly changed or unchanged. It is suggested that the altered muscle spindle sensitivity is because of selective changes in fusimotor control, the consequence of which might be to feed the brain movement trajectory information that is more accurate.

Fusimotor Drive May Adjust Muscle Spindle Feedback to Task Requirements in Humans

The aim of the present study was to investigate whether the fusimotor control of muscle spindle sensitivity may depend on the movement parameter the task is focused on, either the velocity or the final position reached. The unitary activities of 18 muscle spindle afferents were recorded by microneurography at the common peroneal nerve. We compared in two situations the responses of muscle spindle afferents to ankle movements imposed while the subject was instructed not to pay attention to or to pay attention to the movement, both in the absence of visual cues. In the two situations, three ramp-and-hold movements were imposed in random order. In one situation, the three movements differed by their velocity and in the other by the final position reached. The task consisted in ranking the three movements according to the parameter under consideration (for example, slow, fast, and medium). The results showed that paying attention to movement velocity gave rise to a significant increase in the dynamic and static responses of muscle afferents. In contrast, focusing attention on the final position reached made the muscle spindle feedback better discriminate the different positions and depressed its capacity to discriminate movement velocities. Changes are interpreted as reflecting dynamic and static gamma activation, respectively. The present results support the view that the fusimotor drive depends on the parameter the task is focused on, so that the muscle afferent feedback is adjusted to the task requirements.

Changes in intracortical excitability induced by stimulation of wrist afferents in man

1 Inhibitory and facilitatory neuronal circuits may be explored in the human motor cortex by double pulse transcranial magnetic stimulation (TMS). At short interstimulus intervals (2–5 ms), conditioned motor‐evoked potentials (MEPs) are reduced (intracortical inhibition, ICI), whereas they are facilitated at longer interstimulus intervals (8–25 ms; intracortical facilitation, ICF). The aim of this study was to investigate the effects of homonymous and antagonist nerve stimulation on the intracortical inhibition and facilitation in the cortical areas that control the wrist extensor and flexor radialis muscles. 2 Sixteen subjects were asked to contract either their wrist extensor or flexor muscles. The MEP evoked by a test TMS (at 1.2 × MEP threshold) and recorded in the target muscle was then conditioned by subthreshold TMS (at 0.8 × MEP threshold) 2 and 14 ms before the test TMS. The median and radial nerves were stimulated at 0.8 × motor threshold (MT). 3 In both flexor and extensor muscles, antagonist nerve stimulation 40 ms before the test TMS decreased ICI and increased ICF. In contrast, homonymous nerve stimulation had no effect on ICI and ICF. 4 The intensity of the antagonist nerve stimulation required to alter ICI and ICF was as low as 0.6 × MT, which suggests that thick diameter afferents may be involved. The nerve stimulation had to be applied 35–45 ms prior to the test TMS to alter significantly the intracortical excitability. 5 Cutaneous afferents were probably not responsible for the alterations of intracortical excitability, since cutaneous stimulation had no effect on either ICI or ICF at the investigated intervals. 6 The present data suggest that antagonist muscular afferent inputs may evoke reciprocal facilitation or disinhibition at the cortical level. This pattern of antagonist sensory afferent effects may be of significance for control of the wrist extensor and flexor muscles when used as synergists during manipulatory finger movements and gripping tasks.

Mechanical cutaneous stimulation alters Ia presynaptic inhibition in human wrist extensor muscles: a single motor unit study.

Reflex responses were evoked by radial nerve stimulation in 25 single motor units in the extensor carpi radialis muscles of seven subjects during voluntary isometric wrist extension. The responses consisted of narrow peaks in the post‐stimulus time histograms with latencies compatible with monosynaptic activation. When the skin of the palm and finger tips was continuously swept using a soft rotating brush, the purely monosynaptic components of the motor unit responses, as assessed from the contents of the first two 0.25 ms bins of the peak, were found to increase. This increase did not affect the motoneurone net excitatory drive, as assessed by measuring the mean duration of the inter‐spike intervals. The cutaneous inputs activated by the brush may have reduced the tonic presynaptic inhibition exerted on the Ia afferents homonymous to the extensor motor units tested. To further investigate whether Ia presynaptic inhibition was involved, the responses of the extensor motor units were conditioned by stimulating the median nerve 20 ms earlier, using a protocol which is known to induce Ia extensor presynaptic inhibition originating from flexor Ia afferents. The median nerve stimulation did not affect the motoneurone excitatory drive, but led to a decrease in the responses of the extensor motor units to the radial nerve stimulation, especially in the purely monosynaptic components. This decrease was consistent with the Ia presynaptic inhibition known to occur under these stimulation conditions. The cutaneous inputs activated by the brush were found to reduce the Ia presynaptic inhibition generated by the median nerve stimulation, without affecting the distribution of the Ia presynaptic inhibition among the various types of motor units tested. The present data suggest that cutaneous inputs from the palm and finger tips may relieve the Ia presynaptic inhibition exerted on the wrist extensor motor nuclei, and thus enhance the proprioceptive assistance to fit the specific requirements of the ongoing motor task.

Mechanical discrimination of hairy fabrics from neurosensorial criteria

Fabrics with more and more elaborate tactile properties are available on the textile market. However the specifications of the textile products do not feature their touch because this can not be measured precisely and objectively enough. Some measurement methods of the mechanical properties involved in tactile feeling have been developed. Nevertheless, a purely mechanical approach is not sufficient. Therefore, the human being was utilized as a touch sensor. The tactile afferent [i.e. conveyed to the central nervous system, centripetal] messages elicited by the mechanoreceptors of the skin in response to textile stimuli and which were propagated along the sensitive nervous fibers up to the brain were studied. These messages were recorded on conscious human individuals, by a method named microneurography. The aim of this study was to use the neurosensory results in order to improve the mechanical measurement methods for the characterization of the surface state of fabrics. The samples tested had undergone different emery finishing processes. The preliminary results of the microneurographic study highlight the importance of taking account of the effect along/against the main direction of the hairiness. In fact, the discrimination of different hairy fabrics by cutaneous mechanoreceptors is only achieved when the fabrics stroke the skin against the main direction of the hairiness. A friction device developed by the co-authors was modified in term of signal processing in order to measure the surface along and against the main direction of the hairiness separately. Moreover, the probe was improved in order to separate the mechanical behavior information on hairiness from the roughness information. The results obtained with this new method were compared with results obtained using the surface tester of the KES-F.

Distribution of presynaptic inhibition on type-identified motoneurones in the extensor carpi radialis pool in man

The question was addressed as to whether the magnitude of Ia presynaptic inhibition might depend on the type of motor unit activated during voluntary contraction in the wrist extensor muscles. For this purpose, we investigated the effects of applying electrical stimulation to the median nerve on the responses of 25 identified motor units to radial nerve stimulation delivered 20 ms after a conditioning stimulation. The reflex responses of the motor units yielded peaks in the post‐stimulus time histograms with latencies compatible with monosynaptic activation. Although median nerve stimulation did not affect the motoneurone net excitatory drive assessed from the mean duration of the inter‐spike interval, it led to a decrease in the contents of the first two 0.25 ms bins of the peak. This decrease may be consistent with the Ia presynaptic inhibition known to occur under these stimulation conditions. In the trials in which the median nerve was being stimulated, the finding that the response probability of the motor units, even in their monosynaptic components, tended to increase as their force threshold and their macro‐potential area increased and as their twitch contraction time decreased suggests that the median nerve stimulation may have altered the efficiency with which the Ia inputs recruited the motoneurones in the pool. These effects were consistently observed in seven pairs of motor units each consisting of one slow and one fast contracting motor unit which were simultaneously tested, which suggests that the magnitude of the Ia presynaptic inhibition may depend on the type of motor unit tested rather than on the motoneurone pool excitatory drive. The present data suggest for the first time that in humans, the Ia presynaptic inhibition may show an upward gradient working from fast to slow contracting motor units which is able to compensate for the downward gradient in monosynaptic reflex excitation from ‘slow’ to ‘fast’ motor units. From a functional point of view, a weaker Ia presynaptic inhibition acting on the fast contracting motor units may contribute to improving the proprioceptive assistance to the wrist myotatic unit when the contraction force has to be increased.

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.

Is muscle spindle proprioceptive function spared in muscular dystrophies? A muscle tendon vibration study

Muscular dystrophies (MDs) are characterized by the degeneration of skeletal muscle fibers. The aim of the present study was to determine whether the intrafusal fibers of muscle spindles are also affected in MD. The functional integrity of muscle spindles was tested by analyzing their involvement in the perception of body segment movements and in the control of posture. Twenty MD patients (4 with dystrophinopathy, 5 with myotonic dystrophies, 5 with fascioscapulohumeral MD, and 6 with limb‐girdle dystrophies) and 10 healthy subjects participated in the study. The MD patients perceived passive movements and experienced illusory movements similar to those perceived by healthy subjects in terms of their direction and velocity. Vibratory stimulation applied to the neck and ankle muscle tendons induced postural responses in MD patients with spatial and temporal characteristics similar to those produced by healthy subjects. These results suggest that the proprioceptive function of muscle spindles is spared in muscular dystrophies. Muscle Nerve 29: 861–866, 2004

Changes in the tonic activity of wrist extensor motor units induced by stimulating antagonistic group I afferents in humans

Abstract. The question as to whether the firing patterns of low- and high-threshold motor units in the extensor carpi radialis muscles are affected differently by group I afferents from the wrist flexors depending on the motor task being performed was investigated in six subjects. The motor units were voluntarily activated during a task consisting of either selectively contracting the wrist extensor muscles or co-activating the wrist and finger antagonist muscles by clenching the hand around a manipulandum. The motor units (n=40) were identified on the basis of their firing thresholds, their macro-potential areas, and their twitch contraction times. The effects on the motor-unit tonic activity of stimulating the wrist flexor afferents were assessed in terms of the changes in the firing probability, which were analysed after computing peri-stimulus time histograms using the cumulative sum procedure. Median nerve stimulation induced four main changes in the tonic firing pattern of the extensor motor units. An early, short-lasting increase in the firing probability (event E1) was found to occur in the high-threshold motor units, either in both tasks (6/13) or only during hand clenching (2/13). A short-latency decrease in the firing probability (event E2) was found to occur in all the motor units, the amount of which increased from the fast- to slowly contracting motor units, especially during hand clenching. A later decrease (event E3) followed by a large, late increase (event E4) in the tonic activity of the motor units was found to occur in all the motor units, without any task-dependent effects. All these various events were consistently observed in 12 pairs of motor units, each consisting of one slowly and one fast-contracting motor unit, which were tested simultaneously. These findings suggest that median nerve stimulation may selectively alter the tonic firing patterns of identified extensor motor units, depending on their functional characteristics (recruitment threshold, motor unit macro-potential area, contraction time) rather than on the excitatory drive to the motoneurone pool. The possible origins of these various events are discussed, and it is argued that, in the wrist extensor and flexor muscles that act as synergists during manipulatory finger movements and gripping tasks, the spinal pathways which assist the voluntary command may selectively modulate the firing patterns of identified motor units, to fit the requirements of the on-going motor task.

Merkel cells sense cooling with TRPM8 channels

In the skin, Merkel cells connect with keratinocytes and Aβ nerve fibers to form a touch receptor that functions as a slow adapting mechanoreceptor (slow adapting type 1). In human and mouse Merkel cells, we observed an increased concentration of intracellular Ca2+ ions in response to cold temperature and transient receptor potential melastatine 8 (TRPM8) ion channel agonists. A reduction in the response to cooling and TRPM8 agonists occurred after the addition of TRPM8 antagonists, as well as in TRPM8 knockout mice. Cold temperature and TRPM8 agonists also induced a current that was inhibited by a TRPM8 antagonist. Our results indicate that Merkel cells sense cooling through TRPM8 channels. We hypothesized that cooling modulates the slow adapting type 1 receptor response. Cooling mouse skin to 22°C reduced the slow adapting type 1 receptor discharge frequency. Interestingly, we observed no such reduction in TRPM8 knockout mice. Similarly, in human skin, a temperature of 22°C applied to the slow adapting type 1 receptive field reduced the spiking discharge. Altogether, our results indicate that Merkel cells are polymodal sensory cells that respond to mild cold stimuli through the activation of TRPM8 channels. Thermal activation of Merkel cells, and possibly other TRPM8-expressing non-neuronal cells, such as keratinocytes, potentially adapts the discharge of slow adapting type 1 receptors during cooling.