The origin of the premotor potential recorded from the second lumbrical.

PURPOSE: When recording with a palm electrode, a premotor potential (PMP) precedes the compound muscle action potential evoked from the second lumbrical muscle following median nerve stimulation. The origin of the premotor potential has remained uncertain. The aim of this study was to determine whether the PMP-2L is a SNAP derived from antidromically activated digital sensory branches of the median nerve. METHODS: We recorded three active electrodes were placed over the second lumbrical muscle, the third lumbrical muscle, the fourth lumbrical muscle by multi-channel recordings. RESULTS: PMPs are recorded only over the median digital sensory branches after stimulating the median nerve, while they are recorded only over the ulnar branch after stimulating the ulnar nerve. CONCLUSIONS: We conclude that the origin of the PMP is a SNAP arising from antidromically activated digital sensory branches.

EEG-EMG coherence changes in postural tasks.

PURPOSE: Recent studies have investigated the possible function of synchronous oscillatory activity within the sensorimotor cortex of monkeys and humans that is thought to arise from synchronous discharge of large numbers of cortical neurons. There has been found clear task-dependent changes in 15-30 Hz oscillations. In leg muscles, coherence also occurs in the same frequency band during voluntary static contraction. Therefore we investigated changes in coherence in leg muscles during several postural tasks. METHODS: We examined the coherence between EEGs and soleus EMGs during voluntary contraction and in various postural tasks. RESULTS: There was a significant coherence during voluntary static contraction, but not during standing, forward bending, or standing on one foot; whereas, there was significant coherence during stamping the ground. CONCLUSION: These results suggest that the coherence at 15-30 Hz originates from the motor cortex during voluntary contraction, not when doing postural tasks. Coherence analysis indicates that during postural tasks the motor cortex would not produce the synchronous discharge of large numbers of cortical neurons or might not induce soleus EMG activity.

Disorder of phase-related modulation of soleus H-reflex during hip movement in stroke patients.

PURPOSE: The soleus H-reflex during passive hip movement was measured to clarify the modulation of excitability of the soleus monosynaptic reflex during locomotion-like movement in spastic stroke patients. METHOD: The experiment was performed in five patients with spastic hemiparesis. The hip joint was moved passively ranging from 0 to 40 degrees. The knee joint was fixed at full extension and the ankle joint was fixed at the mid-position. During the movement, the soleus M-wave and soleus H-reflex were measured. RESULTS: Flexion caused a decrease in the soleus H-reflex, whereas extension caused an increase symmetrically for both the static and dynamic conditions. In addition, the lowest value was observed at the end of the flexion phase during fast movement. CONCLUSION: These findings indicate that the phase-related modulation of soleus H-reflex during hip movement is partially disordered in stroke patients.

Post-activation depression of the soleus H-reflex in stroke patients.

PURPOSE: We examined post-activation depression in 35 stroke patients and 10 healthy subjects, and investigated whether their clinical symptoms were correlated to post-activation depression. METHODS: Post-activation depression of the soleus H-reflex evoked by electrical stimulation of the common peroneal nerve with supramaximal intensity of motor response on the tibialis anterior muscle. RESULTS: Post-activation depression was decreased on the affected side of the stroke patients. There was a significant difference between the affected and unaffected sides of the patients with post-activation depression, but no difference between the unaffected sides of the patients and healthy subjects. The presence of ankle clonus and the decrease in post-activation depression were correlated. CONCLUSION: Decreased post-activation depression in stroke patients suggests that a mechanism other than postsynaptic inhibition, such as reciprocal Ia inhibition and Ib inhibition, and presynaptic inhibition, may contribute to the exaggerated stretch reflexes, in particular, the manifestation of clonus.

Sensorimotor event-related desynchronization represents the excitability of human spinal motoneurons.

Amplitudes of mu and beta (7-26Hz) oscillations measured by electroencephalography over the sensorimotor areas are suppressed during motor imagery as well as during voluntary movements. This phenomenon is referred to as event-related desynchronization (ERD) and is known to reflect motor cortical excitability. The increased motor cortical excitability associated with ERD during hand motor imagery would induce a descending cortical volley to spinal motoneurons, resulting in facilitation of spinal motoneuronal excitability. Therefore, in the present study, we tested the association of ERD during motor imagery with the excitability of spinal motoneurons in 15 healthy participants. Spinal excitability was tested using the F-wave recorded from the right abductor pollicis brevis muscle. The F-wave results from antidromic activation of spinal motoneurons and is induced by peripheral nerve stimulation. Participants performed 5s of motor imagery of right thumb abduction following 7s of rest. The right median nerve was stimulated at wrist level when the ERD magnitude of the contralateral hand sensorimotor area exceeded predetermined thresholds during motor imagery. The results showed ERD magnitude during hand motor imagery was associated with an increase in F-wave persistence, but not with the response average of F-wave amplitude or F-wave latency. These findings suggest that the ERD magnitude may be a biomarker representing increases in the excitability of both cortical and spinal levels.

Motor unit firing behavior in man.

Studies on motor unit firing behavior in man by the decomposition technique are described. The decomposition technique identifies motor unit firing with 100% accuracy at force levels of greater than 80% maximal voluntary contraction (MVC). In all muscles examined, the higher the recruitment threshold of the motor unit, the lower the rate at which it fired at the target level. Smaller muscles, such as those in the hand, recruit their motor units at 0-50% MVC and rely exclusively on firing rate increases to augment force output at 50-100% MVC. Larger muscles, such as those in the leg or arm, recruit motor units at least to 90% MCV, and possibly higher. Their firing rates have a relatively smaller dynamic swing. Thus, smaller muscles rely primarily on firing rate and larger muscles rely primarily on recruitment to modulate their force. High cross-correlation functions in firing rate behavior within a muscle were observed between individual motor unit firing rates at constant force isometric contraction. Thus the nervous system does not control the firing rate of motor units individually. Instead, it acts on the pool of homonymous motoneurons in a uniform fashion. Electrical stimulation of cutaneous receptors tends to increase the recruitment thresholds of low-threshold motor units and to decrease their firing rates, while high-threshold motor units generally exhibit a decrease in recruitment threshold and an increase in firing rate.

Tonic and kinetic motor units revisited: does motor unit firing behavior differentiate motor units?

OBJECTIVE: We tried to differentiate motor unit into two distinct populations, tonic and kinetic, on the basis of the relationship between the mean inter-spike interval and its variability. METHODS: During voluntary isometric contraction myoelectric activity was recorded with a special quadrifilar electrode from first dorsal interosseous, biceps brachii, soleus, and tibialis anterior muscle. Motor unit action potentials (MUAP) were decomposed into individual MUAP trains, by electromyography (EMG) signal decomposition. The variability in the instantaneous firing rate was assessed at two or more levels of contraction in each muscle. RESULTS: We found each muscle tested had a homogeneous population. There were no tonic and kinetic motor units. But there were differences in the variability in the instantaneous firing rate in the 4 muscles tested. CONCLUSION: Motor unit firing behavior in a muscle may be fitted for its function.

Synchronization of single motor units during voluntary contractions in the upper and lower extremities.

OBJECTIVE: To investigate motor unit synchronization in the time and frequency domains and compare the amount and nature of this synchronization between upper and lower extremity muscles in human subjects. METHODS: A total of 120 motor unit pairs from biceps brachii (BB), first dorsal interosseous (1DI), vastus medialis (VM), and tibialis anterior (TA) on the dominant side were analyzed and compared. Pairs of motor unit spike trains were recorded from two concentric needle electrodes inserted within these muscles in healthy volunteers. Subjects were instructed to maintain a weak isometric contraction of these muscles so that an individual motor unit recorded from each concentric needle discharged at a steady rate of approximately 10 impulses/s. Pairs of motor unit spike trains were cross-correlated in the time domain, and coherence analysis in the frequency domain was performed on the same spike train data. RESULTS: Synchronization was seen in all the muscles studied. Strength of motor unit synchronization, expressed as synchronization index (SI), was greater in 1DI muscles compared to other muscles (P<0.01). Coherence analysis revealed significant association between motor unit firings in the 1--5 and 25--30 Hz frequency ranges in all the muscles studied. The incidence of 25--30 Hz coherence peaks were found to be greater for 1DI muscles compared to other muscles. CONCLUSION: The above results suggest a possible role for corticospinal projections in producing pre-synaptic inputs responsible for synchronization of motor unit firings and 25--30 Hz coherence peaks.

Macro-EMG and motor unit recruitment threshold: differences between the young and the aged.

The relationship between macro-EMG (electromyography) and motor unit recruitment threshold was studied in the first dorsal interosseous (FDI) muscle of normal young and aged subjects. During voluntary isometric contraction, motor unit action potentials (MUAP) were collected by a special quadrifilar electrode and decomposed to each MUAP train (MUAPT) using an EMG signal decomposition technique. Macro-EMG was obtained from the electrode shaft, then triggered and averaged for each MUAPT. A positive linear correlation was observed in both the young and aged subjects. However, the correlation coefficients were significantly lower in the aged individuals than in the young individuals.

Integration-plot test for peri-stimulus time histograms in human motor units.

A statistical test is proposed for peri-stimulus time histograms in human motor units for the case where a test stimulus is delivered at a constant interval after a previous discharge. This mathematically described test included the notion of the multiple comparison and thus achieved higher sensitivity than the previously proposed method. With regard to data acquisition, the interval from a sham stimulus to the next discharge was acquired as a control, and the total number of samples was set to be four times as large as that in the test situation to reduce the statistical scattering noise. A newly defined statistical object, the integration plot (timewise accumulation of the test histogram without control subtraction) was used for this statistical test. The integration plot had less noise than the cumulative sum (CUSUM) plot (1/square root(2) in theory) and thus represented the neural effect. To compare this integration-plot test with that of the CUSUM, a simulation experiment that compared two sample histograms (one of which had a faint structural change from 20 ms) was conducted. As a result, the present test succeeded in detecting the onset of the change point earlier (23 ms on average) than the CUSUM test (27 ms on average), and the detection probability was also higher (9 out of 10) than the CUSUM (6 out of 10). It was therefore confirmed experimentally that the present statistical test had higher sensitivity than that of the CUSUM proposed previously.

Statistical test for peri-stimulus time histograms in assessing motor neuron activity.

The peri-stimulus time histogram is a valuable tool for evaluating neural connections in humans. To detect the degree to which a conditioning stimulus to a sensory nerve modulates motor neuron activity, a histogram of motor unit spike intervals after a conditioning stimulus is measured. This histogram allows the effect of the conditioning stimulus to be visualised. By comparison with a reference histogram of motor unit spike intervals after a sham stimulus, the noise caused by spontaneous firing sway can be removed. However, no valid statistical test has yet been developed to separate the physiological effect from the spontaneous sway and statistical noise. A computational method has been proposed to detect modulation caused by a conditioning stimulus. To clarify the effect of a conditioning stimulus, this new method used reference histograms to calculate a confidence interval. A simulated experiment demonstrated that about 2000 re-samplings were sufficient to estimate a confidence interval for a histogram with 1 ms bin width constructed from 300 triggers. Testing of the experimental data, measured from the tibialis anterior muscles during the elicitation of the excitatory spinal reflex, confirmed that significant peaks were produced at 30, 34, 35 and 38ms after the conditioning stimulus. These correspond appropriately to the delay of the spinal reflex.

Synaptic connections from large muscle afferents to motoneurones in human lower limbs.

OBJECTIVES: Our study was undertaken to investigate reciprocal inhibition in humans both from ankle flexors to extensors and from ankle extensors to flexors. METHODS: Changes in the firing probability of single motor units in response to electrical stimulation of muscle nerves (the peristimulus time histogram technique) were used to derive the reciprocal projections of muscle spindle Ia afferents to the motoneurones of ankle muscles. Discharges of units in ankle flexors (the tibialis anterior muscle [TA]) and extensors (soleus [SOL] and medial gastrocnemius [MG] muscles) were investigated respectively after stimulation of the posterior tibial (PTN) and common peroneal (CPN) nerves (predominantly on the deep branch). In eight normal subjects aged 24 to 40 years, one motor unit per each muscle was studied. RESULTS: CPN stimulation produced reciprocal Ia inhibition in the SOL of 5 of 7 of them and in the MG of 3 of 5, whereas PTN stimulation produced reciprocal Ia inhibition in the TA of only 2 of 6 subjects. CONCLUSIONS: These findings suggest that at low level contraction reciprocal Ia inhibition from ankle flexors to extensors may be stronger than that from ankle extensors to flexors.

Cortical neuromagnetic activity associated with cutaneomuscular reflex.

Cortical activity related to the late component of the cutanomuscular reflex was studied by measuring somatosensory evoked magnetic fields (SEFs) during isometric contraction of the reflex-induced muscle. After electrical stimulation to the index finger with intensity of three times of the perception threshold, three peaks of the cortical activity were detected within the somato-sensory area contralaterally to the stimulation site. Three dimentional location and the amplitude of the equivalent current dipoles for each peak were compared to that of observed SEF without muscle contraction. Significantly increased third component of SEFs (the latency of about 50 ms) always preceeded ca 20 ms against the peak of the late component of the cutaneomusucular reflex observed in the first dorsal interosseous (1DI) muscle. The conduction delay from the primary somatomotor cortex to the 1DI was determined as also ca 20 ms according to the result of the transcranial magnetic stimulation. We conclude from these evidences that all or part of the ingredient of the third component of SEFs may contribute to inducement of the late excitatory component of the cutaneomuscular reflex.

Rise time or rise rate: which should be a criterion for analysing motor unit action potentials?

We measured the rise time (RT) and rise rate (RR) of motor unit action potentials (MUAPs) when sharp sounds are heard in concentric EMG in the first dorsal interosseus muscle (FDI) and biceps brachii muscle (BIC) of normal subjects. MUAPs from FDI muscle with an RT of less than 500 microseconds were 85%, and those of more than 500 microseconds were 15%. In contrast, MUAPs from BIC muscle with an RT of less than 500 microseconds were 65%, and those of more than 500 microseconds were 35%. Distributions of the RR for FDI and BIC were also determined. MUAPs from FDI muscle with an RR more than 0.3 mV/ms were 98.3%, and those of less than 0.3 mV/ms were 1.7%. In contrast, MUAPs from BIC muscle with an RR of more than 0.3 mV/ms were 93%, and those of less than 0.3 mV/ms were 7%. We conclude it is better to use RR than RT when accepting MUAPs in clinical EMG, because even when sharp sounds are heard, MUAPs do not always have an RT of less than 500 microseconds. The use of RT and the sharpness of MUAPS therefore need to be reconsidered, or RR should be used in clinical EMG by automatic program.

The effect of transcranial magnetic stimulation on reciprocal inhibition in the human leg.

The effect of magnetic stimulation on reciprocal Ia inhibition of the human leg was investigated. Stimulation of the common peroneal nerve at the fibula head at the threshold of the alpha motoneuron axons resulted in inhibition of the soleus (SOL) H reflex at a conditioning-test interval of 2 ms. Magnetic stimulation over the contralateral motor cortex resulted in complex modulations of the SOL H reflex, including a short latency facilitation followed by inhibition. This inhibition may have been conveyed by Ia inhibitory interneurons projecting to SOL motoneurons. To test for convergence, whether or not the magnetic stimulation was capable of facilitating disynaptic reciprocal Ia inhibition of the SOL H reflex induced by stimulation of the peroneal nerve, the two stimuli were given together or separately. We observed the inhibition significantly increased when the two stimuli were given together than separately. These results suggest that the Ia inhibitory interneurons projecting to SOL motoneurons in humans might receive convergent input from the motor area of the brain and from Ia afferents of the tibialis anterior (TA) muscle in humans as well as in other animals.

Relation between macro-EMG and muscle Uber conduction velocity.

We investigated the relationship between macro-EMG and muscle Uber conduction velocity in the right biceps brachii in 5 normal subjects using a single-Uber macroelectrode and surface array electrodes. The macro-EMG signal and the surface electrode signals were triggered and averaged by the single-Uber action potential. The amplitude and area of the macro-EMG was correlated with the muscle Uber conduction velocity, suggesting that the macro-EMG can serve as a parameter of the size principle in both the clinical and research settings.

Passive movement of hip and knee joints decreases the amplitude of soleus H-reflex in stroke patients.

OBJECTIVES: The study aims to investigate the changes of the H-reflex in soleus (Sol-H-reflex) during hip and knee joint movements in stroke patients. METHODS: The experiments were carried out on five stroke patients with spastic hemiplegia (2 males and 3 females, 48 to 71 years old). Sol-H-reflexes were measured 200 times for each joint movement speed Stimulus was given at random intervals (4 to 5 seconds) during the joint movement. Two movement speeds were used to investigate the effects of movement speed. RESULTS: For both fast and slow movements, the amplitude of the Sol-H-reflex decreased in the middle flexion phase. In contrast, the amplitude of the Sol-H-reflex increased in the middle extension phase. For the fast movement, the Sol-H-reflex was smaller in only a small angle range during the flexion and extension phase in comparison to the slow movement. The Sol-H-reflex during the flexion phase was significantly smaller than during the extension phase at almost all angles for both speeds. The Sol-H-reflex for both speeds was smaller than the Sol-H-reflex at rest for the whole angle range. CONCLUSION: These results demonstrate the differential effects of Sol-H-reflex modulation in stroke patients when compared with normal subjects.

Changes of reflex size in upper limbs using wrist splint in hemiplegic patients.

We evaluated the effect of prolonged wrist extension on H reflex in the flexor carpi radialis (FCR) muscle and tendon jerk (T) reflex in the biceps brachii (BB) muscle of 17 chronic hemiplegic patients. H reflex of the FCR and T reflex of the BB were assessed every 5 minutes within 20 minutes during prolonged wrist extension and post-20 minutes after the extension. As a result, H reflex in the FCR was reduced by passive wrist stretch in 82% of the spastic limbs. The effect was larger in the higher spastic group. In 45% of the spastic limbs, T reflex in the BB also was reduced by passive wrist stretch. The inhibitory effects had a tendency to strengthen in accordance with the grade of muscle tone. We considered from these results, prolonged wrist extension generated inhibitory projections via probably group II afferents of the FCR in the homonym and in the transjoint in spastic limbs.

A new method of superficial peroneal nerve conduction studies.

Herein, we report a new method for obtaining sensory nerve conduction velocities (SCVs) in the distal segment of the superficial peroneal nerve (SPN). Twenty lower extremities from 10 normal subjects (mean age: 33.4 years) were evaluated. The recording electrodes were placed on the dorsal surfaces of the ankle and foot. We stimulated the SPNs on the anterior edge of subjects' fibulas, and evoked sensory nerve action potentials (SNAPs) antidromically. SCVs were calculated based upon the distances and the latencies. The mean SCV was 41.3 +/- 4.3 m/s in the distal segment, which was slower than in the proximal segment (51.7 +/- 3.9 m/s). We were able to stimulate only the SPN with certainty. In conclusion, the described technique should be of clinical value in diagnosing peripheral neuropathy.

Quantitative EMG and motor unit recruitment threshold using a concentric needle with quadrifilar electrode.

According to Henneman's size principle, small motor units are recruited before large ones. We used the electromyographic (EMG) signal decomposition technique to determine the quantitative relationships between five motor unit action potential (MUAP) parameters (amplitude, duration, area, thickness, and size index) and the recruitment threshold of the motor units recruited up to 50% of the maximum voluntary contraction in the first dorsal interosseous, biceps brachii, rectus femoris, and tibialis anterior muscles of 5 healthy young men. In each muscle, the amplitude, duration, area, and size index had significant, positive high correlations with the motor unit recruitment thresholds. We conclude that the size principle applies to recordings made with concentric needle EMG electrodes under special recording conditions, and therefore that more importance should be attached to the patient's contraction force during EMG examinations in order to evaluate MUAPs for electrodiagnostic purposes.