Delayed-onset muscle soreness in human masticatory muscles increases inhibitory jaw reflex responses.

The effects of masticatory muscles' overloading on jaw-motor control are not yet fully clarified. Therefore, it was tested whether eccentric and concentric exercises of the human masticatory muscles would influence inhibitory jaw reflex responses. Eleven participants (6 males, 5 females) performed 6, 5-minutes bouts of eccentric-concentric contractions. Before, immediately after, 24 hours, 48 hours and 1 week afterwards, visual analogue scale (VAS) scores for jaw muscle fatigue and pain, maximum voluntary bite force (MVBF) and inhibitory jaw reflexes were recorded. Reflex data were analysed with the cumulative sum control chart error box method. Immediate and delayed masticatory muscle fatigue and pain were provoked. Further, 24 hours after the exercises, MVBF tended to decrease (P = .056), suggesting that delayed-onset muscle soreness (DOMS) was provoked in the masticatory muscles. In addition, the inhibitory jaw reflex showed a delayed increase in size 24 hours after the exercise (P < .05). In conclusion, DOMS provoked in the masticatory muscles alters jaw motor control by inducing a delayed increase in the size of the inhibitory jaw reflex.

Reflex responses of human masseter motor units to mechanical stimulation of the teeth.

Our aim was to investigate the jaw reflexes using both the probability- and the discharge rate-based analysis methods. Twelve consenting volunteer subjects participated in this study. Subjects bit gently on bite bars that carried the impression of their teeth. Surface and intramuscular electrical activity of the masseter was recorded. With the help of audio feedback from one motor unit, each subject bit to discharge the unit at a fixed rate. While the subject continuously activated the selected motor unit, 4-N stimuli were delivered to the upper right central incisor either at a rapid or a slow rate. For each trial, ≥300 stimuli were delivered, and, once a trial was completed, local anesthetic block was applied around the stimulated tooth, and the experiment was repeated. While preceding local anesthesia, the rapid-rate stimuli ("tap") induced substantial inhibitory reflex responses; during local anesthetic block, the same stimulus induced excitatory and inhibitory reflex responses. Slow-rate stimuli ("push"), on the other hand, usually generated a combination of inhibitory and excitatory responses that disappeared completely during the local anesthetic block. This study discovered that the strength of the inhibitory reflex response to a tooth-tap stimulus was much larger than previously reported. This study also found that whereas the probability-based analyses were better for illustrating the existence and latency of small earlier responses, the discharge rate-based method was better for indicating the duration of earlier responses and the existence, sign, and duration of later responses.

Estimates of EPSP amplitude based on changes in motoneuron discharge rate and probability.

When motor units are discharging tonically, transient excitatory synaptic inputs produce an increase in the probability of spike occurrence and also increase the instantaneous discharge rate. Several researchers have proposed that these induced changes in discharge rate and probability can be used to estimate the amplitude of the underlying excitatory post-synaptic potential (EPSP). We tested two different methods of estimating EPSP amplitude by comparing the amplitude of simulated EPSPs with their effects on the discharge of rat hypoglossal motoneurons recorded in an in vitro brainstem slice preparation. The first estimation method (simplified-trajectory method) is based on the assumptions that the membrane potential trajectory between spikes can be approximated by a 10 mV post-spike hyperpolarization followed by a linear rise to the next spike and that EPSPs sum linearly with this trajectory. We hypothesized that this estimation method would not be accurate due to interspike variations in membrane conductance and firing threshold that are not included in the model and that an alternative method based on estimating the effective distance to threshold would provide more accurate estimates of EPSP amplitude. This second method (distance-to-threshold method) uses interspike interval statistics to estimate the effective distance to threshold throughout the interspike interval and incorporates this distance-to-threshold trajectory into a threshold-crossing model. We found that the first method systematically overestimated the amplitude of small (<5 mV) EPSPs and underestimated the amplitude of large (>5 mV EPSPs). For large EPSPs, the degree of underestimation increased with increasing background discharge rate. Estimates based on the second method were more accurate for small EPSPs than those based on the first model, but estimation errors were still large for large EPSPs. These errors were likely due to two factors: (1) the distance to threshold can only be directly estimated over a limited portion of the interspike interval and (2) the distance to threshold can be affected by the EPSP itself. Both methods provide the most accurate EPSP estimates for EPSP amplitudes less than 5 mV and moderate background discharge rates (~15 imp/s).

Periodontal-masseteric reflexes decrease with tooth pre-load.

The responses of incisal periodontal mechanoreceptors to increasing mechanical stimulation are known to follow a hyperbolic-saturating course. The implications of these properties for the reflexive control of bite-force have not been examined directly. In line with the above mentioned receptor characteristics, we hypothesized that the periodontal-masseteric reflex will reduce as a function of increasing incisal pre-load. In 10 participants, a central incisor was repeatedly tapped (0.4 N). We measured the modulation by pre-load (0.2-2.0 N) of the reflex frequency-response at and between 3 and 20 Hz. The entrainment of the reflex increased with frequency up to 20 Hz and diminished with increasing pre-load. Importantly, the hyperbolic relationship shown here between the periodontal-masseteric reflex and tooth pre-load agreed with the load/response relationships predicted by single-receptor and tooth movement studies. This study demonstrated that periodontal mechano-receptors are able to contribute to the ongoing control of only small bite-forces.

Black box revisited: a technique for estimating postsynaptic potentials in neurons.

Our understanding of the operation of the brain depends on knowledge of its wiring. Currently, the wiring of the human brain is estimated by counting the number of neuron discharges that occur at specific times following a stimulus. There is now strong evidence that this approach generates significant errors. Recently, the accuracy of this 'count' method has been compared directly with an alternative 'rate' method in rat brain slices. The results confirmed that the count method generates significant errors that are minimized by the rate method, because the rate of discharge of a neuron accurately displays its excitability at the time of discharge. Therefore, it is now crucial that the rate method be used to reassess previous estimates of the characteristics of wiring in the brain.

Standardization of H-reflex analyses.

Variability in the H-reflex can make it difficult to identify significant changes using traditional pooled analysis techniques. This study was undertaken to introduce a normalisation approach to calculate both the relative size and the relative stimulus intensity required to elicit the H-reflex response so that comparisons can be made not only with results obtained during different experimental session but also between different subjects. This normalisation process fits the size of the measured M-responses and H-reflexes over the entire stimulus range with model curves to better facilitate the calculation of important parameters. This approach allows normalisation of not only the size of the response but also the relative stimulus intensity required to elicit the response. This eases the comparison of the reflex responses under various situations, and is capable of bringing out any genuine differences in the reflex in a reliable manner not previously possible. This study illustrates that comparison of the reflex between days is problematic, even in the same subject, as both the reflex size and the relative stimulus intensity required to obtain this reflex changed in all subjects. We suggest that H-reflex studies need to use normalisation not only for size of the reflex but also for the stimulus intensity, and also that all experiments for a single subject should be performed in the same session or during the same day using some level of background muscle activity in the muscle concerned as the variability of the muscle at rest was found to be larger.

Triceps surae stretch and voluntary contraction alters maximal M-wave magnitude.

UNLABELLED: Reliability of the motor response (M-wave) is fundamental in many reflex studies; however it has recently been shown to change during some investigations. The aim of this investigation was to determine if triceps surae stretch and voluntary contraction, or recording and analysis techniques, affect the maximal M-wave magnitude. The maximal M-wave was investigated in human gastrocnemius and soleus during different foot positions and during triceps surae contraction. Both bipolar and monopolar-recoding methods, and area and peak-to-peak (PTP) amplitude analysis methods were used. RESULTS: Maximal M-wave magnitude changed significantly between test muscle conditions, and is largest during dorsiflexion, probably due to changes in muscle bulk and recording electrode relationship. The maximal M-wave was up to 88% smaller when recorded by bipolar electrodes compared to monopolar electrodes, which is discussed in relation to signal cancellation. Area analysis provided more significant differences in M-wave magnitude between test muscle conditions than did PTP amplitude analysis, and the maximal M-wave shape changed significantly between test muscle conditions. This study suggests that maximal M-wave magnitude can vary depending on muscle condition, it highlights the importance of using correct recording and analysis techniques, and questions the reliability of using M-wave magnitude to monitor the relationship between the nerves and stimulating electrodes.

A new method to estimate signal cancellation in the human maximal M-wave.

A new method is introduced that estimates EMG signal cancellation in surface recorded investigations. Its usefulness is demonstrated when determining changes in the maximal motor response (M-wave) magnitude during rest and voluntary contraction. The accuracy of recording and analysis methods and the reliability of the maximal M-wave were assessed in the human gastrocnemius and soleus. The maximal M-wave was recorded by bipolar surface electrodes placed 2 cm, 3 cm and 4 cm apart, and by monopolar (one active and one indifferent reference) surface electrodes. Up to 85% of the maximal M-wave was lost due to signal cancellation during bipolar recording. The maximal M-wave magnitude decreased consistently and significantly during triceps surae contraction compared to rest when recorded by monopolar electrodes, but not when recorded by bipolar electrodes. Area and peak-to-peak (PTP) amplitude analysis methods provided similar results when determining the magnitude of the maximal M-wave. This provides evidence that monopolar recording is superior to bipolar recording as it removes the signal cancellation error and allows the genuine changes in maximal M-wave magnitude to be observed.

Tonic activity of the human temporalis muscle at mandibular rest position.

OBJECTIVE: There are two theories on the control of the human mandibular rest position; the visco-elastic theory and the muscular theory. In this study, we have searched for evidence for the muscular theory. DESIGN: We have investigated the activity of the anterior temporalis muscle during various positions of the mandible using intramuscular electrodes. RESULTS: In nine out of ten subjects tonic activity in the anterior temporalis muscle during the mandibular rest position was observed. Most of these units ceased firing under the jaw dropped condition of the mandible. CONCLUSIONS: Current findings support the muscular theory since single motor unit activity during the rest position of the mandible is observed in the anterior temporalis. We do not however have any evidence against the visco-elastic theory which may contribute in part to maintain the rest position of the mandible.

Jaw tremor as a physiological biomarker of bruxism.

OBJECTIVE: To determine if sleep bruxism is associated with abnormal physiological tremor of the jaw during a visually-guided bite force control task. METHODS: Healthy participants and patients with sleep bruxism were given visual feedback of their bite force and asked to trace triangular target trajectories (duration=20s, peak force <35% maximum voluntary force). Bite force control was quantified in terms of the power spectra of force fluctuations, masseter EMG activity, and force-to-EMG coherence. RESULTS: Patients had greater jaw force tremor at ∼8 Hz relative to controls, along with increased masseter EMG activity and force-to-EMG coherence in the same frequency range. Patients also showed lower force-to-EMG coherence at low frequencies (<3 Hz), but greater coherence at high frequencies (20-40 Hz). Finally, patients had greater 6-10 Hz force tremor during periods of descending vs. ascending force, while controls showed no difference in tremor with respect to force dynamics. CONCLUSION: Patients with bruxism have abnormal jaw tremor when engaged in a visually-guided bite force task. SIGNIFICANCE: Measurement of jaw tremor may aid in the detection/evaluation of bruxism. In light of previous literature, our results also suggest that bruxism is marked by abnormal or mishandled peripheral feedback from the teeth.

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.

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.

A device for investigating neuromuscular control in the human masticatory system.

A new apparatus has been developed to study the control of mastication in humans. The subject places his/her teeth on fixed upper and mobile lower bite plates; the device then enables opening and closing movements of the lower jaw against a controlled resistance. It is also possible to vary the number of teeth in contact with the device during an experiment from the entire dental arcade to a single tooth. The specially designed lower bite plate is dynamic and allows for both rotation and translation of the lower jaw during movement, thus, permitting the natural curvilinear trajectory of the jaw. The lower bite plate can follow chewing initiated by the subject without resisting the movement ('no force' mode) via a dedicated microprocessor controlled compensation mechanism. Another function of the device is to inject a constant predetermined load onto the lower bite plate so that the subject 'chews' against a fixed resistance simulating rapidly yielding food bolus ('fixed force' mode). The device can be programmed to increase or decrease the force during the closing or opening phase of chewing by feeding the position information into the force compensation system so both position and force change in parallel, hence, simulating a bite onto a non-yielding, or sticky, food bolus ('normal chewing' mode). By use of a jaw position compensation mechanism, the device can actively move the lower jaw, following any imposed position pattern ('position controlled' mode). The chewing simulator also has a mode that holds the position at a fixed level and allows the force to change ('position hold' mode). Furthermore, the device can inject additional rapid or slow forces or displacements onto the lower bite plate in order to elicit reflexes so that the response of jaw muscles to such stimuli can be examined at various jaw positions, force levels, phases of motion and velocities. The different modes of the apparatus can be used to study the operation and feedback control of human mastication; in particular whether modulations in jaw muscle activity and reflexes are due to changes in force, velocity, position, chewing cycle phase or a combination of these factors.

Climbing fiber lesions disrupt conditioning of the nictitating membrane response in the rabbit.

The pattern of acquisition of the classically conditioned nictitating membrane response was determined in rabbits which had recovered from a surgical lesion of the olivary decussation. In comparison with sham-operated controls, the animals with climbing fiber lesions showed delayed and incomplete conditioning. This observation supports the hypothesis of Marr and Albus that motor learning occurs in the cerebellar cortex when conjunctive inputs arrive along both the mossy and climbing fiber pathways.

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.

Compound group I excitatory input is differentially distributed to motoneurons of the human tibialis anterior.

The distribution of the compound group I excitatory input to various-sized motoneurones in the human tibialis anterior muscle was studied, using low-intensity electrical stimulation of the common peroneal nerve. The stimulation initiated the H-reflex response in all motor units with a latency of approximately 40 ms (range 30-45 ms). In each experiment, the amplitude of the H-reflex responses in a pair of simultaneously active motor units were assessed. It was shown that, although the stimulus intensity was identical, the amplitude of the H-reflex response was bigger in the motor unit with the higher recruitment threshold of the pair compared with the size of the reflex in the unit that had a relatively lower recruitment threshold. The present results are compared and contrasted with the findings in animals and one human study that suggested that the smaller-sized motoneurones receive larger group I excitatory input.

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.

Harmaline disrupts acquisition of conditioned nictitating membrane responses.

A basic tenet of the Marr-Albus theory of motor learning is that the learning process involves concurrent activation of the climbing fibre and mossy fibre inputs to the cerebellum. This theory was tested by administering harmaline, a drug which causes climbing fibres to fire at their maximal rate of 8-12 Hz, to rabbits during a classical conditioning protocol. Harmaline significantly reduced the proportion of conditioned nictitating membrane responses on each of 4 successive training days. When harmaline was given to already-conditioned rabbits, the proportion of conditioned responses fell sharply to the day 1 level: however, this was still a higher proportion of conditioned trials than that which was seen with naive animals trained for 4 days with harmaline. It is therefore concluded that harmaline disrupts the acquisition of classically-conditioned nictitating membrane responses but does not prevent the expression of an already learnt response. These findings are therefore consistent with the proposal in the Marr-Albus theory that the climbing fibres play an essential part in motor learning.

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.

The effect of temperature on the contraction characteristics of jaw muscles in the cat.

The effect of muscle temperature on the speed of contraction and on the strength of contraction of the cat jaw muscles was studied. Muscles contracted faster when their temperatures were increased. The tetanic and twitch tension decreased significantly as the animal was heated from 30 to 40 degrees C.