Kemal S. Türker

Ia reflexes and EPSPs in human soleus motor neurones

SummaryThe reflex responses of single motor units in the soleus muscle to electrical stimulation of the tibial nerve were recorded in human volunteers. A feature of the experiments was the stimulation paradigm used. In order to control the peristimulus firing rate, a computer triggered the stimulus isolator only when 2 interspike intervals of specified duration occurred in succession. In addition, the timing of the stimulus in relation to the preceding action potential was controlled in a manner similar to a conditioning/testing paradigm. The general pattern of response was an initial, “H-reflex” excitation at monosynaptic latency, followed first by a silent period due to the refractoriness of the motor neurone, then by other phases of reduced activity. When the stimulus intensity was increased, the intensity of the excitation and the duration of the silent period increased in parallel. When the pre-stimulus firing rate of the motor unit was varied, the amplitude of the H-reflex response, normalized to the number of stimulus trials, was similar at 6, 8 and 10 Hz, but was greater at 4 Hz in most units tested. These findings were consistent with a simple model of the events occurring at the cell membrane in this reflex which was proposed by Ashby and Zilm (1982a), although some modification of the model was necessary to account for the different response at 4 Hz. The improved stimulation paradigm enabled a direct estimate to be made of the amplitude and shape of the rising phase of the Ia EPSP in human motor neurones.

Reflex responses of motor units in human masseter muscle to electrical stimulation of the lip

SummaryThe reflex responses of single motor units in human masseter muscle to electrical stimulation of the lip were recorded. The subject maintained a contant mean level of pre-stimulus excitation of the parent motor neurone by biting in such a way that the unit fired at either 10 or 15 Hz during each trial. When firing at 10 Hz, most units were reflexly inhibited for up to 90 ms by electrical stimuli at intensities that were perceived to be mildly uncomfortable. In many units, the inhibition consisted of 2 phases which were separated from each other by a few spikes occurring about 30 ms after the stimulus. It was occasionally possible to evoke only the later phase (latency about 40 ms) with stimuli at intensities near the response threshold. In these instances, the inhibitory response became biphasic at higher stimulus intensities with the emergence of a shorter (10–15 ms) component. Still higher intensities caused the 2 phases of inhibition to merge, giving the appearance of a single, prolonged, inhibitory response. When the pre-stimulus firing frequency of the unit was changed from 10 Hz to 15 Hz, the inhibitory responses to the same stimuli were decreased, with the longer-latency component usually surviving beyond the shorter-latency phase. The pattern of reflex responses observed can be explained by a model based on information derived from intracellular recordings in animal experiments.

A method for quantifying reflex responses from intra-muscular and surface electromyogram.

Measuring human reflex responses from electromyogram (EMG) traces in an accurate, repeatable and reliable way with a high degree of specificity has traditionally been a difficult task. This paper describes a new method that can be used to quantify reflex responses from both surface and intra-muscular EMG. This technique extends the classical cumulative sum (CUSUM) calculations by defining precise points for the calculation of latencies, durations and strengths to facilitate automatic reflex detection and permit the strength of a reflex to be defined in absolute units. The effect of varying the pre-stimulus time, the number of trials averaged and the amount of filtering used on the identification and classification of reflex parameters are also investigated. Furthermore, the effect of noise on these values, and how to remove it, is discussed. The new method, which is an expansion of the CUSUM analysis, is compared and contrasted with the more common threshold-crossing method in two different muscles: masseter and first dorsal interosseous (FDI), in experiments utilizing both mechanical and electrical stimulation. There are a number of advantages to using the new method; not only does the modified CUSUM method detect reflexes earlier than threshold-crossing methods but also the strength and duration are less susceptible to averaging and filtering parameters while giving a better indication of the reflex size. The data suggests that a pre-stimulus analysis period of at least 100 ms be used to correctly identify the variability inherent in EMG traces. It is also concluded that for subtle reflexes, 50 stimuli should be the minimum number used when spike trigger averaging is employed as lower numbers are associated with much greater pre-stimulus variability. Zero-phase filtering the rectified averaged EMG traces is recommended as this makes it easier to identify significant changes in the electrical activity of the muscle in question. In addition, noise estimation and removal from averaged rectified EMG recordings yields results that are a more accurate representation of the synaptic activity of the motor units in question.

Does reflex inhibition of motor units follow the "size principle"?

SummaryThe “size principle” is known to dictate the sequence of recruitment of motor neurons during voluntary or reflex activation of muscles. The present study sought to determine whether the size principle also determined the sequence of reflexly-elicited inhibition of motor neurones. It was found that all masseter motor units were about equally inhibited by a mildly-noxious stimulus to the lip, providing that they were tested at the same level of pre-stimulus excitability, i.e., firing frequency. At higher prestimulus firing frequencies, more intense stimuli were required to inhibit motor unit activity. Thus it is the firing frequency of individual motor neurones, rather than their size, which determines their susceptibility to inhibition.

Mechanoreceptors around the tooth evoke inhibitory and excitatory reflexes in the human masseter muscle.

1. The reflex responses evoked in the human masseter muscle by controlled mechanical stimulation of an incisor tooth were examined electromyographically. The stimuli were (slow) pushes and (brisk) taps of about 0.5‐3 N peak force, applied orthogonally to the labial surface. 2. The brisk taps elicited a short‐latency inhibitory reflex that was often followed by an excitatory peak, as has been described earlier. The inhibition increased as the taps became stronger. 3. Slow pushes evoked a long‐latency, primarily excitatory response. The excitation increased with stronger, faster rise‐time pushes; however, with the stronger stimuli, the short‐latency inhibitory response often became evident before the onset of the excitation. 4. The reflex responses to 3 N pushes and 2 N taps were abolished when the receptors around the tooth were blocked with local anaesthetic, indicating that the response was elicited from receptors located within the periodontal area. 5. Prolonged, rapid‐onset displacements evoked a complex reflex response that combined the characteristics of the taps and the pushes. 6. The most likely explanation for the different responses evoked by the pushes and taps is that the patterns of afferent activity elicited by the slow and fast tooth displacements activated different interneuronal pathways to motoneurones. 7. The inhibitory response to taps is essentially a protective reflex which probably serves to reduce the activity of the jaw‐closing muscles when one bites unexpectedly on hard objects. It is suggested that the excitatory response may contribute to the muscle activity required to hold food between the teeth during chewing, or may act as a load compensation reflex to control chewing force.

Motor-unit firing frequency can be used for the estimation of synaptic potentials in human motoneurones

This paper describes a new method that uses the frequency of firing of motor units to estimate the stimulus-induced net post-synaptic potential (PSP) and the synaptic noise in the membrane of voluntarily active human motoneurons. Unlike the peri-stimulus time histogram (PSTH) which is the most commonly used method for assessing stimulus-induced synaptic potentials in human motoneurones, this new approach overcomes contamination of the results caused by the synchronizing effect of the stimulus on the firing pattern of the motor units. However, even after overcoming the contamination by synchronized firing, the new method does not directly represent the true net synaptic potential in the motoneurone membrane. Therefore, a new term estimated net synaptic potential (ENSP) has been introduced. This term highlights the fact that the stimulus-induced net synaptic potential has been determined indirectly and that the size and the shape of this synaptic potential may depend on the level of activity of the recording medium (i.e., pre-stimulus firing frequency of the motor unit). This paper also puts forward a normalization procedure that allows the value of the ENSP and the amplitude of the synaptic noise to be read from the ENSP graph. The normalization procedure, therefore, allows comparisons of those values within and between subjects.

Decomposition of the human electromyogramme in an inhibitory reflex

SummaryThe activity of up to 4 motor units was recorded simultaneously with electrodes placed in the masseter muscle in human subjects. Mildly noxious electrical shocks were applied to the ipsilateral lip while one of the units was kept firing at a steady frequency by voluntary control. The pattern of reflex responses of each of the units was determined, and spike-triggered averaging was used to measure the potential that each action potential contributed to the rectified surface electromyogramme (EMG). Finally, the average contribution made to the surface EMG by each unit throughout the whole course of the reflex was determined. The contribution of each unit to the reflex response in the surface EMG was found to depend on its firing frequency throughout the course of the response, and on its amplitude measured at the surface. The timing of the various phases of inhibition and activation of different units depended on their pre-stimulus firing frequency. In a given bite, the lowest-threshold units were more likely to be firing most rapidly, and these were least susceptible to the inhibitory stimulus. Higher-threshold units tended to produce larger potentials at the muscle surface but, because they fired more slowly in a given bite, they were more powerfully inhibited by the stimulus. Most units showed the same general pattern of inhibitory and excitatory activity that can be seen in the surface EMG. However, the timing of these various phases in the surface EMG did not necessarily correspond with the timing of inhibition and activation in the records of individual units. Rather, the surface signal is the sum of many similar, but out-of-phase frequency changes in the motor units of which it is constituted.

Effect of gender, age, fatigue and contraction level on electromechanical delay

OBJECTIVE The aim of this study was to determine electromechanical delay (EMD) using supramaximal stimuli and to investigate its variation with gender, age, contraction level and fatigue. METHODS Fifteen male and 15 female healthy subjects (aged between 18 and 60) participated in our study. Electromyogram (EMG) recordings were taken from triceps surae muscle. While subjects contracted their muscles voluntarily at specified percentages of maximum voluntary contraction, 10 supramaximal stimuli were applied to the tibial nerve. The time lag between the onset of the EMG response (M-wave) and the onset of force generation was calculated as EMD. RESULTS EMD was found to be 8.5+/-1.3 ms (at rest condition), which is much shorter than those reported in previous studies. Although EMD did not significantly vary with gender (P>0.05), it decreased significantly with escalating muscle contraction level (P<0.05) and increased significantly with advancing age and with fatigue (P<0.05). CONCLUSIONS EMD was found to be considerably shorter than those reported in previous studies, and hence we discuss the possible reasons underlying this difference. We suggest that supramaximal nerve stimulation and high resolution EMG and force recording may have generated this difference. SIGNIFICANCE Current findings suggest that EMD is very sensitive to the method used to determine it. We discuss the reasons for the short EMD value that we have found in the present study.

Evidence for strong synaptic coupling between single tactile afferents and motoneurones supplying the human hand.

1 Electrical stimulation of digital nerves elicits short‐latency excitatory and inhibitory spinal reflex responses in ongoing EMG in muscles acting on the fingers and thumb. Similar responses are elicited by stimulating a population of muscle spindles but not when a single muscle spindle is activated. The current study investigated whether short‐latency EMG responses could be evoked from the discharge of a single cutaneous afferent. 2 Thirty‐three tactile afferents were recorded via tungsten microelectrodes in the median nerve of awake humans. Spike‐triggered averaging revealed EMG events time‐locked to the afferent discharge. The afferents were activated by an external probe and the EMG was elicited by a weak voluntary contraction. 3 Eleven cutaneous afferents (33 %) showed a short‐latency response in the ongoing EMG. Overt increases or decreases in EMG were observed for seven afferents (onset latency 20.0‐41.1 11hms1h). For four slowly adapting (SA) type II afferents, EMG showed a periodicity that was correlated to the afferent interspike interval (r= 0.99). 4 The EMG associated with two rapidly adapting (FA) type I afferents (29 %) showed a short‐latency excitation while five showed neither excitation nor inhibition. Seven SA II afferents (39 %) showed excitation and 11 no response; and none of the six SA I afferents showed any response. 5 We conclude that, unlike muscle spindle afferents, the input from a single cutaneous afferent is strong enough to drive, via interneurones, motoneurones supplying muscles acting on the digits. The potent short‐latency response we found supports the important role of cutaneous mechanoreceptors in fine motor control of the human hand.

Cross-talk from other muscles can contaminate EMG signals in reflex studies of the human leg

The extent of contamination of the whole muscle surface electromyogram (SEMG) was studied using the classical H-reflex preparation of the human leg. SEMG and single motor unit activity were recorded from the soleus and the anterior tibial muscles in response to a low intensity stimulation of the tibial nerve at the popliteal fossa. The SEMGs of individual motor units were established to compare with the whole muscle SEMG in order to determine the presence of the cross-talk. It was shown that the whole muscle SEMG can be contaminated by the activity from the antagonists especially at high stimulation intensities.