The influence of experimental muscle pain on the human soleus stretch reflex during sitting and walking.

OBJECTIVES: The stretch reflex is functionally important during human locomotion. Muscle pain has been found to increase the stretch reflex amplitude during sitting, possibly due to an altered fusimotor drive. To further study the importance of altered fusimotor activity due to muscle pain we investigated the combined effect of muscle pain and motor task on the soleus stretch reflex. METHODS: Stretch reflexes were elicited before, during and after experimentally induced muscle pain in soleus (i.m. infusion of 6% saline) in 3 experiments: (1) in the relaxed soleus muscle and before, during and after an isometric ramp contraction (500 ms, 0-10 Nm), (2) at 3 different time periods during walking, and (3) at matched pain intensity and soleus activity during sitting and walking. RESULTS: Infusion of hypertonic saline into the soleus muscle caused a significant facilitated stretch reflex in the relaxed muscle (P<0.01), but not during walking or during sitting and walking at matched soleus EMG and matched pain levels. The infusion of isotonic saline (non-painful) did not cause any changes (P = 0.75). CONCLUSIONS: The main findings of the present study were that experimental muscle pain facilitated the stretch reflex during pain in the relaxed muscle, but caused no changes in stretch reflex amplitude during sitting and walking at higher "functional" background EMG levels.     

Spinal stretch reflex and cortical evoked potential amplitudes versus muscle stretch amplitude in the monkey arm

While investigating operant conditioning of the primate spinal stretch reflex (SSR), we studied SSR amplitude and cortical somatosensory evoked potential (SEP) amplitude as stretch amplitude changed in the monkey arm. Initial muscle length and background EMG activity remained constant. With change in stretch amplitude (and proportional change in stretch velocity and acceleration), changes in SSR and SEP amplitudes were respectively 0.75 and 0.66 as great. The lesser change in SSR amplitude may reflect saturation of Ia afferents, while that in SEP amplitude may also reflect participation of other peripheral receptors.     

Effects of fatigue on the stretch reflex in a human muscle.

The effects of fatigue on the electromyographic (EMG) reflex activities were compared during sustained voluntary contractions and contractions evoked by electrical stimulation (30 Hz) in the human first dorsal interosseus (FDI). Short latency (SL), medium latency (ML) and long latency (LL) reflex responses to a ramp-and-hold stretch of the muscle were recorded and analysed in 27 healthy subjects of both sexes. The amplitude of the reflex components was normalized as function of the amplitude of the surface action potential (SAP) recorded in response to the supramaximal stimulation of the motor nerve. The results indicate that for a similar reduction of force, SL and ML are significantly reduced after fatigue induced by voluntary contractions but they are not when the fatigue test is performed by electrical stimulation at the motor point. In voluntary fatigue experiments, the LL component showed no significant decrease below control values, but an enhancement was observed during electrically evoked contraction. This enhancement remained above control values for at least 15 min during the recovery period, whereas SL and ML decreases returned to control within 5 min after the fatigue tests. The electrical stimulation applied to the skin overlying the FDI at an intensity lower than the motor threshold did not affect SL and ML, but enhanced LL for about 15 min. On the contrary, the anaesthesia of the skin overlying the FDI induced a decrease in LL without significant change of SL and ML. It is concluded that muscle reflex fatigue is present during sustained voluntary contractions and decreases SL and ML responses to quick stretches.(ABSTRACT TRUNCATED AT 250 WORDS)     

Skeletal muscle tone and the misunderstood stretch reflex.

This review presents a revision of long-accepted tenets regarding the genesis of muscle tone in humans. Most discussions liken muscle tone in humans to the reflex tone described by Sherrington in decerebrate animals. That tradition presumes that muscle tone is fully determined by the monosynaptic stretch reflex, that tonic fusimotor activity is necessary for its production in normal humans, and that tonic muscle tone in antigravity leg muscles is responsible for the maintenance of posture. Data reviewed here show that nonreflex, mechanical mechanisms are involved in the maintenance of resting muscle tone; that spinal cord reflex responses are not stereotyped responses, but depend upon the ongoing activity in interneurons upon which inputs from a variety of peripheral sensory receptors and descending fiber systems converge; that long-latency transcortical responses are elicited when a muscle is stretched, and these responses effectively deal with large displacements; and that inertial components and viscoelastic muscle forces can counterbalance small amounts of body sway during quiet standing. In sum, the response to muscle stretch is not fixed and inflexible, but can be adjusted according to the demands of the moment.     

Distribution of the human jaw stretch reflex response elicited by percutaneous, localized stretch of jaw-closing muscles

In the present study homonymous and heteronymous projections of muscle spindle afferent fibers in the human jaw-closing system were investigated. Stretch reflex responses were elicited with percutaneous displacement stimuli applied to the belly of the anterior superficial divisions of temporalis and masseter muscles. The distribution of reflex responses in jaw-closing muscles was assessed. Reflex responses to this stimulus were observed in all jaw-closing muscles ipsilateral to the stimulated muscle, suggesting that the projection of spindle afferent fibers to jaw-closing muscles in humans is diffuse. Comparisons of homonymous and heteronymous reflex response amplitudes indicated that homonymous response amplitudes tended to be larger. However, the relative strength of heteronymous inputs to masseter motoneurons appeared to be stronger than those to temporalis motoneurons.     

Influence of sex on reflex jaw muscle activity evoked from the rat temporomandibular joint

Injection of glutamate into the rat temporomandibular joint (TMJ) evoked a concentration-dependent increase in jaw muscle activity. We investigated whether there are sex-related differences in glutamate-evoked jaw muscle activity that are mediated by sex hormones and whether prior injection of glutamate into the TMJ alters the magnitude of jaw muscle activity evoked by a subsequent injection of the algesic and inflammatory compound mustard oil (MO) into the TMJ. The magnitude of glutamate-evoked digastric and masseter muscle activity was significantly greater in female than male rats when 1000 mM glutamate was injected into the TMJ. Gonadectomy significantly reduced the magnitude of glutamate-evoked digastric muscle activity in female rats. Treatment of gonadectomized female rats with estrogen (20 microg/day) increased the magnitude of glutamate-evoked digastric muscle activity. Glutamate-evoked jaw muscle activity in gonadectomized and estrogen-treated gonadectomized males was not significantly different from intact males. Prior injection of glutamate over a concentration range of 10-1000 mM significantly increased digastric muscle activity evoked by MO injection into the TMJ 30 min later. In contrast, MO-evoked masseter muscle activity was significantly increased by prior injection of 250 mM glutamate only. There were, however, no sex-related differences in the enhancement of MO-evoked jaw muscle activity by prior injection of glutamate. These findings indicate that there are sex-related differences in glutamate-evoked jaw muscle activity that are dependent on female sex hormones, and increased glutamate concentrations sensitize the TMJ to noxious chemical stimuli.     

The human stretch reflex and the motor cortex.

The spinal stretch reflex, exemplified by the tendon jerk, appears to be less important in humans than a delayed 'long-latency' response. This is easily observed when muscles of the hand are stretched while they are already contracting voluntarily. On limited evidence, many have long held that the delayed response is a transcortical reflex and have tended to neglect alternative possibilities, particularly that it might be a spinal reflex dependent upon slow afferents. New experiments have now eliminated the alternatives, leaving the transcortical hypothesis in command of the field.     

Effects of muscle relaxant E-0646 on human stretch reflex and responses

The experiment examined the effects of a muscle relaxant E-0646 on short latency stretch reflex (SLR) and long latency stretch responses (LLRs) of lower leg muscles in 5 healthy male subjects, aged 24-41 years. An AC servomotor produced an ankle rotation of comfortably seated subjects in the direction to stretch the triceps surae muscle for the SLR and the tibialis anterior muscle for the LLRs. Following control measurements, after the oral administration of 50, 100, 150, and 300 mg of E-0646, the SLR and LLRs were recorded every 2 hours for 4 or 6 hours with simultaneous blood sampling to measure E-0646 plasma levels. Electromyograms (EMGs) of the soleus and tibialis anterior muscles were recorded with surface electrodes on each muscle belly. A precision potentiometer was used to measure the ankle angle. The soleus SLR, evoked by a torque pulse (7 msec duration) of one of 5 different amplitudes, decreased 2 hours after the 300 mg administration. A SLR decrease was also found in a simple relationship between the integrated SLR EMG and the angular velocity at 20 msec after the torque onset, in which the slope and intercept of the relationship changed. The plasma E-0646 level roughly correlated with the SLR decrease. No significant change of the SLR was observed when dosage was less than 150 mg. The LLRs of the tibialis anterior muscle were tested under the instruction "resist when perturbed" during a weak isometric contraction against a constant plantar flexion torque.(ABSTRACT TRUNCATED AT 250 WORDS)     

Participation of the stretch reflex in human physiological tremor.

Microneurographic studies were made of normal human muscle spindle afferent activity to evaluate the role played by these endings, and by the segmental stretch reflex, in various types of physiological tremor. Primary endings are sufficiently sensitive to respond to the minute tremulous movements normally seen in contracting muscles. Human muscle spindle endings appear to possess the same non-linear sensitivity to small stretches as has been reported for cat muscle spindles. The findings also support the notion that the segmental stretch reflex plays an important role in enhanced physiological tremor, in so far as it tends to synchronize the motor outflow in such a way as to produce rhythmical contractions properly timed to reinforce the mechanical resonant properties of muscle spindle endings, the stretch reflex also serves to damp the larger oscillations of finger or hand which result from a brisk tap or muscle twitch. There is no evidence for alpha-gamma co-activation in these enhanced physiological tremors.     

Hamstrings stretch reflex in human spasticity

In 16 patients with spastic paralysis the hamstrings stretch reflex was found to increase as the velocity of stretch increased, and generally to subside after movement ceased. These effects are attributable to the dynamic property of the primary spindle ending. The stretch reflex commonly appeared in only the last third of the stretching movement and was maximal as the knee became fully extended. This is consistent with the static properties of the primary and secondary spindle endings, and accounts for the absence of the clasp-knife phenomenon in the spastic hamstrings. The difference in the nature of the stretch reflex in spastic flexor and extensor muscles is best explained by the differential reflex effects of group II afferent fibres which facilitate flexor motoneurones and inhibit extensor motoneurones.     

A method for studying jaw muscle activity during standardized jaw movements under experimental jaw muscle pain

This paper describes a method for studying superficial and deep jaw muscle activity during standardized jaw movements under experimental jaw muscle pain. In 22 healthy adults, pain was elicited in the right masseter muscle via tonic infusion of 4.5% hypertonic saline and which resulted in scores of 30-60 mm on a 100-mm visual analogue scale. Subjects performed tasks in five sessions in a repeated measures design, i.e., control 1, test 1 (during hypertonic or isotonic saline infusion), control 2 (without infusion), test 2 (during isotonic or hypertonic saline infusion), control 3 (without infusion). During each session, subjects performed maximal clenching and standardized jaw tasks, i.e., protrusion, lateral excursion, open/close, chewing. Mandibular movement was recorded with a 6-degree-of-freedom tracking system simultaneously with electromyographic (EMG) activity from the inferior head of the lateral pterygoid muscle with fine-wire electrodes (verified by computer tomography), and from posterior temporalis, the submandibular muscle group and bilateral masseter muscles with surface electrodes. EMG root mean square values were calculated at each 0.5 mm increment of mandibular incisor movement for all tasks under each experimental session. This establishes an experimental model for testing the effects of pain on jaw muscle activity where the jaw motor system is required to perform goal-directed tasks, and therefore should extend our understanding of the effects of pain on the jaw motor system.     

Effect of low-level clenching and subsequent muscle pain on exteroceptive suppression and resting muscle activity in human jaw muscles.

OBJECTIVE: To investigate the effects of muscle fatigue induced by low-level isometric jaw-clenching and subsequent glutamate-evoked muscle pain on the exteroceptive suppression (ES) response and resting electromyographic (EMG) activities in human jaw muscles. METHODS: The resting EMG activity and the ESs were recorded before (baseline), after low-level jaw-clenching (Post1), after subsequent glutamate or isotonic saline injections into the left masseter (Post2), and 60 min after the clenching (Post3) in 23 healthy volunteers. RESULTS: The late ES (ES2) showed more inhibition at Post1 compared with baseline (P<0.05). It was less inhibited after both types of injections (Post2), and increased at Post3 again (P<0.05) with no significant difference between the glutamate and isotonic saline sessions. The resting EMG activity increased at Post1 and Post2 (P<0.05). The glutamate injection further increased the resting EMG activity in the injected muscle (P<0.01). CONCLUSIONS: Muscle fatigue influences inhibitory reflex pathways in jaw-closing muscles and subsequent acute muscle pain potentiates the local increase in the resting EMG activity of the painful muscle. SIGNIFICANCE: Muscle fatigue which can be observed in patients with oral dysfunctions may interact with nociceptive regulation and influence the clinical presentation of jaw symptoms and function.     

Temporomandibular-evoked jaw muscle reflex: role of brain stem NMDA and non-NMDA receptors.

This study investigated the possible involvement of brain stem excitatory amino acid receptor mechanisms and the trigeminal subnucleus caudalis (Vc) in temporomandibular joint (TMJ)-evoked reflex jaw muscle activity. Glutamate injected into the TMJ of anesthetized rats reflexly evoked activity in the jaw muscles. Application of lidocaine, but not saline, to the surface of the caudal brainstem overlying Vc significantly suppressed TMJ-evoked jaw muscle activity, while application of NMDA or non-NMDA receptor antagonists also significantly attenuated jaw muscle activity. These results provide evidence that Vc is a critical relay in the TMJ-evoked reflex activation of the jaw muscles, and that both NMDA and non-NMDA receptor mechanisms may contribute to these effects.     

Normalization reduces the spatial dependency of the jaw‐stretch reflex activity in the human masseter muscle

The jaw‐stretch reflex is the short‐latency response in the jaw‐closing muscles after a sudden stretch. The hypothesis whether normalization of the jaw‐stretch reflex amplitude with respect to prestimulus electromyographic (EMG) activity will make the amplitude more independent of the location of the electrodes over the masseter muscle was tested. A 5 × 6 electrode grid was used to record the jaw‐stretch reflex from 25 sites over the right masseter muscle of 15 healthy men. The results showed that there was a significant site dependency of the prestimulus EMG activity and the reflex amplitude. High cross‐correlation coefficients were found between the spatial distribution of mean prestimulus EMG activities and reflex amplitude. When the reflex amplitude was normalized with respect to the prestimulus EMG activity, no site dependency was found. In conclusion, normalization of the jaw‐stretch reflex amplitude by the prestimulus EMG activity strongly reduces its spatial dependency. Muscle Nerve, 2010     

Quantitation of the stretch reflex

The stretch reflex should ideally be quantitated for better clinical use by standardizing the muscle stretch and measuring the resulting muscle contraction. Quantitation of muscle contraction can be done by force measurements or electromyographic recordings. The electromyographic response to stretch consists of one component (short latency response) for short stretches (less than 15 ms) and of 2 or 3 components (short and long latency responses) for longer stretches (greater than 40-50 ms). The magnitude of the phasic stretch reflex is reflected by the short latency response, whereas the magnitude of the tonic stretch reflex is reflected by both the short and the long latency responses. In clinical studies of upper motor neuron syndromes, the knee jerk and the muscle tone correlated with the magnitude of the short latency response. In patients with paralysis agitans an increased long latency response, which correlated to the rigidity, was found.     

Sex-related suppression of reflex jaw muscle activity by peripheral morphine but not GABA.

The present study examined the effect of peripherally applied morphine and GABA on jaw muscle electromyographic activity reflexly evoked by co-injection of glutamate into the temporomandibular joint (TMJ) of lightly anesthetized Sprague-Dawley rats of both sexes. In male but not female rats, morphine significantly suppressed glutamate-evoked jaw muscle activity in a dose-dependent and naloxone-reversible manner. The median suppressive dose (+/- s.e.) for male rats was 12.7 +/- 3.1 microg (digastric muscle) and 12.6 +/- 1.3 microg (masseter muscle). GABA (5 micromol) significantly reduced glutamate-evoked muscle activity in both sexes. These data suggest that female rats are considerably less sensitive than male rats to the suppressive effects of peripherally applied morphine, but both sexes are equally affected by peripherally applied GABA.