Biomedical subjects

T S Miles

Publications and source records attributed to T S Miles.

At least 37 records · Page 2Linked to original sources

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↗

PubMed1993

Simple reaction-time responses to mechanical and electrical stimuli in human masseter muscle.

The latencies of the simple reaction-time responses for opening and closing of the jaws in response to taps and pushes on teeth and lip shocks were measured in human adults. The reaction times were scored from both the integrated electromyographic (EMG) signal in individual trials, and from the cumulative sum (CUSUM) of the averaged EMG response to 50 stimuli. The mean reaction times for both closing and opening were about 80-90 ms for taps and electrical lip shocks, and about 140 ms for push, measured from the CUSUMs. The reaction times in individual trials were difficult to measure from the EMG signal because of the unsteady baseline. In contrast, the reaction times measured from the CUSUMs were clearly defined by the point at which the record began to move sharply above or below the baseline. However, because of the delays inherent in the CUSUM procedure, they were systematically longer than the means of the trial-by-trial measurements.

Adult↗

PubMed1993

An apparatus for controlled stretch of human jaw-closing muscles.

The design of a special-purpose muscle stretcher for reflex studies of the human jaw-closing muscles is described. The device is based on a servo-controlled electromagnetic vibrator which imposes controlled displacements on the lower jaw. The mechanics of the device keep jaw movements coaxial with the temporomandibular joint during the stretches. The design incorporates important safety features including mechanical stops and electronic cut-outs to protect the jaws from excessive stretches.

Electromyography↗

PubMed1993

Dental pain: self-observations by a neurophysiologist.

This is a description of a dentally trained neurophysiologist's first-hand experience of the slow and progressive deterioration of the pulp of one of his posterior teeth. He describes his sensations throughout the course of the pulpal pathosis and his experience as a patient undergoing endodontic treatment for the first time. His subjective experiences are put into the context of the current understanding of the physiology of pain.

Adult↗

PubMed1993

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.

Adolescent↗

PubMed1992

Quantitative analysis of reflex responses in the averaged surface electromyogram.

The relationship between the averaged surface electromyogram (EMG) and the activity of motoneurones in reflex studies is analysed mathematically. This analysis reveals that, subject to certain conditions being met, the integral of the average of the unrectified EMG is linearly related to the activity of motor units in the muscle. This was tested with experimental data. The activity of 2 motor units, and the surface EMG, were recorded directly in a reflex paradigm. A close match was found between the integral of the average of the unrectified EMG and the summed peristimulus time histograms (PSTHs) of the units' activity. This analysis gives more quantitative measurements of the timing of motoneuronal activity in reflexes than the conventional analysis of the EMG. It also offers the potential for making quantitative measurements of changes in motor unit activity evoked by different stimuli. This analysis avoids the pitfall of the artefactual peaks that can occur in the average of the rectified EMG as a consequence of the full-wave rectification. It is concluded that the integral of the unrectified average offers a valuable adjunct to the conventional analysis in many reflex studies. The analysis can also be applied to quantitative studies of neurograms.

Action Potentials↗

PubMed1992

A simple stimulator for periodontal mechanoreceptors in human subjects.

Mechanoreceptors in the periodontal ligament are activated by tooth movements as little as 1-3 microns. It has been difficult in the past to move a tooth only over this small distance in order to activate the periodontal receptors without at the same time activating other receptors in related structures. This paper describes a simple solution to this problem based on the use of von Frey hairs (in the form of nylon filaments) rather than a rigid probe, to move the tooth. Filaments of different stiffness are mounted on the moving coil of a loudspeaker to apply different forces to the tooth. The timing of the contact of the filament with the tooth is measured precisely with a simple impact detector. The sensation elicited by this stimulator is abolished by local alveolar anaesthesia, indicating that the stimulation was localised to the periodontal area. The flexible probe is an important safety feature, as it limits the force applied to the tooth, and therefore the distance that the tooth can be displaced. The stimulator, suitable for psychophysical studies on the thresholds of periodontal receptors, on the reflex responses to stimulation of periodontal mechanoreceptors, and teaching applications, can be constructed from readily available components at minimal cost.

Adult↗