Biomedical subjects

T S Miles

Publications and source records attributed to T S Miles.

At least 55 records · Page 3Linked to original sources

PubMed1991

Instability of motor unit firing rates during prolonged isometric contractions in human masseter.

The firing patterns of up to 4 concurrently active masseter motor units were studied with intramuscular electrodes during a continuous isometric contraction of 15 min duration, in which the subject maintained the mean firing rate of one selected unit at 10 Hz. With this paradigm the net excitation (i.e. mean firing rate) of one unit in the muscle was controlled. This served as the reference for the functional state of other active units during the prolonged contraction. With the mean firing rate of one unit in the muscle fixed, 58% of other active units showed a slow, statistically-significant change in mean firing rate over the 15 min. The initial firing rate of the units did not influence the change in rate. The original firing rate hierarchy, which in short-term contractions reflects the recruitment order, was altered during the prolonged contraction. The explanation for these differential changes in motoneuron net excitation is not clear; they could be intrinsic to the motoneurons or perhaps mediated by reflex pathways. The selective facilitation or suppression of some motor units with continuous activation means that the original size-structured combination of motor units can be modified during a prolonged contraction.

Adolescent↗

PubMed1991

Discharge variability and physiological properties of human masseter motor units.

Intramuscular electrodes were used to study discharge variability in motor units of human masseter whose physiological properties were determined using spike-triggered averaging. Subjects voluntarily controlled the mean firing rate of a selected motor unit at 10 Hz for 15 min of continuous activation. Discharge variability was assessed at the beginning and end of this period. In 81% of units, the discharge variability at a mean interspike interval (ISI) of 100 ms increased after 15 min of continuous activity. There was a wide range of discharge variability within the population of masseter units studied, but no significant correlations were found between initial discharge variability and recruitment threshold, twitch tension or time-to-peak tension (TTP). There was, however, a significant correlation between motor unit fatigability and its initial discharge variability. This represents a link between the motoneuron and the functional properties of the muscle fibers in innervates.

Adolescent↗

PubMed1991

Threshold depolarization measurements in resting human motoneurones.

Threshold depolarization is defined as the amplitude of the depolarization that will just bring a neurone to its firing threshold. Here, a novel technique is described for the measurement of threshold depolarization in resting human motoneurones. The tibial nerve was stimulated at an intensity that was just sufficient for the resulting Ia afferent volley to activate a single motor unit in the soleus muscle. The same motor unit was then recruited by voluntary activation, and the amplitude of the EPSP elicited by this stimulus was determined by a manner described earlier. The value found for the threshold depolarization was around 5 mV for all 6 motoneurones that were successfully tested.

Action Potentials↗

PubMed1990

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.

Action Potentials↗

PubMed1990

Silent periods in dysfunction patients: a preliminary study using a novel approach.

Strong peri-oral stimulation is known to elicit a transient inhibition in the activity of the jaw-closing muscles. Several reports have claimed that this 'silent period' (SP) is prolonged in patients with temporomandibular joint dysfunction syndromes. In the present preliminary study, the biphasic SP evoked by electrical lip stimulation was tested in single motor units in the masseter muscles of dysfunction patients. In comparison with normal subjects, the pattern of the SP was different in the patients in that the shorter-latency phase predominated over the later phase, whereas the converse is true for normals.

Electric Stimulation↗

PubMed1990

Synchronization of motor units in human masseter during a prolonged isometric contraction.

1. The action potentials of concurrently active motor units in human masseter were recorded on several fine-wire intramuscular electrodes and synchronization was assessed by cross-correlation of their firing times. 2. Long recording sessions and a test of significance for peaks in histograms were used to enhance detection of synchrony. 3. Synchrony in masseter units was found to be very weak, and was observed consistently only in histograms with a large number of counts. However, synchrony was widespread within the motoneurone pool; of the unit pairs whose cross-correlation histograms contained more than 100 counts per 1 ms bin, 89% were significantly synchronized. 4. The strength of synchrony between pairs of units did not change consistently during 15 min of continuous activity. 5. It was concluded that most, if not all, low-threshold masseter motoneurones have some common synaptic inputs, although the origin of those inputs remains unclear.

Action Potentials↗

PubMed1990

Fatigue of single motor units in human masseter.

The spike-triggered averaging technique was used to determine the time course and extent of fatigue of single motor unit twitches in the human masseter. This is the first report of a fatigue test having been applied to masseter motor units in either animals or humans. The human masseter was found to be comprised predominantly of fast-twitch motor units with a broad spectrum of fatigability. Very few physiological type S units were found, despite histochemical evidence for a substantial population of type I fibers in the masseter. In addition, there was no significant correlation between fatigability and either twitch amplitude or contractile speed in the motor units studied. The latter observations are consistent with the unusual histological features of the masseter. Comparison with other human fatigue data suggests that the extent of fatigue in the present population of masseter motor units after approximately 3,000 activations is similar to that reported for populations of units in first dorsal interosseous and medial gastrocnemius.

Adolescent↗

PubMed1989

Modulation of an inhibitory reflex in single motor units in human masseter at different joint angles.

The inhibitory reflex evoked in single units in human masseter by lip stimulation was recorded at several joint angles. The pre-stimulus firing frequency of the unit was kept constant. Some modulation of the inhibitory response at different vertical jaw positions (i.e. temporomandibular joint angle) was observed in most masseter units, with the majority of the modulated units being inhibited less when the teeth were closer together.

Action Potentials↗

PubMed1989

Effect of motor unit firing pattern on twitches obtained by spike-triggered averaging.

The spike-triggered averaging technique (STA) was used to determine the twitch profile of single motor units in human masseter during a voluntary isometric contraction. The effect of the immediate firing pattern of the unit on the twitch was assessed with a computer program which scanned the unit discharge records and selected valid trigger spikes for the averager on the basis of the interspike intervals preceding and following the trigger spike. Successive averages from the same data using different interval parameters revealed progressively more fusion of twitches as the instantaneous firing rate increased. When the data were averaged with interval parameters similar to those used in earlier studies, some fusion of the twitch profile was also evident. It is therefore likely that the degree of fusion of human motor unit twitches obtained by STA has been underestimated in the past. It is further concluded that masseter motor units are sufficiently fast-contracting to allow a relatively unfused twitch profile to be obtained with STA, provided trigger spikes are subjected to appropriate rate control.

Adolescent↗

PubMed1989

Ia reflexes and EPSPs in human soleus motor neurones.

The 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 peri-stimulus 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.

Action Potentials↗

PubMed1989

Biphasic inhibitory responses and their IPSPs evoked by tibial nerve stimulation in human soleus motor neurones.

A novel stimulating protocol was used to demonstrate the existence of a biphasic inhibitory reflex in single soleus motor neurones in response to weak stimulation of the tibial nerve in human subjects. The stimulus intensity was sufficient to evoke a small M-response but was subthreshold for the H-reflex. Stronger stimulation caused the 2 phases of inhibition to fuse. The experimental protocol allowed excitatory and inhibitory responses to be recognized unambiguously. The criterion for excitation was stimulus-induced advance in the timing of the next action potential in certain trials: for inhibition, the criterion was stimulus-induced delay in the timing of the next action potential. The shorter-latency inhibitory response began at about 45-50 ms and lasted for about 20 ms. With the stimulus intensity used, only inhibitory responses were evoked at short latency in tibialis motor neurones, which suggests that the soleus inhibition resulted from recurrent inhibition via the Renshaw pathway rather than by activation of Ib afferents in the tibial nerve. A novel approach to the determination of the waveform of the IPSP underlying the shorter-latency (presumably Renshaw-mediated) inhibition revealed only part of the waveform owing to the sharp fall in the membrane potential at the onset of (presumably-Renshaw) IPSP. This approach was more successful when applied to the longer-latency, slower IPSP. The longer-latency inhibitory response in soleus motor neurones began at about 80-120 ms. Overall, the latency of this inhibition was less when it followed an M-response than when it followed an H-reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1989

A comparison of the masseteric silent period in temporomandibular joint dysfunction and normal human subjects by surface electromyography and single motor-unit recordings.

Single electric stimuli were delivered to the lower lip and the reflex effects were monitored in the ipsilateral masseter. Two strengths of stimulus were delivered at two levels of background excitation (defined in terms of the firing frequency of a single motor unit). Although the threshold stimulus intensity required to produce a silent period in the single motor units was the same for the patients with temporomandibular joint dysfunction and normal subjects, there were qualitative differences in the pattern of reflex responses between the two groups. Of the two phases of inhibition elicited by the stimulus, the short-latency inhibitory period was the more prominent and more resistant to changes in the background excitation levels in the dysfunction group. In normal subjects, however this was true of the long-latency inhibition. This difference may be due to the changes in the inputs to one or more of the interneurones linking the afferent limb of the reflex to the masseteric motoneurones.

Adult↗

PubMed1989

Surface electromyography, force and single motor-unit data for inhibitory reflex responses in human masseter at two levels of excitatory drive.

The inhibitory reflex response evoked by mildly noxious lip stimulation was tested in up to 100 trials at two different levels of pre-stimulus excitation. To regulate that excitation, the subject voluntarily controlled the firing rate of one motor neurone at 10 Hz, and then at 15 Hz while the reflex was tested. When the frequency of the motor-unit action potential (MUAP) in the controlled unit increased or decreased, the frequency of other active units' MUAPs also changed in parallel: that is, the frequency of one MUAP was an index of whole muscle excitation. The inhibitory response in the surface electromyogram (EMG) was quantitated by rectifying and averaging the signal, and by measuring its depth and area. The best correlation between the responses evoked in single units and the response in the whole muscle EMG at the two levels of excitation was obtained when the area of inhibition in the surface record was normalized to the pre-stimulus EMG level. The inhibitory response to lip stimulation in the masseter EMG decreased when the level of pre-stimulus excitation in the muscle was increased. This was confirmed using the reflex response in the averaged force data. The findings suggest that controlling one motor unit's firing rate is a useful method for controlling the whole-muscle excitation in quantitative reflex studies.

Bite Force↗

PubMed1988

The lip-clip: a simple, low-impedance ground electrode for use in human electrophysiology.

A simple and effective ground electrode is described for use in human electrophysiological recordings. This electrode consists of a wire clip that holds a silver disc of 1 cm diameter which is clipped to the lower lip of the subject. It is found that this simple ground electrode reduced the background noise and/or stimulus artifact at least as effectively as other conventional grounding techniques.

Electric Stimulation↗

PubMed1988

On the long-latency reflex responses of the human flexor digitorum profundus.

1. Electromyography (surface and intramuscular) has been used to study the reflex responses of the human flexor digitorum profundus (FDP) to angular rotation of the distal interphalangeal joint of the 4th finger. This has been done with the hand in three separate positions which, owing to the arrangement of the various tendons, allow the movement to be transmitted to (a) both the flexor and extensor muscles, (b) FDP alone (extensors disengaged) and (c) neither flexor nor extensor muscles (all muscles disengaged, but cutaneous and joint receptors still potentially activated). The stimuli were applied while the subject was voluntarily contracting FDP to produce a constant level of EMG activity; this remained possible when the muscle was disengaged from the joint. 2. With all muscles connected, FDP behaved similarly to the analogous long flexor of the thumb. 'Stretch' elicited a prolonged complex response starting with a short-latency component corresponding to the tendon jerk. Unloading of the contracting muscle caused a pronounced reduction of its on-going EMG activity. The latency of this latter effect was approximately 20 ms greater than that of the initial stretch-evoked response, thereby demonstrating that it was not due to a disfacilitation via the short-latency pathway (on reduction of the tonic spindle afferent firing from FDP as it shortened). 3. With all muscles disengaged, movement of the joint in either direction evoked simply a weak, variable excitatory response, with a latency somewhat greater than that of the normal unloading response. This was attributed to the activation of cutaneous and/or joint receptors. The effectiveness of the disengagement of the flexor was demonstrated by the abolition of its normal stretch-evoked short-latency response. 4. With the flexor engaged and the extensors disengaged both stretch and release evoked their normal types of response. In control experiments, surface EMG recordings from the interosseus muscles confirmed that the procedure used for extensor disconnection was effective. These findings exclude the possibility that the reduction of EMG activity of the unloading response of FDP might be attributable to an inhibition evoked by the concomitant stretch of its antagonists. 5. The long-latency unloading response (whether with the extensors engaged or disengaged) remained when the sensory receptors in the finger itself were inactivated, confirming that these were not responsible.(ABSTRACT TRUNCATED AT 400 WORDS)

Electromyography↗

PubMed1987

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

The reflex responses of single motor units in human masseter muscle to electrical stimulation of the lip were recorded. The subject maintained a content 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 latter phase (latency about 40 ms) with stimulus 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.

Action Potentials↗

PubMed1987

Decomposition of the human electromyogramme in an inhibitory reflex.

The 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1987

The cortical control of motor neurones: some principles of operation.

There are several ways in which the motor control areas of the brain can elicit action potentials in motor neurones. In this article, it is argued that the basis for the cortical control of motor neurones is that each corticospinal cell gives excitatory inputs to all or most motor neurons innervating a given muscle. When a muscle contracts under voluntary control with a steadily-increasing force, the asynchronous activity in many corticospinal cells will effectively result in a steadily-increasing injection of current into most or all motor neurones. When this current exceeds the level necessary to bring individual cells to threshold, they will begin to fire. The threshold current for each cell is determined by the cell's size and the biophysical properties of its membrane. As the level of this "common drive" from the cortex increases, all active cells will tend to increase their firing frequency proportionately: decreases in the common drive will cause their individual frequencies to fall proportionately. Damage to corticospinal pathways (as in strokes) will decrease the total excitatory input to all motor neurones. This will mean that a larger effort is necessary to produce more activity in cortical cells, resulting in an increased sense of exertion and an increase in the subjective sense of muscle fatigue.

Action Potentials↗