Muscle-joint unit transfer function derived from torque and surface mechanomyogram in humans using different stimulation protocols.

UNLABELLED: Torque and laser detected surface mechanomyogram (MMG) analysis after electrical stimulation of human tibialis anterior (TA) of 14 male subjects was aimed to: (a) obtain the dynamic responses of TA muscle-joint unit from a long (LP, about 1h) and short (SP, 12.5s) stimulation protocol; (b) compare the resulting transfer function parameters from the two signals. The sinusoidal amplitude modulation of a 30 Hz stimulation train (SST) changed the number of the recruited motor units, and hence the isometric torque and the TA surface position in the same fashion. Subject instrumentation and SST amplitude range definition took about 25 min. SP: seven consecutive modulation frequencies (0.4, 6.0, 1.0, 4.5, 1.8, 3.0, and 2.5 Hz). LP: fourteen 5s long isolated frequencies (0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.5, 3.0, 4.0, 5.0, and 6.0 Hz), 5 min rest in between. Poles position (Hz) and added delay (ms) for phase correction with respect to the input sine (parameters of a critically damped II order system) were: torque 2.44+/-0.27 Hz (SP) or 2.32+/-0.33 Hz (LP) and 18.3+/-2.2 ms (SP) or 17.2+/-4.5 ms (LP); MMG 2.28+/-0.30 Hz (SP) or 2.30+/-0.44 Hz (LP) and 17.4+/-5.6 ms (SP) or 17.4+/-6.4 ms (LP). Differences were never statistically significant. CONCLUSION: it is possible to characterise the in vivo mechanics of muscle-joint unit with a short (few seconds) stimulation protocol affordable in clinical environment using both torque and MMG signals.

Spike-firing resonance in hypoglossal motoneurons.

During an inspiration the output of hypoglossal (XII) motoneurons (HMs) in vitro is characterized by synchronous oscillatory firing in the 20- to 40-Hz range. To maintain synchronicity it is important that the cells fire with high reliability and precision. It is not known whether the intrinsic properties of HMs are tuned to maintain synchronicity when stimulated with time-varying inputs. We intracellularly recorded from HMs in an in vitro brain stem slice preparation from juvenile mice. Cells were held at or near spike threshold and were stimulated with steady or swept sine-wave current functions (10-s duration; 0- to 40-Hz range). Peristimulus time histograms were constructed from spike times based on threshold crossings. Synaptic transmission was suppressed by including blockers of GABAergic, glycinergic, and glutamatergic neurotransmission in the bath solution. Cells responded to sine-wave stimulation with bursts of action potentials at low (<3- to 5-Hz) sine-wave frequency, whereas they phase-locked 1:1 to the stimulus at intermediate frequencies (3-25 Hz). Beyond the 1:1 frequency range cells were able to phase-lock to subharmonics (1:2, 1:3, or 1:4) of the input frequency. The 1:1 phase-locking range increased with increasing stimulus amplitude and membrane depolarization. Reliability and spike-timing precision were highest when the cells phase-locked 1:1 to the stimulus. Our findings suggest that the coding of time-varying inspiratory synaptic inputs by individual HMs is most reliable and precise at frequencies that are generally lower than the frequency of the synchronous inspiratory oscillatory activity recorded from the XII nerve.

Changes in the recruitment curve of the soleus H-reflex associated with chronic low back pain.

OBJECTIVE: We investigated whether patients with chronic low back pain (CLBP) manifest changes in the excitability of the soleus H-reflex. METHODS: H-reflex stimulus-response curve was studied in 14 CLBP patients and 14 age-matched healthy subjects. H-threshold, H-maximum size, H-steepness and H-latency were determined for both legs. Homosynaptic depression (HD), following a train of H-reflexes, and presynaptic inhibition (PI) from flexor afferents onto soleus Ia afferents were also evaluated. RESULTS: H-threshold was significantly increased, H-size as a function of stimulus intensity was significantly different, and H-recruitment curve steepness was significantly lower in CLBP patients compared to healthy subjects. No significant difference in the amount of HD and PI of the H-reflex was found between the two groups. H-latency and Hmax/Mmax ratio was comparable between the subjects groups. CONCLUSIONS: In CLBP there is a reduced excitability of group Ia afferent fibres from the soleus muscle to which presynaptic factors do not seem to contribute and that presumably depend on changes in the peripheral sensory input. SIGNIFICANCE: Changes in H-reflex excitability may underlie a decrease in the gain of a peripheral sensor in CLBP. Estimation of soleus H-threshold and H-recruitment curve may contribute to the diagnostic evaluation of CLBP and may be used to monitor the efficacy of treatment.

Absence of linear correlation between fluctuations in area of simultaneous recorded monosynaptic responses and Hoffmann's reflexes in the rat.

In this study we analyze the possible relationship between fluctuations in area of monosynaptic reflex responses (MSR) and Hoffmann's reflex (H reflexes) in the plantar closed loop pathway of the anesthetized rat. These reflexes were evoked by low-frequency stimuli applied to the sciatic nerve or lateral plantar nerve and then concurrently recorded on the distal tibial nerve or lateral plantar nerve, respectively as well as the lateral plantar muscles in the foot of the anesthetized rat. From trial to trial, H reflexes showed higher variability in area than MSR, whether the latter was recorded in the distal tibial nerve (n=8 experiments) or in the lateral plantar nerve (n=5 experiments). No linear correlation was found between changes in area of concurrently evoked MSR and H reflexes (r(MSR-H,n=8)=0.11+/-0.03 and r(MSR-H,n=5)=0.08+/-0.09, respectively). These findings suggest that trial-to-trial fluctuations in area of H reflexes may involve interaction of several sources of variation, among others to MSR variability (due to pre-, and post-synaptic factors influencing the excitability of spinal motoneurons) in combination with those related to peripheral mechanisms, such as trial to trial activation of a different number of muscle fibers, either by the probabilistic transmitter release from neuromuscular junctions, by activation of motor units of variable size or to fluctuations in excitability of muscle fibers.

H-reflex and motor responses to acute ischemia in apparently healthy individuals.

The authors examined the effect of acute ischemia on peripheral nerve function in healthy subjects. It was hypothesized that acute ischemia would interfere with the ability of sensory and motor nerves to propagate an impulse. Twelve young, apparently healthy adults participated in the study. Soleus H-reflex and motor recruitment curves were determined for subjects during a control condition, after 5 minutes of ischemia by femoral artery occlusion, and after a 5-minute recovery. During ischemia, the stimulus intensity required to evoke an H-reflex or M-wave was reduced by 18.3% and 18.4%, respectively. Hmax/Mmax ratios were significantly reduced with acute ischemia (mean +/- standard error) 66.29% +/- 5.4% and 58.81% +/- 6.7% for control and ischemia, respectively, owing to a decrease in Hmax during acute ischemia with no change in Mmax. After ischemia, the Hmax/Mmax returned to control values, as did the M-threshold. However, although the H-threshold slightly recovered, it failed to return to control threshold after 5 minutes of recovery. The results suggest that acute ischemia decreases motor and H-reflex thresholds in healthy individuals with a longer lasting effect for the H-reflex. In addition, a decrease in Hmax/Mmax ratio was observed, suggesting that acute ischemia has differential effects on sensory nerve propagation and synapse transmission.

Motor control of the costal and crural diaphragm--insights from transcranial magnetic stimulation in man.

The costal and crural parts of the diaphragm differ in their embryological development and physiological function. It is not known if this is reflected in differences in their motor cortical representation. We compared the response of the costal and crural diaphragms using varying intensities of transcranial magnetic stimulation of the motor cortex at rest and during submaximal and maximal inspiratory efforts. The costal and crural motor evoked potential recruitment curves during submaximal inspiratory efforts were similar. The response to stimulation before, during and at 10 and 30 min after 44 consecutive maximal inspiratory efforts was also the same. Using paired stimulations to investigate intra-cortical facilitatory and inhibitory circuits we found no difference between the costal and crural response with varying interstimulus intervals, or when conditioning and test stimulus intensity were varied. We conclude that supraspinal control of the costal and crural diaphragm is identical during inspiratory tasks.

Dopaminergic effects on electrophysiological and functional MRI measures of human cortical stimulus-response power laws.

Power laws have been widely used to formulate relationships between objective intensity of stimulation and subjective intensity of sensation. We investigated the effects of dopaminergic drug treatment (sulpiride) on the relationship between somatosensory stimulus intensity and cortical response measured electrophysiologically by somatosensory-evoked potentials (SEP) and functional magnetic resonance imaging (fMRI). The intensity of stimulation was related by a simple power law to both electrophysiological and fMRI measures of cortical response, with overlapping confidence intervals for both power law exponents. Sulpiride did not modulate the power law exponent, but significantly attenuated the "gain" of both stimulus-response functions. Using path analysis we decomposed dopaminergic effects on fMRI data into an indirect component (16%), predictable by drug effects on SEP, and a direct component (84%), not explained electrophysiologically. Results indicate that sulpiride has comparable effects on power law parameters estimated from SEP and fMRI, but fMRI has superior sensitivity to detect drug effects on somatosensory cortical recruitment by graded stimulation.

Interaction of a combination of morphine and ketamine on the nociceptive flexion reflex in human volunteers.

Experimental studies in animals have suggested that a combination of morphine and N-methyl-D-aspartate (NMDA) receptor antagonists may have additive or synergistic analgesic effects. To further study the nature of the interaction between these two classes of analgesic agents, we analyzed the effects of morphine, ketamine and their combination on electrophysiological recordings of the nociceptive flexion RIII reflex in 12 healthy volunteers. Morphine (0.1 mg/kg), ketamine (0.1 mg/kg followed by 4 microg/kg/min) or their combination were administered intravenously according to a double-blind, placebo controlled and cross-over design. The RIII reflex was recorded from the biceps femoris and elicited by electrical stimulation of the sural nerve. The effects of the drugs were tested on: (1) the stimulus-response curves of the reflex up to the tolerance threshold (frequency of stimulation: 0.1Hz); (2) the progressive increase of the reflex and painful sensations (i.e. wind-up phenomenon) induced by a series of 15 electrical stimuli at a frequency of 1Hz (intensity: 20% above threshold). The stimulus-response curve of the nociceptive RIII reflex was significantly reduced after injection of a combination of ketamine and morphine, but was not modified when placebo or each of the active drugs was administered alone. The wind-up of the RIII reflex and painful sensation was not significantly altered after the injection of placebo, ketamine, morphine or their combination. In conclusion, the present electrophysiological results in humans demonstrate a synergistic interaction between morphine and ketamine, which tends to confirm the interest of using this type of combination in the clinical context. The differential effects observed on the recruitment curve and wind-up indicate, however, that the mechanisms of the interaction between opiates and NMDA receptor antagonists are not univocal but depend on the modality of activation of the nociceptive afferents.