Biomedical subjects

T S Miles

Publications and source records attributed to T S Miles.

At least 19 recordsLinked to original sources

PubMed1999

Observations on the variability of the H reflex in human soleus.

H reflexes were evoked in human soleus by stimulating the tibial nerve at a constant intensity. Each trial was then assigned to one of three groups on the basis of the amplitude of its H reflex; all trials in each group were then full-wave rectified and reaveraged. There was a strong positive relationship between the amplitude of the H reflex and the level of electromyographic activity in the muscle at the time of onset of the H reflex, which reflects the activity of the motoneuronal pool when the afferent volley arrived. Thus, much of the variability of the H reflex is due to small changes in the level of activation of the motoneuronal pool during repeated trials. The steady torque preceding the H reflex was a poor predictor of the H-reflex amplitude, presumably because of the delay between the changes in the electrical activity of motoneurons and the mechanical outcome thereof.

Adult↗

PubMed1998

Movements modulate the reflex responses of human flexor pollicis longus to stretch.

The reflex responses to brisk, ramp stretch perturbations of the human flexor pollicis longus muscle (FPL) were recorded during isometric and slow concentric or eccentric contractions at similar levels of muscle excitation. The subjects flexed their thumb to push down against a thumb-rest, whose position was controlled by a servo-controlled motor. In different runs, the stretch perturbations were imposed when the thumb-rest was stationary (isometric) or was flexing or extending the interphalangeal joint of the thumb at a constant velocity, i.e. during concentric or eccentric contractions of FPL. The latency of the most prominent component of the electromyographic reflex in the isometrically contracting muscle was about 60 ms, measured from the command signal. The amplitude of this response was sharply reduced during the non-isometric contractions. While not dependent on the direction, this modulation of the reflex response increased with the speed of active movement of the interphalangeal joint (flexion or extension). The response was greatly reduced during concentric or eccentric movements as slow as 1.6 mm x s[-1] (approximately 5 degrees x s (-1) at the joint). When the force rather than the position of the thumb-rest was servo-controlled, the stretch response to perturbation again diminished with speed in a self-paced flexion task, compared with an isometric "hold" condition.

Adult↗

PubMed1997

Control of motor units in human flexor digitorum profundus under different proprioceptive conditions.

1. Changing the posture of the human fingers can functionally 'disengage' the deep finger flexor muscle from its normal action on the terminal phalanx of the fourth (or third) finger. This enables the activity of the muscle to be studied both with and without its normal proprioceptive inputs. 2. Spike trains of long duration from pairs of concurrently active motor units in this muscle were recorded in both the engaged and disengaged hand postures. Subjects voluntarily kept one of the motor units (the 'controlled' unit) discharging at the same target frequency in both postures. The strength of short-term synchrony, the strength of common drive, and the variability of discharge of these pairs of motor units were determined in both postures. 3. All subjects reported that the effort required to activate the motor units in the disengaged hand posture was substantially greater than in the normal engaged posture. 4. Short-term synchrony, which is a function of common corticospinal inputs to pairs of motor units, was similar in both hand postures. However, the strength of common drive was significantly decreased when the muscle was disengaged. Although the neural substrate for common drive is not known, this observation suggests that proprioceptive feedback is involved either directly or indirectly. 5. Although the discharge rate of the 'uncontrolled' motor units increased when the muscle was disengaged, the variability of discharge of these and the 'controlled' motor units increased significantly. This supports the idea that the precision with which fine motor tasks can be performed is improved when proprioceptive feedback is intact.

Action Potentials↗

PubMed1997

Estimating post-synaptic potentials in tonically discharging human motoneurons.

The activity of single motor units in human muscles can be recorded with relative ease, and the spike train of a single motor unit precisely reflects the spike train of the parent motoneurone. This has led to the proposal of a number of methods to estimate stimulus-evoked post-synaptic potentials in human motoneurones. All of these methods rely on manipulating the spike trains of motor units over a number of trials. All are based on a number of assumptions, all have limitations, and none so far have passed the test of a direct comparison of the estimate of the shape of the post-synaptic potential with a direct intracellular measurement of it. These techniques are summarised in this review.

Electric Stimulation↗

PubMed1997

Symmetric force response of human masticatory muscles to stretch and unloading.

During chewing, the force exerted by the jaw-closing muscles must constantly adapt to changing resistances between the teeth, as the food is broken down. In the present study, the changes in biting force resulting from small, controlled displacements imposed on isometrically contracting jaw-closing muscles were measured. We found that the force changes resulting from small loading and unloading movements were normally highly symmetrical. The initial force change was linear, and preceded the onset of reflex changes in muscle activity. Later changes in force were the result of both short- and long-latency reflexes in the jaw-closing muscles, the long-latency component being quantitatively greater. The long-latency unloading reflex in the jaw-closing muscles has not been described hitherto. The symmetry of force increase with loading and decrease with unloading was absent in one subject with atypical stretch and H-reflexes.

Adult↗

PubMed1997

Responses of human single motor units to transcranial magnetic stimulation.

Transcranial electromagnetic brain stimuli elicit a complex response in the electromyogram of active human hand muscles. Relatively weak stimuli evoke a short-latency primary response via a presumably monosynaptic corticospinal path. This is followed by a silent period that is terminated by a second peak at a latency of 50-80 ms. The responses evoked in single motor units in flexor digitorum profundus (FDP) were recorded. Responses were elicited at the second-peak latency only in trials in which no primary response was elicited in that unit, and only when the stimulus was given during the first half of the interspike interval (ISI). When given during the second half of the ISI, the same stimulus evoked a primary response but no second peak response. Stronger stimuli suppressed the second peak by evoking a primary response in more trials. Having discharged at about 20 ms latency, the parent motoneurone was unable to discharge again at second-peak latency, 30-60 ms later. The response at second-peak latency was not modified by disengaging both FDP and the extensors of the distal interphalangeal joint. Hence, this response is not secondary to a stretch reflex provoked by activation of the finger extensors, nor is it the result of a cutaneous signal resulting from movement of the finger. The latencies suggest that the corticospinal volley evokes a beta-motoneurone-mediated twitch in FDP muscle spindles, which elicits an afferent volley that activates the motoneurone reflexly. The first 100 ms or so of the silent period is due to the realignment of the first post-stimulus spike in most trials to corticospinal latency; i.e. this is not necessarily the result of an inhibitory or disfacilitatory process. Still stronger stimuli increase the duration of the ISI in which the stimulus is given, indicating the presence of an inhibitory/disfacilitatory process.

Adult↗

PubMed1997

Influence of muscle blood flow on fatigue during intermittent human hand-grip exercise and recovery.

1. The influence of muscle blood flow on fatigue and recovery was studied in the forearm muscles of eight male subject performing a powerful isometric hand-grip exercise. The exercise was performed with the exercising forearm normally perfused and, on a separate occasion, with its blood flow occluded with a sphygmomanometer cuff. 2. In the no cuff condition, peak force declined to an initial plateau at 40-50% of the maximal voluntary grip force (MVC). When perfusion was occluded, the force decline was similar during the first minute of exercise, then force fell rapidly to exhaustion. 3. In a separate experiment to investigate the mechanisms underlying the plateau in force loss, occlusion of blood flow during the force plateau phase resulted in a rapid decline in force to exhaustion. 4. Recovery of peak force after the cuff exercise was significantly greater during the initial 3.5 min of recovery than after no-cuff exercise. After this time, recovery was similar for both conditions. 5. Muscle blood flow occlusion during intermittent exercise profoundly reduces endurance without prolonging recovery. Recovery time may depend on the duration and energy cost of the exercise rather than on the degree of force loss. 6. The present study suggests that the fall in muscle force induced by a continuous MVC is a combination of profound short-term fatigue in anaerobic muscle fibres due to the consumption of their short-term energy supplies, plus a decline in force production by aerobic muscle fibres that is the consequence of hypoxia. Thus, MVC may not be a good model of fatigue occurring under submaximal conditions, as hypoxia of type I fibres is unlikely to occur under physiological conditions in which muscle contractions are usually intermittent.

Adult↗

PubMed1997

Electrophysiological observations on an unusual, task specific jaw tremor.

A patient with no other neurological signs or symptoms presented with a prominent tremor restricted to the mandible. This 5-6 Hz tremor was interesting in that it was normally confined to the digastric muscles and was highly task specific. In the course of her normal daily activities, it began only when the patient drank from a cup or glass. The localisation of this tremor to a muscle that has no muscle spindles and no reciprocal inhibitory reflexes suggests that such tremors must be capable of being generated centrally.

Electromyography↗

PubMed1995

Responses of human masseter motor units to stretch.

1. The reflex responses to stretch were studied in single motor units and the surface electromyogram in human masseter. 2. Controlled stretches of the isometrically contracting jaw-closing muscles evoked short-latency (10-15 ms) and long-latency (35-70 ms) excitatory reflex responses in the masseter surface electromyogram. 3. The majority (65%) of tonically active masseter motor units were excited in both short- and long-latency phases of the reflex. The timing of the stimulus determined whether the unit discharged in the short- or long-latency phase. If a non-tonically active motor unit was recruited by the stimulus, it invariably discharged in the long-latency phase. 4. Although short-latency responses were strongly time-locked to the stimulus, there was very little shortening of interspike intervals (ISIs) in this phase of the reflex. The shortening of ISIs was more prominent and prolonged during the long-latency phase, which explains why this phase produces most of the reflex force changes following the stretch. 5. Within pairs of concurrently active motor units there was a tenfold range in the size of the short-latency response to the same stretch. 6. A substantial proportion (35%) of the twenty-two masseter motor units tested had no statistically significant short-latency reflex response. 7. In contrast to other human muscles, there was no functional connection between a population of Ia afferents and some masseter motoneurons. There are two possible explanations for this result. The short-latency, presumably monosynaptic, Ia afferent inputs may not be uniformly distributed to human masseter motoneurons. Alternatively, these inputs may be subject to tonic presynaptic inhibition that is not uniformly distributed throughout the masseter motoneuron pool.

Adult↗

PubMed1995

Fatigue of jaw muscles and speech mechanisms.

Histochemical studies show that the distribution of fiber types in human jaw muscles is different from that in various limb muscles, no doubt representing different functional demands as well as a different embryological derivation. Jaw-closing muscles appear more resistant to fatigue than limb muscles with intermittent maximal contractions. Endurance of continuous isometric biting is limited by pain. Masseter motor unit fatigability in sub-maximal contractions is similar to the limb muscles. There are few physiological data for the jaw-opening muscles. The distribution of fiber types in human speech muscles is consistent with the high speeds of contraction that must be used in phonation. Although clinical syndromes of fatigue of speech muscles are recognized, there is little direct information on the fatigability of the muscle fibers themselves.

Animals↗

PubMed1995

Discharge patterns of tonically firing human motoneurones.

We have attempted to reconcile the different patterns of distribution of interspike intervals that are found in motoneurones made to discharge by intracellular injection of constant current in reduced animal preparations and by voluntary control in human subjects. We recorded long spike trains from single motor units in three human muscles made to discharge at constant mean frequencies with the help of auditory and visual feedback. The distribution of interspike intervals in each spike train was analysed quantitatively. We found that the different pattern of discharge of the human motor units could be accounted for when due allowance was made for the variability of the drive to the human motoneurone which arose because of the feedback process used to maintain the target frequency. A model testing this hypothesis gave results that were qualitatively consistent with the human data.

Acoustic Stimulation↗

PubMed1995

Spike-train acquisition, analysis and real-time experimental control using a graphical programming language (LabView).

A solution is described for the acquisition on a personal computer of standard pulses derived from neuronal discharge, measurement of neuronal discharge times, real-time control of stimulus delivery based on specified inter-pulse interval conditions in the neuronal spike train, and on-line display and analysis of the experimental data. The hardware consisted of an Apple Macintosh IIci computer and a plug-in card (National Instruments NB-MIO16) that supports A/D, D/A, digital I/O and timer functions. The software was written in the object-oriented graphical programming language LabView. Essential elements of the source code of the LabView program are presented and explained. The use of the system is demonstrated in an experiment in which the reflex responses to muscle stretch are assessed for a single motor unit in the human masseter muscle.

Action Potentials↗

PubMed1994

Stretch reflexes in human masseter.

The reflex response to stretch in most contracting human muscles includes both a short-latency, probably monosynaptic, excitatory component, and a longer-latency, polysynaptic excitation. However, it has been claimed that stretch of the jaw-closing muscles evokes only the short-latency response in masseter. This question was re-examined, using controlled stretches of varied rates and durations. Very brief, rapid stretches analogous to the stimuli used to investigate the 'jaw-jerk' reflex in earlier studies evoked a prominent excitatory peak in the electromyogram at monosynaptic latency excitation, but little or no longer-latency excitation. This response could be produced even by stimuli that were barely detectable by the subject. However, this prominent electrical response did not produce a measurable increase in biting force. In contrast, slower stretches evoked both a short- and a longer-latency excitatory response in the surface electromyogram, as in most limb muscles. It is shown that the absence of a long-latency excitatory response in earlier studies can be explained by the powerful reflex disfacilitation of the motoneurones that occurred at the end of the brief stretches used. Depending on the duration of the stretch, this disfacilitation is often sufficient to mask or abolish the long-latency reflex. The reflex response to stretches was not markedly affected by blocking the activation of mechanoreceptors around the teeth with local anaesthetic, indicating that receptors around the teeth cannot be playing more than a minor role in the response. The stretch-induced increase in force became greater as the velocity of the stretch decreased.

Adolescent↗

PubMed1994

Reflex responses of motor units in human masseter muscle to mechanical stimulation of a tooth.

The reflex responses evoked by controlled mechanical stimulation of an upper central incisor tooth in single motor units in the human masseter muscle were examined. The stimuli were (brisk) taps and (slow) pushes of about 2 N peak force, applied orthogonally to the labial surface of the ipsilateral upper central incisor tooth. The reflex responses of the motor units were characterised by analysis of the changes in the durations of the first and second interspike intervals (ISIs) immediately following the stimulus. A significant increase in the duration of these ISIs in comparison with pre-stimulus ISIs indicated inhibition, and significant shortening indicated excitation. Twenty masseter motor units were tested with both the pushes and the taps. The brisk taps elicited a significant reflex inhibition in 16 of the 20 motor units at a latency of 13 ms and duration of 37 ms. This inhibition was followed by significant excitation in 11 of the 20 units at latencies of 71 ms, lasting for 29 ms. The short-latency response to slow pushes was significant inhibition in four units: significant excitation in one unit and no response in 15 units. The slow pushes evoked a significant long-latency excitatory reflex response in 12 of the 20 units at latencies of 77 ms and lasting for 40 ms. The shapes and amplitudes of the compound post-synaptic potentials underlying the reflex responses in the motoneurones were estimated. It is concluded that stimulation of periodontal mechanoreceptors usually activates an excitatory reflex pathway to the jaw-closing motoneurones. This probably helps to grip the food bolus between the teeth during chewing. However, when the rate of application of the stimulus is large enough, a short-latency inhibitory response is evoked which, if of sufficient duration, may over-ride the subsequent excitatory response. Inhibition of the jaw-closing muscles will tend to protect the teeth and soft tissues when one bites unexpectedly on a hard object while chewing.

Adult↗

PubMed1994

A new approach to the estimation of post-synaptic potentials in human motoneurones.

A new method is described for estimating the shape of the compound post-synaptic potentials evoked by stimuli in human motoneurones. The method is based on changes in the duration of the interspike intervals in motor-unit spike trains that are time-locked to the stimulus. This is particularly helpful in estimating the profile of long-latency slow rise-time post-synaptic potentials that are difficult to estimate with other methods. The method is simple to apply and the analysis is readily implemented on a personal computer.

Electric Stimulation↗

PubMed1994

Reflex responses to periodontal and auditory stimulation in human masseter.

An investigation was made of the reflex responses evoked in the human masseter by periodontal mechanoreceptors. Weak taps were applied to the labial surface of a central incisor tooth by an electromechanical stimulator with a flexible probe (von Frey hair). Forces as low as 0.2 N evoked inhibitory reflex responses in the surface electromyograms of both masseter muscles. These reflexes were modulated to markedly different extents in different subjects by auditory white noise, which always reduced the amplitude of the inhibition. The reflexes were abolished when local anaesthesia was infiltrated around the stimulated tooth and white noise was played into the ears. Evidence is presented that the sound of the tap on the tooth, transmitted through the air, is in itself sufficient to evoke an inhibitory reflex in masseter which is qualitatively similar to that from the periodontal receptors. Thus, in the absence of auditory masking, the total reflex evoked by tooth taps is the result of the summation of the inputs from both periodontal mechanoreceptors and auditory receptors.

Acoustic Stimulation↗

PubMed1994

The control of human motor units.

1. The motor unit, consisting of a single motor neuron and the skeletal muscle fibres that it innervates, is the final output pathway of the motor system. 2. Much is now known about the way that human motor neurons are recruited and controlled during voluntary and reflex movements. This review briefly summarizes some of the recent experimental data that has contributed to our present understanding. The review is largely limited to data obtained in human experiments. While much of what we know about the organization of the nervous system has come from studies of the anatomy and physiology of experimental animals, there are some questions that cannot be addressed in reduced animal preparations. The development of new techniques has made it possible to investigate the human nervous system at a level of detail that has not hitherto been possible.

Animals↗

PubMed1993

Position sense at the elbow after fatiguing contractions.

Fatigue is often associated with increased clumsiness. One possible explanation for this is that the proprioceptive signals from receptors in and around muscles change during muscle fatigue. Thirteen human subjects were tested for their ability to match the elbow angle of one arm with the contralateral arm, before and after a fatiguing contraction of one arm. Contractile fatigue was induced by a series of maximal voluntary contractions of the elbow flexors of the dominant arm. While fatigue of either the target arm or the matching arm usually changed the ability of individual subjects to match arm position, this effect varied markedly from one subject to another and no consistent pattern was discerned. In particular, there was no reciprocal change when the fatigued arm was the matching arm compared with when the nonfatigued arm was the matching arm. The absence of a consistent reciprocal effect indicates that the fatigue-related changes in the ability to match arm position are not solely due to changes in proprioceptive signals and that central fatigue processes are probably involved.

Adult↗