Non-nociceptive upper limb afferents modulate masseter muscle EMG activity in man

Recent electrophysiological data obtained in anaesthetized rats evidenced jaw muscle excitatory responses to the electrical stimulation of type II limb somatosensory afferents. In the present work, we describe an inhibitory reflex evoked in human masseter muscles by stimulation of non-nociceptive fibres travelling in the median and radial nerves (MED and RAD, respectively). Eighteen healthy volunteers participated in the study. Subjects were seated on a comfortable chair, with the complex head-mandible-neck-trunk and the limbs securely fixed to the chair. Attempts were made to minimize possible interferences due to the activation of afferents other than the stimulated ones. The subjects were instructed to contract masseter muscles at a submaximal level and to maintain a stable level of muscle contraction during all trials. EMG voluntary activity was recorded from both masseter muscles by means of coaxial needle electrodes before and after the electrical stimulation of MED and/or RAD at intensities below pain threshold. In all subjects, MED stimulation induced bilaterally a marked depression of masseter EMG activity, which occurred at a latency of 23.6 +/- 2.1 ms and lasted 27.8 +/- 6.6 ms. RAD stimulation also induced a marked reduction in masseter EMG activity, but this effect was clearly observed in 9 out of 18 subjects, and it showed latency (30.2 +/- 7.5 ms) and duration (44.9 +/- 5.4 ms) significantly longer in comparison with the MED-induced effect. All subjects exhibited the inhibitory period in masseter EMG following the simultaneous stimulation of both nerves; this one appeared at a latency not significantly different (25.3 +/- 5.9 ms) and lasted much more (37.4 +/ - 8.2 ms) than EMG depression evoked by MED stimulation. The duration of masseter muscle inhibition, induced by MED and/or RAD stimulation, was inversely related to the level of EMG activity, while latency was not related to it. Significant increases in the inhibitory period duration were also observed by increasing stimulus intensity, within a subthreshold range for the activation of nociceptive fibres. In all cases, the inhibitory period was followed by a later excitatory rebound activity, whose latency and duration depended on the duration of the preceding EMG inhibition and on the background level of masseter activation. In conclusion, results evidenced that the activation of arm somatosensory fibres modulates masseter muscle activity in normal man. This might lead to a coordination between limb and masticatory muscle activity, which is required in several complex motor acts.     

Modulation of exteroceptive suppression periods in human jaw-closing muscles induced by summation of nociceptive and non-nociceptive inputs

Convergence of various afferent inputs onto brain-stem neurons may play an important role in the regulation of trigeminal motor function. The present study examined the efficacy of nociceptive and non-nociceptive perioral stimulation for the modulation of the two exteroceptive suppression periods (ES1 and ES2) in human jaw-closing muscles. The inhibitory jaw-reflexes, ES1 (10-15 ms onset) and ES2 (40-50 ms onset), were recorded as the surface electromyogram of masseter and temporalis muscles in 14 healthy subjects. Non-painful electrical stimuli were applied to the right mental nerve while the subject was biting at 50% of the maximal voluntary force. Five conditions were compared: baseline, repetitive tactile stimulation (1 Hz) on the right cheek, topical application of capsaicin (5%) on the right cheek, repetitive tactile stimulation plus capsaicin, and postbaseline. The perceived intensity of the electrical stimuli was evaluated by the subjects on a 0-10 rating scale. Capsaicin alone failed to induce significant changes of ES1 and ES2; tactile stimulation alone induced a significant delay in the onset of ES2 (P < 0.001). During repetitive tactile stimulation plus capsaicin, the duration of ES1 and ES2 was shortened (ES1 and ES2: P < 0.001) and the degree of suppression was reduced (ES1: P < 0.05; ES2: P < 0.005). Perceived intensity of the electrical stimuli was unchanged during the different experimental conditions. The present results suggest that summation of simultaneous nociceptive and non-nociceptive inputs plays an important role in the modulation of the neural pathways involved in the short-latency ES1 and long-latency ES2.     

Jaw muscle response to stimulation of type II somatosensory afferents of limbs in the rat

Convergence of various afferent inputs onto brainstem neurones may play an important role in the regulation of trigeminal motor activity. In particular, previous studies suggest that, besides sensory inputs arising from the orofacial region, extratrigeminal information may modulate jaw muscle function. In the present study the actions exerted on masseter and digastric muscles by the activation of somatosensory afferents coming from fore- and hind limbs were examined. The electromyographic activity (EMG) of masseter and digastric muscles was recorded in 20 anaesthetised rats, and EMG responses to single and paired electrical stimulation of common radial and sciatic nerves, at a threshold intensity for the activation of group II afferent fibres, were studied. The stimulation induced an excitatory response in both masseter and digastric muscles bilaterally. Ipsi- and contralateral radial nerve stimulation evoked masseter responses at latencies of 13.8±2.4 ms and of 18.0±2.6 ms, respectively, and digastric responses 1.6±0.4 ms later. Ipsi- and contralateral sciatic nerve stimulation elicited masseter responses at latencies of 21.4±2.6 ms and of 23.3±2.0 ms, respectively, and digastric responses 2.0±0.2 ms later. The same masseter and digastric motor units were excited by both radial and sciatic nerve stimulation; this suggests a convergence of somatosensory inputs arising from fore- and hind limbs on the same pool of masseter and digastric motoneurones. Paired stimulation of the two nerves did not induce any summation of the responses; this finding suggests that the two inputs, reaching a common relay station, could give rise either to occlusion or to inhibitory interactions. Spinotrigeminal relationship evidenced in this study may be involved in the coordination of jaw and limb movements. Electronic Publication     

Long-loop reflex from arm afferents to remote muscles in normal man

The present study was designed to examine the effects of median nerve stimulation on motoneurones of remote muscles in healthy subjects using H-reflex, averaged EMG and PSTH methods. Stimulation of the median nerve induced facilitation of soleus H-reflex from about 50 ms and it reached a peak at about 100 ms of conditioning-test interval. Afferents that induced the facilitation consisted of at least two types of fibres, the high-threshold cutaneous fibres and the low-threshold fibres. When the effects were examined by the averaged surface EMG and PSTH, no facilitation but rather inhibition or inhibition-facilitation was induced in all tested muscles except for the upper limb muscles on the stimulated side. The inhibition latency was shortest in masseter muscle and longest in leg muscles, while values for the contralateral upper limb muscles were in the middle, indicating that the onset of inhibition was delayed from rostral to caudal muscles. Inputs from the median nerve converged to inhibitory interneurones, which mediate the masseter inhibitory reflex. Our findings suggested that inputs from the median nerve initially ascend to the brain, at least to the brainstem, and then descend to the spinal cord. Therefore, inhibition induced by median nerve stimulation was not considered as an interlimb reflex mediated by a propriospinal pathway, but long-loop reflex, at least via the pons. The discrepancy between the results of reflex and motor units suggests that facilitation of soleus H-reflex following median nerve stimulation was mainly due to reduced presynaptic inhibition.     

Evidence for bilateral innervation of certain homologous motoneurone pools in man.

1. Surface EMG recordings were made from left and right homologous muscle pairs in healthy adults. During each recording session subjects were requested to maintain a weak isometric contraction of both the left and right muscle. 2. Cross-correlation analysis of the two multiunit EMG recordings from each pair of muscles was performed. Central peaks of short duration (mean durations, 11.3-13.0 ms) were seen in correlograms constructed from multiunit EMG recordings obtained from left and right diaphragm, rectus abdominis and masseter muscles. No central peaks were seen in correlograms constructed from the multiunit EMG recordings from left and right upper limb muscles. 3. To investigate descending pathways to the homologous muscle pairs, the dominant motor cortex was stimulated using a focal magnetic brain stimulator whilst recording from homologous muscle pairs. 4. Following magnetic stimulation of the dominant motor cortex, a response was recorded from both right and left diaphragm, rectus abdominis and masseter muscles. In contrast, when recording from homologous upper limb muscles, a response was only seen contralateral to the side of stimulation. 5. The finding of short duration central peaks in the cross-correlograms constructed from multiunit recordings from left and right diaphragm, rectus abdominis and masseter, suggests that muscles such as these, that are normally co-activated, share a common drive. The mechanism is discussed and it is argued that the time course of the central correlogram peaks is consistent with the hypothesis that they could be produced by a common drive that arises from activity in last-order branched presynaptic fibres although presynaptic synchronization of last-order inputs is also likely to be involved. 6. The results of the magnetic stimulation experiments suggest that this common drive may involve the corticospinal tract. 7. We saw no evidence for a common drive to left and right homologous muscle pairs that may be voluntarily co-activated but often act independently.     

Monosynaptic and oligosynaptic contributions to human ankle jerk and H-reflex

Studies were undertaken in normal subjects to determine whether it is possible for oligosynaptic reflex pathways to affect motoneuron discharge in the ankle jerk and H-reflex of the soleus. It is argued that if the rising phase of the increase in excitability of the soleus motoneuron pool produced by tendon percussion or by electrical stimulation of the peripheral nerve lasts more than a few milliseconds and if the increase in excitability takes several milliseconds to reach the threshold for motoneuron discharge, these reflexes are unlikely to be exclusively monosynaptic. In relaxed subjects, changes in excitability of the soleus motoneuron pool produced by tendon percussion and by electrical stimulation of the tibial nerve were examined using conditioning stimuli just below threshold and a test H-reflex just above threshold for a reflex response. The increase in excitability due to tendon percussion had an average rise time of 10.8 ms and a total duration of approximately 25 ms. With electrical stimulation the rising phase appeared shorter, but it could not be measured accurately due to afferent refractoriness. In single motor units, the rise times of the composite excitatory postsynaptic potentials (EPSPs) set up by subthreshold tendon percussion and by subthreshold electrical stimulation of the tibial nerve were estimated from changes in the probability of discharge of voluntarily activated single motor units. Rise times were significantly longer with tendon percussion (mean +/- SD, 7.1 +/- 2.3 ms; n = 34) than with electrical stimulation (2.4 +/- 1.4 ms; n = 32). In four experiments in which a number of motor units were studied using identical mechanical and identical electrical stimuli, the poststimulus time histograms (PSTHs) for each stimulus were pooled to provide an estimate of the rise time of the excitability change in the motoneuron pool. The mean rise times of these four samples were 10.5 ms with mechanical stimulation and 4.5 ms with electrical stimulation. The spontaneous variability in latency of reflexly activated single motor units was 0.8-3.1 ms (average SD, 0.34 ms) in the tendon jerk, and 0.6-1.4 ms (average SD, 0.19 ms) in the H-reflex. Comparison of these figures with the measurements of rise time given above suggests that the composite EPSPs are larger than the background synaptic noise. With six motor units, the timing of reflex discharge in the tendon jerk when the subject was relaxed was compared with the timing of the change in probability of discharge due to apparently identical percussion when the units were activated voluntarily.(ABSTRACT TRUNCATED AT 400 WORDS)     

A Short Latency Vestibulomasseteric Reflex Evoked by Electrical Stimulation Over the Mastoid in Healthy Humans

We describe EMG responses recorded in active masseter muscles following unilateral and bilateral electrical vestibular stimulation (EVS, current pulses of 5 mA intensity, 2 ms duration, 3 Hz frequency). Averaged responses in unrectified masseter EMG induced by unilateral EVS were examined in 16 healthy subjects; effects induced by bilateral (transmastoid) stimulation were studied in 10 subjects. Results showed that unilateral as well as bilateral EVS induces bilaterally a clear biphasic response (onset latency ranging from 7.2 to 8.8 ms), that is of equal amplitude and latency contra- and ipsilateral to the stimulation site. In all subjects, unilateral cathodal stimulation induced a positive—negative response termed p11/n15 according to its mean peak latency; the anodal stimulation induced a response of opposite polarity (n11/p15) in 11/16 subjects. Cathodal responses were significantly larger than anodal responses. Bilateral stimulation induced a p11/n15 response significantly larger than that induced by the unilateral cathodal stimulation. Recordings from single motor units showed that responses to cathodal stimulation corresponded to a brief (2–4 ms) silent period in motor unit discharge rate. The magnitude of EVS-induced masseter response was linearly related to current intensity and scaled with the mean level of EMG activity. The size of the p11/n15 response was asymmetrically modulated when subjects were tilted on both sides; in contrast head rotation did not exert any influence. Control experiments excluded a possible role of cutaneous receptors in generating the masseter response. We conclude that transmastoid electrical stimulation evokes vestibulomasseteric reflexes in healthy humans at latencies consistent with a di-trisynaptic pathway.     

Magnetic motor evoked potentials (MEPs) in masseter muscles

Electromyographic responses of the masseter muscles and orbicularis oris muscles following transcranial magnetic stimulations were recorded with surface and needle electrodes. MEPs in masseter muscles (latency 6.9 +/- 0.71 ms, mean +/- SD) due to activation of controlateral cortico-nuclear connections were evoked by magnetic stimulations at 4 cm laterally to the vertex on the biauricular line. These MEPs were followed bilaterally by a silent period lasting about 20 ms and, less constantly, by a later silent period lasting up to 80 ms. The ipsilateral responses to the same stimuli presented shorter latencies and higher amplitudes and they were ascribed to direct stimulation of the trigeminal nerve, probably its intracisternal portion. Ipsilateral masseter "cortical" MEPs could be elicited only by a lower output setting (70% of the maximum output) of the stimulator. Orbicularis oris MEPs were polyphasic and dispersed with latencies ranging from 7 to 11 ms. In patients with hemispheric or capsular ischemic lesions "cortical" MEPs were absent when stimulating the affected hemisphere and present when stimulating the unaffected one. We suggest that the direct corticomotoneuronal projections for the masseter are mainly crossed.     

Chronic hypoxia abolishes expiratory prolongation following carotid sinus nerve stimulation in the anesthetized rat

In anesthetized rats, increases in phrenic nerve (PN) amplitude and frequency during brief periods of hypoxia or electrical stimulation of the carotid sinus nerve (CSN) are followed by an increase in expiratory duration. We investigated the effects of chronic exposure to hypoxia on PN responses to CSN stimulation. In Inactin anesthetized (100 mg/kg) Sprague-Dawley rats PN discharge and arterial pressure responses to 10-120 s of CSN stimulation (20 Hz, 0.2 ms duration pulses) were recorded after 7-10 days exposure to hypoxia (10 +/- .5% O2). In normoxic rats, the degree of CSN-evoked expiratory prolongation was dependent upon the duration of CSN stimulation. CSN-evoked increases in PN burst amplitude were not different comparing chronic hypoxic rats to rats maintained at normoxia while CSN-evoked increases in PN burst frequency were greater in chronic hypoxic rats (p<.05). CSN-evoked expiratory prolongation was abolished in chronic hypoxic rats. Following chronic hypoxia, changes occur within the central processing of arterial chemoreceptor inputs so that CSN stimulation evokes an enhanced PN frequency response and no expiratory prolongation.     

Glucose infusion attenuates muscle fatigue in rat plantaris muscle during prolonged indirect stimulation in situ

Carbohydrate ingestion increases both endurance time to exhaustion during prolonged exercise, and the ability to perform resistance exercise. The mechanism(s) underlying the increased performance following glucose ingestion remain(s) unclear. The purpose of the present study was to verify the hypothesis that glucose infusion could attenuate peripheral muscle fatigue in the anaesthetized rat during prolonged indirect electrical stimulation in situ. For this purpose the plantaris muscle was electrically stimulated (50 Hz for 200 ms every 2.7 s; 5 V; pulse width, 0.05 ms) in situ through the sciatic nerve to perform concentric contractions for 60 min while infusing intravenously either saline alone (7.25 ml kg(-1) h(-1)), or saline and glucose (1 g kg(-1) h(-1): plasma glucose 11 +/- 1.1, vs. 4.9 +/- 0.2 mm with infusion of saline) (8 rats per group). Glucose infusion attenuated the reduction in submaximal peak dynamic force (55% decrease vs. 70% decrease in rats infused with saline alone, P < 0.05). In a third group of rats (n = 8), infusion of glucose 30 min after the start of stimulation partially restored submaximal peak dynamic force (P < 0.05). Maximum dynamic and isometric forces at the end of the period of stimulation were also higher (P < 0.05) in rats infused with glucose (4.0 +/- 0.2 and 4.3 +/- 0.2 N, respectively) than saline alone (3.0 +/- 0.2 and 3.5 +/- 0.2 N, respectively). The beneficial effect of glucose infusion on peripheral muscle force during prolonged stimulation was not associated with a reduction in muscle glycogen utilisation, nor with a reduction of fatigue at the neuromuscular junction, as assessed through maximal direct muscle stimulation (200 Hz for 200 ms; 150 V; pulse width, 0.05 ms). However, changes in M-wave peak-to-peak amplitude, duration and total area suggest that glucose infusion, and/or the associated increase in plasma insulin concentration, may prevent the deterioration of electrical properties of the muscle fibre membrane.     

The bilateral reflex control of the trapezius muscle in humans

Abstract. Electromyographic recordings were made from the trapezius muscle in 18 healthy subjects using surface electrodes placed bilaterally. A mechanical tap applied to the insertion of the lower fibres of trapezius evoked a reflex at 11.9 +/- 1.4 ms (mean plus minus SD) in the ipsilateral trapezius muscle. In addition and surprisingly, short latency, facilitatory reflexes were also seen on the contralateral side at a latency of 14.3 +/- 1.7 ms. Electrical stimulation of the afferent nerve to trapezius, the cervical nerve of C3/4, also evoked short latency, facilitatory reflexes from both the ipsilateral and contralateral muscles. Since this nerve carries the Ia fibres from trapezius, this reflex would appear to be the equivalent of the H reflex seen in the soleus muscle. The latency of this reflex was 10.9 +/- 1.4 ms ipsilaterally and 11.9 +/- 1.5 ms contralaterally, the difference being only 1.0 +/- 0.7 ms. Voluntary activity of the ipsilateral trapezius increased the amplitude of both mechanically and electrically evoked ipsilateral and crossed reflexes, and vibration of the ipsilateral trapezius decreased the amplitude of both reflexes. These results together suggest that the earliest parts of both ipsilateral and crossed reflexes are monosynaptic in origin. If the crossed reflex is, in part, due to a common Ia presynaptic input, correlation of bilateral trapezius activity should produce a peak in the cross-correlogram. Indeed, cross-correlation of the bilateral trapezius activity during elevation of the arms produced a short duration peak. Moreover, this was significantly larger when compared to that constructed from activity of the upper and lower fibres of trapezius from one side. These results imply that muscle spindle afferents from the ipsilateral trapezius monosynaptically activate motoneurones contralaterally.     

Neurochemical and electrophysiological studies on the functional significance of burst firing in serotonergic neurons

We have previously described a population of 5-hydroxytryptamine neurons which repetitively fires bursts of usually two (but occasionally three or four) action potentials, with a short (<20 ms) interspike interval within a regular low-frequency firing pattern. Here we used a paradigm of electrical stimulation comprising twin pulses (with 7- or 10-ms inter-pulse intervals) to mimic this burst firing pattern, and compared the effects of single- and twin-pulse electrical stimulations in models of pre- and postsynaptic 5-hydroxytryptamine function. Firstly, we measured the effect of direct electrical stimulation (2 Hz for 2 min) of rat brain slices on efflux of preloaded [3H]5-hydroxytryptamine. In this in vitro model, twin-pulse stimulation increased the efflux of tritium by about twice as much as did single-pulse stimulation. This effect was evident in the medial prefrontal cortex (area under the curve: 2. 59+/-0.34 vs 1.28+/-0.22% relative fractional release), as well as in the caudate-putamen (3.93+/-0.65 vs 2.17+/-0.51%) and midbrain raphe nuclei (5.42+/-1.05 vs 2.51+/-0.75%). Secondly, we used in vivo microdialysis to monitor changes in endogenous extracellular 5-hydroxytryptamine in rat medial prefrontal cortex in response to electrical stimulation (3 Hz for 10 min) of the dorsal raphe nucleus. In this model, twin-pulse stimulation of the dorsal raphe nucleus increased 5-hydroxytryptamine by approximately twice as much as did single-pulse stimulation at the same frequency (area under the curve: 50.4+/-9.0 vs 24.2+/-4.4 fmol). Finally, we used in vivo extracellular recording to follow the response of postsynaptic neurons in the rat medial prefrontal cortex to 5-hydroxytryptamine released by dorsal raphe stimulation. Electrical stimulation of the dorsal raphe nucleus (1 Hz) induced a clear-cut poststimulus inhibition in the majority of cortical neurons tested. In these experiments, the duration of poststimulus inhibition following twin-pulse stimulation was markedly longer than that induced by single-pulse stimulation (200+/-21 vs 77+/-18.5 ms).Taken together, the present in vitro and in vivo data suggest that in 5-hydroxytryptamine neurons, short bursts of action potentials will propagate along the axon to the nerve terminal and will enhance both the release of 5-hydroxytryptamine and its postsynaptic effect.     

Differential regulation of crossed cutaneous effects on the soleus H-reflex during standing and walking in humans

Although sensory inputs from the contralateral limb strongly modify the amplitude of the Hoffmann (H-) reflex in a static posture, it remains unknown how these inputs affect the excitability of the monosynaptic H-reflex during walking. Here, we investigated the effect of the electrical stimulation of a cutaneous (CUT) nerve innervating the skin on the dorsum of the contralateral foot on the excitability of the soleus H-reflex during standing and walking. The soleus H-reflex was conditioned by non-noxious electrical stimulation of the superficial peroneal nerve in the contralateral foot. Significant crossed facilitation of the soleus H-reflex was observed at conditioning-to-test intervals in a range of 100–130 ms while standing, without any change in the background soleus electromyographic (EMG) activity. In contrast, the amplitude of the soleus H-reflex was significantly suppressed by the contralateral CUT stimulation in the early-stance phase of walking. The background EMG activity of the soleus muscle was equivalent between standing and walking tasks and was unaffected by CUT stimulation alone. These findings suggest that the crossed CUT volleys can affect the presynaptic inhibition of the soleus Ia afferents and differentially modulate the excitability of the soleus H-reflex in a task-dependent manner during standing and walking.     

Subcortical interactions between somatosensory stimuli of different modalities and their temporal profile

Interactions between inputs of different sensory modality occur along the sensory pathway, including the thalamus. However, the temporal profile of such interaction has not been fully studied. In eight patients who had been implanted an intrathalamic electrode for deep brain stimulation as symptomatic treatment of tremor, we investigated the interactions between mechanical taps and electrical nerve stimuli. Somatosensory evoked potentials (SEPs) were recorded from Erb's point, cervical spinal cord, nucleus ventrointermedialis of the thalamus, and parietal cortex. A handheld electronic reflex hammer was used to deliver a mechanical tap to the skin overlying the first dorsal interosseous muscle and to trigger an ipsilateral digital median nerve electrical stimulus time-locked to the mechanical tap with a variable delay of 0 to 50 ms. There were significant time-dependent interactions between the two sensory volleys at the subcortical level. Thalamic SEPs were decreased in amplitude at interstimulus intervals (ISIs) from 10 to 40 ms with maximum effect at 20 ms (-42.8 +/- 10.5%; P < 0.001). A similar decrease was also seen in the number and frequency of the high-frequency components of thalamic SEPs (-25 +/- 4%). A smaller reduction (-18.1 +/- 5.8%; P < 0.001) was present in upper cervical response at ISI = 20 ms. There were no changes in peripheral responses. Cortical SEPs were almost completely absent in some subjects at ISIs from 20 to 50 ms. There were no changes in SEP latencies. Our results indicate that significant time-dependent interactions between sensory volleys occur at the subcortical level. These observations provide further insight into the physiological mechanisms underlying afferent gating between sensory volleys of different modality.     

Types of afferents from the knee joint evoking sympathetic reflexes in cat inferior cardiac nerves

In anesthetized cats electrical stimulation of the medial articular nerve of the knee joint evoked sympathetic reflex discharges in inferior cardiac nerves. Low intensity single stimuli elicited early reflex discharges (A-reflexes, latency 70-90 ms, duration 110-200 ms) whereas short tetanic stimulation at higher intensities evoked, in addition, late reflexes (C-reflexes, latency 390-480 ms, duration 230-400 ms). An analysis of the relation between the conduction velocity and the electrical threshold of 231 single medial articular nerve fibers revealed that the A-reflex is mainly due to activation of Group II units, whereas the C-reflex is evoked by activity in unmyelinated Group IV fibers.     

Remote noxious stimuli modulate jaw reflexes evoked by activation of periodontal ligament mechanoreceptors in man

The purpose of the study was to investigate whether jaw reflexes evoked by selective stimulation of periodontal ligament me canoreceptors are susceptible to modulation by remote noxious stimulation. Experiments were performed on 10 volunteer subjects. Skin surface recordings were made from the jaw-closing masseter muscle.The subjects activated the muscle to approximately 10% of maximum by biting on a rubber impression of their molar teeth while they received visual feedback of the electromyogram (EMG) of the muscle. Reflexes were produced by the application of gentle mechanical stimuli to an upper central incisor tooth. The stimuli were in the form of 'ramp and hold' forces with a 5 ms rise-time and a 1.5 N plateau which lasted 350 ma. The resulting reflexes were recorded both under control conditions and while the subjects received a remote noxious stimulus (immersion of a hand in water at 3 degrees C). In all 10 subjects, the stimuli produced a single period of inhibition of masseteric activity (latency, 12.8 t 04 ms; duration, 18.1+/-1.3 ms; means +/- S.E.M.), which was usually followed by a period of increased masseteric activity. The period of inhibition constituted a downward wave in full-wave rectified, averaged signals. The integrals of such waves were significantly smaller (by 17+/- 6.5 %; P = 0.027; Student's t test) when the reflex was evoked during remote noxious stimulation rather than under control conditions. As such reflexes are beLieved to play a modulatory role during normal oral function this finding maybe relevant to disorders of mastication associated with pain.     

Cardiopulmonary sympathetic afferent excitation of lower thoracic spinoreticular and spinothalamic neurons

1. Responses of spinoreticular (SRT) and spinothalamic (STT) neurons located in the T7-T9 segments to cardiopulmonary sympathetic afferent (CPS) stimuli were studied in 27 cats that were anesthetized with alpha-chloralose. 2. CPS stimulation excited 32 SRT and 10 STT neurons. Each neuron was also excited by stimulation of the left greater splanchnic nerve (SPL) and had a somatic receptive field that was most commonly located on the upper abdomen and over the lower rib cage. An additional 12 SRT and 3 STT neurons received input from SPL and somatic structures but failed to respond to CPS stimulation. 3. CPS stimulation evoked early responses (23 cells) or both early and late responses (19 cells) that had average latencies of 12.7 +/- 1.8 and 88.2 +/- 13.1 (SE) ms, respectively. Latencies of responses to SPL stimulation were significantly shorter and averaged 8.1 +/- 0.9 and 46.1 +/- 7.1 ms. Magnitudes of early responses to SPL stimulation were significantly greater than responses to CPS stimulation; however, late responses were not different. 4. Responses to CPS stimulation were inhibited by a prior conditioning stimulus applied to SPL. Greatest inhibition occurred at a conditioning-test interval of 40 ms, and inhibition lasted for at least 300 ms. Inhibition of responses to SPL stimulation could be evoked by conditioning stimuli applied to CPS; however, the inhibition was significantly less than that evoked by SPL stimulation on responses to CPS stimulation. 5. Thirty-eight neurons were tested for responses to injection of bradykinin (4 micrograms/kg) into the left atrium. Discharge rate of 17 cells increased from 5 +/- 2 to 12 +/- 4 Hz. Four cells were tachyphylactic to repeated injections. Injections of bradykinin into the thoracic aorta did not significantly alter cell activity. Bilateral cervical vagotomy had no effect on responses to intracardiac bradykinin. 6. The results indicate that lower thoracic SRT and STT neurons are excited by CPS stimuli including noxious stimulation of the heart. However, comparison of these responses with previously reported responses of upper thoracic SRT and STT neurons indicate that there is a decrease in effectiveness of CPS stimuli from upper to lower thoracic segments. Convergence of CPS and abdominal inputs onto lower thoracic pain pathways could explain abdominal pain that is occasionally associated with cardiac disease.     

H-reflexes in masseter and temporalis muscles in man

In contrast with limb muscles, studies on H-reflexes in the trigeminal system are scarce. The present report aimed at reevaluating the responses obtained in the masseter and temporalis muscles after electrical stimulation of their nerves. Twenty-four subjects participated in the experiments. The reflexes were elicited in the masseter and temporal muscles by monopolar stimulation and recorded using surface electrodes. Stimulation of the masseteric nerve evoked an M-response in the masseter and an H-reflex in both the masseter and the temporal muscles. In contrast with the masseter muscle, where the homonymous H-reflex disappeared at higher stimulation intensities, the heteronymous temporal H-reflex remained and reached a plateau. Simultaneous stimulation of the masseteric and deep temporal nerves resulted in an M-response and an H-reflex in both the masseter and temporal muscles. Increasing stimulus intensitites led to disappearance of the H-reflex in both muscles. The results were compared with those obtained by others on limb muscles. As in these muscles, the presence of heteronymous H-reflexes in the jaw muscles can be used in future studies of motoneuronal excitability.     

Inhibition of nerve conduction by electromagnetic induction of the frog sciatic nerve-gastrocnemius muscle preparation

The effect of electromagnetic induction (EMI) on impulse conduction and muscle contraction was studied in isolated sciatic nerve-gastrocnemius muscle preparation of the frog. Electrical stimulation (ES) of the sciatic nerve, at 0.5 Hz with 0.6 V (supramaximal) and 1-ms pulse duration, produced twitch contractions (3.5 +/- 0.4 g tension, mean +/- S.E., n = 8 frogs), which were reduced or blocked by EMI, applied to the nerve via an induction coil, from a d.c. source of 1.5-4 V, at a frequency of 100 min-1, for 2- to 4-min duration. Recovery of the blocked twitches was obtained within 4-5 min, after the cessation of the EMI and washing out the preparation in Ringer solution. The inhibition of the twitch tension by EMI was compared to that produced by an effective concentration of a local anaesthetic, lignocaine (1 microM), which is known to block conduction, by blocking ionic fluxes across the nerve membrane. It is possible that EMI also interferes with the ionic fluxes, and in prolonged duration, may produce changes in the myelin sheath (or the Schwann cells) of the nerve membrane. A comparison of ES with EMI was made, and it was concluded that EMI inhibited electrically induced neuromuscular transmission at the frog neuromuscular junction.     

Evidence that peaks in EMG averages can sometimes be caused by inhibition of motoneurons

1. It has been reported that an excitatory response occurs before strong inhibition in masticatory muscles. We tested the hypothesis that this small monopolar wave in the EMG, called by us the early exteroceptive component (EEC), is in fact the first response to inhibition. 2. A mapping of the electrical activity of the masseter muscle was performed using a 3 x 4 matrix of surface electrodes with reference to the back of the neck. Subjects sat with the jaw closing muscles relaxed or contracting at approximately 75% of the maximum voluntary level. The chin was tapped to evoke a jaw jerk reflex and the EEC was elicited by electrical stimulation to the palate. 3. In addition, bipolar EMGs and jaw position were recorded at minimal bite forces and at contraction levels of 5, 10, 15, or 20 N. 4. Data were computer average, with and without rectification, for 32 stimuli. 5. The EEC [latency 12.1 +/- 1.0 (SD)ms] was found to have the same polarity, shape, and duration as the repolarizing wave of the stretch reflex. 6. After electrical stimulation, an increase of bite force was never observed during or immediately after the EEC. Instead, bite force began to decrease 5-7 ms after the onset of the EEC. 7. The amplitude of the EECs never exceeded the level of the peaks in the preceding background EMG, even when the left and right palatal electrodes were stimulated simultaneously at high intensity.(ABSTRACT TRUNCATED AT 250 WORDS)