Biomedical subjects

T Graven-Nielsen

Publications and source records attributed to T Graven-Nielsen.

At least 19 recordsLinked to original sources

PubMed2003

Pharmacological modulation of experimental phasic and tonic muscle pain by morphine, alfentanil and ketamine in healthy volunteers.

BACKGROUND: Muscle pain is a major clinical problem but the underlying mechanisms and its pharmacological modulation need further investigation. This study on 15 volunteers evaluates if two experimental muscle pain models are sensitive to micro -receptor agonists and to an N-methyl-D-aspartate (NMDA)-receptor antagonist. METHODS: In the left tibialis anterior, intramuscular electrical (IMES) pain thresholds were determined for single (SPTmuscle) and five (RPTmuscle) repeated stimuli. Also pain to suprathreshold stimulation at 150% of RPTmuscle, 10 s, was assessed on a visual analog scale (VAS) as AUCimes (area under the VAS curve). In the right TA muscle, pain intensity on infusion of 0.5 ml of hypertonic saline, 5% (AUCsaline) and pain distribution indicated as local and referred were evaluated. Pain variables were assessed before, during and after intravenous infusions of morphine (10 microg x kg-1 min-1, 10 min), alfentanil (target-controlled infusion, plasma concentration; 60 ng ml-1, 60 min) and ketamine (10 microg x kg-1 min-1, 60 min). All data were normalized to baseline pain values (before drug infusions were initiated) and compared with placebo (midazolam, 2 microg x kg-1 min-1, 10 min). RESULTS: SPTmuscle increased (log mean values +/- SD, mA) with morphine (0.11 +/- 0.17, P < 0.05), alfentanil (0.28 +/- 0.24, P < 0.001) and ketamine (0.19 +/- 0.18, P < 0.01) as compared with placebo (-0.03 +/- 0.12). Alfentanil and ketamine also increased RPTmuscle (0.25 +/- 0.21, P < 0.01 and 0.21 +/- 0.19, P < 0.05, respectively) as compared with placebo (0.00 +/- 0.17). Pain to IMES (AUCimes) was reduced (median values [25th-75th percentiles], cm x s) by alfentanil and ketamine (-19.7 [-14.6 - -29.6] and-12.8 [-8.3 - -27.8], P < 0.05, respectively) vs. placebo (-0.8 [1.6 - -12.3]). Similar drug effects were seen when pain to infusion of hypertonic saline (AUCsaline) was assessed (alfentanil:-388 [-99 - -677] and ketamine:-326 [-227 - -573], P < 0.05 compared with placebo: 150 [449--240]). Ketamine also reduced the size of the local pain area (-58.4 [-21.2 - -176.1], < 0.05) as compared with placebo (-0.4 [70.6 - -13.4]). The frequency of referred pain was also lower when ketamine was given (3/13, P < 0.05) vs. placebo (9/14). CONCLUSION: The study demonstrates that experimental muscle pain induced in humans by electrical stimulation and infusion of hypertonic saline is sensitive to pharmacological modulation similar to preclinical animal tests and clinical trials. The data suggest that these models can be valuable tools in analgesic drug development.

Adult↗

PubMed2003

Lumbar epidural fentanyl: segmental spread and effect on temporal summation and muscle pain.

BACKGROUND: Despite extensive use, different aspects of the pharmacological action of epidural fentanyl have not been clarified. We applied a multi-modal sensory test procedure to investigate the effect of epidural fentanyl on segmental spread, temporal summation (as a measure for short-lasting central hyperexcitability) and muscle pain. METHODS: Thirty patients received either placebo, 50 or 100 micro g single dose of fentanyl epidurally (L2-3), in a randomized, double-blind fashion. Heat pain tolerance thresholds at eight dermatomes from S1 to fifth cranial nerve (assessment of segmental spread), pain threshold to transcutaneous repeated electrical stimulation of the sural nerve (assessment of temporal summation) and pain intensity after injection of hypertonic saline into the tibialis anterior muscle (assessment of muscle pain) were recorded. RESULTS: Fentanyl 100 micro g, but not 50 micro g, produced analgesia to heat stimulation only at L2. Surprisingly, no effect at S1 was detected. Both fentanyl doses significantly increased temporal summation threshold and decreased muscle pain intensity. CONCLUSIONS: The findings suggest that a single lumbar epidural dose of fentanyl should be injected at the spinal interspace corresponding to the dermatomal site of pain. Increased effect on L2 compared with S1 suggests that drug effect on spinal nerve roots and binding to opioid receptors on the dorsal root ganglia may be more important than traditionally believed for the segmental effect of epidurally injected fentanyl. Epidural fentanyl increases temporal summation threshold and could therefore contribute to prevention and treatment of central hypersensitivity states. I.M. injection of hypertonic saline is a sensitive technique for detecting the analgesic action of epidural opioids.

Adult↗

PubMed2002

Temporal summation of pain from skin, muscle and joint following nociceptive ultrasonic stimulation in humans.

This study investigated the phenomenon of temporal summation in response to repetitive focused ultrasound stimulation of skin, muscle and joint in human volunteers. Stimulation was carried out using a custom-designed, focused ultrasonic stimulator with a resonant frequency of 1.66 MHz. A series of stand-off attachments were used to ensure that the focal region of the ultrasound beam projected either cutaneously, within the distal interphalangeal joint of the index finger, or within the first dorsal interosseous muscle. Stimulation was carried out using single pulses and trains of five pulses of different pulse durations (25 ms, 50 ms, 75 ms, 100 ms), and using single pulses and trains of five pulses (50 ms duration) at different frequencies (0.5 Hz, 1 Hz, 2 Hz, 3 Hz, 4 Hz, 5 Hz). Tactile perception thresholds, pain thresholds and summation pain thresholds were recorded. Temporal summation of pain could be elicited by stimulation of both skin, joint and muscle, although the influence of temporal summation appeared to be more pronounced for muscle stimulation. Muscle stimulation also required greater ultrasound intensity compared with joint and skin stimulation. Temporal summation could not be elicited by tactile, low-intensity stimulation. Focused ultrasound is a potent, noninvasive technique with which to investigate temporal summation from somatic structures. A number of factors may account for the higher intensities required to elicit pain in muscle and the increased rate of temporal summation. It is clear, however, that if temporal summation is more pronounced in muscle than other tissues then this may be an important factor contributing to pain in musculoskeletal syndromes.

Adult↗

PubMed2002

Thermosensitivity of muscle: high-intensity thermal stimulation of muscle tissue induces muscle pain in humans.

Small-calibre afferent units responding to thermal stimuli have previously been reported to exist in muscle. The question as to whether these receptors in humans mediate subjective thermal sensations from muscle remains unresolved. The aims of the present study were to determine in humans whether intramuscular injection of warm and cold isotonic saline elicits temperature sensations, muscle pain or any other sensations. In 15 subjects, no thermal sensations assessed on a temperature visual analogue scale (VAS) could be detected with intramuscular injections of isotonic saline (1.5 ml) into the anterior tibial muscle at temperatures ranging from 8 to 48 degrees C. The same subjects recorded strongly increasing scores on a temperature VAS when thermal stimuli in the same intensity range were applied to the skin overlying the muscle by a contact thermode. However, I.M. isotonic saline of 48 degrees C induced muscle pain with peak scores of 3.2 +/- 0.8 cm on a VAS scale ranging from 0 to 10 cm. Using the the McGill pain questionnaire a subgroup, of subjects qualitatively described the pain using the 'thermal hot' and 'dullness' word groups. Temperature measurements within the muscle during the stimulating injections showed that the time course of the pain sensation elicited by saline at 48 degrees C paralleled that of the intramuscular temperature and far outlasted the injection time. The present data show that high-intensity thermal stimulation of muscle is associated with muscle pain. High-threshold warm-sensitive receptors may mediate the pain following activation by temperatures of 48 degrees C or more. Taken together, the data indicate that thermosensation from a given volume of muscle is less potent than nociception.

Adult↗

PubMed2001

Topographic effects of tonic cutaneous nociceptive stimulation on human electroencephalograph.

To examine the specific effects of cutaneous pain on electroencephalographic (EEG) activities, tonic painful and non-painful sensations in left forearm were induced by intradermal injection of capsaicin 100 microg/20microl and the same volume of vehicle, respectively, in 15 healthy males. The EEG data acquired in five experimental conditions: (i) baseline A; (ii) non-painful vehicle injection; (iii) baseline B; (iv) painful capsaicin injection and (v) waning pain, were analyzed and compared with analysis of variance. Only the painful capsaicin injection evoked significant decreases of theta, alpha-1 and alpha-2 powers over the centro-parieto-occipital regions compared with baseline B. No significant difference in EEG activation between the non-painful vehicle injection and painful capsaicin injection was found. This implicates that the observed topographic EEG activation is not specific for pain but probably related to the cutaneous stimulation.

Adult↗

PubMed2001

Conditioning of heteronymous H reflex in human temporalis muscle by stimulation of perioral afferents.

A heteronymous H reflex in the temporalis muscle can be elicited by selective stimulation of the masseteric nerve. The present study aimed at defining the optimal amplitude of the H reflex to detect inhibitory changes induced by stimulation of the perioral afferents and at providing new information on the control of masticatory muscles. Sixteen healthy volunteers participated in the experiment. A conditioning stimulus (CS) to the perioral skin was applied at various delays before an ipsilateral selective masseteric nerve stimulation (test stimulus: TS) while the subject was clenching the teeth at 25% of the maximal voluntary contraction. Two intensities of CS and TS were employed, high and low. The peak-to-peak amplitude of the H reflex (TS) and the root-mean-square value of the preceding electromyography were measured and the data analyzed by three-way analysis of variance and Tukey's posthoc tests. For both intensities used the heteronymous H reflex in the temporalis muscle was significantly decreased by prior activation of perioral afferents for delays from 5 to 60 ms. With a delay of 5 and 35 ms the preceding EMG level was not changed, while it was reduced at 20 and 60 ms delay. The intensities used to elicit the heteronymous H reflex of the temporalis muscle were appropriate to detect a reduction in motoneuron excitability. The reduction in the H reflex without a change in the preceding EMG at 5 and 35 ms delays could be due to presynaptic inhibition of the masseteric afferents exerted by the ipsilateral perioral afferents.

Adult↗

PubMed2001

Different EEG topographic effects of painful and non-painful intramuscular stimulation in man.

To clarify the specific effects of muscle pain on electroencephalogram (EEG) activation in man, painful and non-painful sensations were produced by intramuscular injections of capsaicin and vehicle solution in the left brachioradialis muscle, with identical procedures in 15 male volunteers. Thirty-one channel EEG data acquired before, during and after the two injections were analysed and compared in respect of topography and power spectrum. Although the painful and non-painful muscular stimulations evoked similar EEG topographic patterns, statistics demonstrated that distinct EEG activation over different areas of the head were induced by the painful and non-painful stimulation compared with the baselines. The decreases in theta and alpha-1 (8-10.5 Hz) activity in central and posterior parietal parts were evoked by non-painful stimulation, but the decreases in alpha-1 and alpha-2 (11-13.5 Hz) activities in the posterior part of the head were induced by painful stimulation. The alpha-2 activity augmented during the waning pain following a decrease in the overt pain. Comparing the EEG changes between baseline, non-painful and painful stimulations as well as waning pain, we found that the increase in beta-2 activity during muscle pain was significant over the extensive areas of the head, whereas a significant increase in alpha-2 activity took place at the posterior part of the head during waning pain following a marked decrease in overt pain. These results may imply that the painful and non-painful muscular stimulations evoke distinct EEG activation in different neural networks of the human brain and the intensity of nociceptive input from muscle may encode the variety of topographic EEG changes.

Adult↗

PubMed2001

An investigation of how acute muscle pain modulates performance during computer work with digitizer and puck.

The purpose was to investigate the influence of muscle pain on work performance during computer work with digitizer and puck. Muscle pain was induced by infusion of hypertonic saline in the trapezius and the extensor carpi ulnaris (ECU) muscles on two separate days. Twelve healthy subjects participated. A computer task was performed in three 6 min sessions: baseline, pain, after pain. The computer task comprised production of drawings at maximal work pace. One drawing was defined as a work cycle. Work cycle time, number of puck button clicks, and screen pixels the cursor had moved per cycle were assessed. Shoulder pain did not influence these variables. Cycle time decreased from 13.8 (SD 2.2) to 13.0 s (SD 1.9) compared to baseline (p < 0.05) during ECU muscle pain. The increased or unchanged performance suggests that acute moderate muscle pain has minor influence on performance during computer work with digitizer and puck.

Adult↗

PubMed2001

Motor potentials evoked by transcranial magnetic stimulation during isometric and dynamic masseter muscle contraction in humans.

The facilitation of muscle motor potentials evoked by transcranial magnetic stimulation (TMS) has been demonstrated convincingly during both isometric and dynamic activity in the limbs but not in the jaw muscles. An experimental design involving TMS, surface electromyography and controlled muscle-activity was employed to investigate the motor response of the human masseter during voluntary isometric and dynamic voluntary conditions. During the isometric condition, an increase in muscle facilitation resulted in a progressive increase in motor-evoked potential (MEP) amplitude that was consistently greater on the side contralateral to that subjected to TMS (P < 0.05). No difference in MEP amplitude or laterality of response was revealed for the two dynamic conditions. The sample size may have been too small to reveal any differences. The modulation of MEPs during isometric activity was probably due to cortical and brainstem mechanisms. Putative variation in masseter MEPs during dynamic conditions cannot be discounted.

Adult↗

PubMed2001

Inhibition of motor system excitability at cortical and spinal level by tonic muscle pain.

OBJECTIVE: To assess whether the motor system excitability can be modified by experimental tonic pain induced either in muscles or in subcutis. METHODS: Transcranial magnetic stimulation of the left primary motor cortex was used to record motor evoked potentials (MEPs) from the right abductor digiti minimi (ADM) muscle. Recordings were made before, during and after experimental pain induced by (1) injection of hypertonic (5%) saline into the right ADM, the right first dorsal interosseum (FDI) and the left ADM muscles, and (2) injection of hypertonic saline in the subcutaneous region of the right ADM. Both MEPs and H-reflex were recorded also from the right flexor carpi radialis (FCR) before, during and after muscle pain. RESULTS: MEPs recorded from the ADM muscle were significantly reduced in amplitude during pain induced in the right ADM and right FDI muscles, but not during pain in the left ADM muscle or during subcutaneous pain. This inhibitory effect was observed during the peak-pain and persisted also after the disappearance of the pain sensation. In the FCR muscle, the MEP inhibition was observed during the peak-pain, while a significant reduction of the H-reflex's amplitude was observed starting 1 min after the peak-pain. CONCLUSIONS: Tonic muscle pain can inhibit the motor system. The motor cortex inhibition observed at an early phase is followed by a reduction of the excitability of both cortical and spinal motoneurones.

Adult↗

PubMed2001

Non-painful and painful surface and intramuscular electrical stimulation at the thenar and hypothenar sites: differential cerebral dynamics of early to late latency SEPs.

Little is known about somatosensory evoked potentials (SEPs) from muscle stimulation compared to that from skin stimulation. The current study examined this issue in the full SEP spectrum (0-440 ms). The aims of the study were to (1) establish the dynamics of early to late latency SEPs from intramuscular stimulation in contrast to surface stimulation, (2) compare the effect of non-painful and painful stimuli on SEP latencies and amplitudes of the two methods, and (3) investigate to which extent these results can be shared between the median nerve innervated thenar site and ulnar nerve innervated hypothenar site. Stimuli were delivered (2 Hz) at a non-painful and a painful intensity above or within the thenar and hypothenar muscles of the hand. Maximas of the SEPs were extracted by a combination of global field power and visual inspection of the topographies. Amplitudes and latencies of the maximas were analysed by a two-way ANOVA with repeated measures. In the early phase (0-50 ms) the topographic patterns showed different dynamics between surface and intramuscular stimulation and in the late phase (100- 440 ms) prolonged latencies were found for intramuscular stimulation. Apart from this, similar topographic patterns and time sequences were obtained. Significant higher SEP amplitudes for most of the isolated components (C4'/P25, Fz/N35, C4'/P45, Fc2/N65, P4/P90, T4/N137, F3/P150, Cz/P240-P270) were found with surface stimulation compared to intramuscular stimulation. In contrast to surface stimulation, intramuscular stimulation at a stimulation frequency of 2 Hz did not result in a differentiation in amplitude for any of the isolated components. These results indicate differences in the early and late processing of sensory input from skin and muscle.

Adult↗

PubMed2001

Post-exercise muscle soreness after eccentric exercise: psychophysical effects and implications on mean arterial pressure.

The aim of the study was to examine the time course of changes in pressure pain threshold (PPT), visual analogue scale (VAS) pain and tenderness scores, McGill Pain Questionnaire (MPQ) descriptors, pain areas, skin temperature and mean arterial pressure (MAP) following intensive eccentric exercise. In 11 healthy male subjects, eccentric exercise of the first dorsal interosseous muscle (FDI) of the right hand with 114% maximum voluntary contraction weight (MVC) was used to induce post-exercise muscle soreness (PEMS) in the right hand, while the left hand served as a control. At 24 h to 48 h all the pain profiles indicated the presence of PEMS in the right hand when compared to before exercise (P<0.05). MPQ and pain area assessments also indicated PEMS immediately after the exercise, while the pain assessment by PPT and tenderness VAS showed insignificant pain immediately after the exercise. Skin temperature measured in the first web space of the hand did not change at any time. MAP was significantly reduced at 48 h. It is concluded that eccentric exercise of a small hand muscle is followed by PEMS and a reduced MAP after 48 h that may suggest a role of central mechanisms in the PEMS, thereby giving further insight into clinical aspects of muscle pain.

Adult↗

PubMed2001

Computer-controlled pneumatic pressure algometry--a new technique for quantitative sensory testing.

Hand-held pressure algometry usually assesses pressure-pain detection thresholds and provides little information on pressure-pain stimulus-response function. In this article, a cuff pressure algometry for advanced pressure-pain function evaluation is proposed. The experimental set-up consisted of a pneumatic tourniquet cuff, a computer-controlled air compressor and an electronic visual analogue scale (VAS) for constant pain intensity rating. Twelve healthy volunteers were included in the study. In the first part, hand-held algometry and cuff algometry were performed over the gastrocnemius muscle with constant compression rate. In the second part, the cuff algometry was performed with different compression rates to evaluate the influence of the compression rate on pain thresholds and other psychophysical data. Pressure-pain detection threshold (PDT), pain tolerance threshold (PTT), pain intensity, PDT-PTT time and other psychophysical variables were evaluated.Pressure-pain detection thresholds recorded over the gastrocnemius muscle with a hand-held and with a cuff algometer, were 482 +/- 19 kPa and 26 +/- 1.6 kPa, respectively. Pressure and pain intensities were correlated during cuff algometry. During increasing cuff compression, the subjective pain tolerance limit on VAS was 5.6 +/- 0.95 cm. There was a direct correlation between the number of compressions, the compression rate and pain thresholds. The cuff algometry technique is appropriate for pressure-pain stimulus-response studies. Cuff algometry allowed quantification of psychophysical response to the change of stimulus configuration.

Adult↗

PubMed2001

The peripheral apparatus of muscle pain: evidence from animal and human studies.

The peripheral apparatus of muscle pain consists of nociceptors that can be excited by endogenous substances and mechanical stimuli. Histologically, the nociceptors are free nerve endings supplied by group III (thin myelinated) and group IV (nonmyelinated) afferents with conduction velocities less than 30 m/s. At the molecular level, nociceptors have receptors for algesic substances, such as bradykinin, serotonin, and prostagladin E2. The purinergic receptors and tetrodotoxin-resistant sodium channels might be new important targets for the treatment of muscle pain. Algesic substances (capsaicin, bradykinin, serotonin, potassium chloride, and hypertonic saline) and other stimuli (ischemia, strong mechanical stimuli, and electrical stimuli) have been shown to induce nociception from muscle in animals and muscle pain in humans. Muscle nociceptors can be sensitized to chemical and mechanical stimuli. Contrary to a former belief, the sensitization is not an unspecific process; rather, it is caused by endogenous algesic substances binding to highly specific receptor molecules in the membrane of the nociceptive ending. For example, animal studies showed that serotonin sensitizes muscle nociceptors to chemical and mechanical stimuli. Later, human studies showed that serotonin combined with bradykinin induces muscle hyperalgesia to pressure. The sensitization process by endogenous substances that are likely to be released during trauma or inflammatory injury is probably the best established peripheral mechanism for muscle tenderness and hyperalgesia.

Animals↗

PubMed2001

Craniofacial muscle pain: review of mechanisms and clinical manifestations.

Epidemiologic surveys of temporomandibular disorders (TMD) have demonstrated that a considerable proportion of the population--up to 5% or 6%--will experience persistent pain severe enough to seek treatment. Unfortunately, the current diagnostic classification of craniofacial muscle pain is based on descriptions of signs and symptoms rather than on knowledge of pain mechanisms. Furthermore, the pathophysiology and etiology of craniofacial muscle pain are not known in sufficient detail to allow causal treatment. Many hypotheses have been proposed to explain cause-effect relationships; however, it is still uncertain what may be the cause of muscle pain and what is the effect of muscle pain. This article reviews the literature in which craniofacial muscle pain has been induced by experimental techniques in animals and human volunteers and in which the effects on somatosensory and motor function have been assessed under standardized conditions. This information is compared to the clinical correlates, which can be derived from the numerous cross-sectional studies in patients with craniofacial muscle pain. The experimental literature clearly indicates that muscle pain has significant effects on both somatosensory and craniofacial motor function. Typical somatosensory manifestations of experimental muscle pain are referred pain and increased sensitivity of homotopic areas. The craniofacial motor function is inhibited mainly during experimental muscle pain, but phase-dependent excitation is also found during mastication to reduce the amplitude and velocity of jaw movements. The underlying neurobiologic mechanisms probably involve varying combinations of sensitization of peripheral afferents, hyperexcitability of central neurons, and imbalance in descending pain modulatory systems. Reflex circuits in the brain stem seem important for the adjustment of sensorimotor function in the presence of craniofacial pain. Changes in somatosensory and motor function may therefore be viewed as consequences of pain and not factors leading to pain. Implications for the diagnosis and management of persistent muscle pain are discussed from this perspective.

Afferent Pathways↗

PubMed2000

Duration and distribution of experimental muscle hyperalgesia in humans following combined infusions of serotonin and bradykinin.

The present study examined distribution and duration of muscle hyperalgesia to pressure stimuli after intramuscular bolus-infusions of serotonin (5-HT, 20 nmol) and bradykinin (BKN, 10 nmol) in 10 volunteers. Infusions were given into the tibialis anterior (TA) muscle over 20 s with an inter-infusions interval of 3 min. Infusions of isotonic saline (NaCl, 0.9%) were given as control. Pain intensity was continuously scored on a visual analogue scale (VAS), and subjects drew the distribution of the pain areas on an anatomical map. Pressure pain thresholds (PPTs) were assessed with an electronic algometer at the injection site (10 cm below the patella), 2, 5, and 10 cm distal from the injection site, and at the ankle. Control assessments of PPTs were done at the contralateral TA and ankle. Skin sensibility was assessed with a Von Frey hair at the same sites. All measurements were done before and 5, 20, 40, and 60 min after infusions. The VAS-peak after BKN was significantly higher (P<0.05) compared with 5-HT and the second infusion of NaCl. The duration of the increase in VAS after 5-HT+BKN was significantly longer (P<0.05) compared with the infusions of NaCl. The local pain area after infusion of BKN was significantly larger (P<0.05) compared with 5-HT and control infusions. Cutaneous sensibility to tactile stimuli was not affected by any of the combinations. PPTs at the injection site and 2 cm (5, 20, and 40 min) were significantly decreased (P<0.05) after 5-HT+BKN compared with baseline and isotonic saline. In addition, PPTs were significantly decreased (P<0.05) after 5-HT+BKN at 5 cm (5 and 20 min) and 10 cm (5 min). Serotonin may enhance the effect of bradykinin in producing experimental muscle pain and muscle hyperalgesia to mechanical stimuli. The combination of serotonin and bradykinin can produce muscle hyperalgesia, lasted for up to 40 min and located within the muscle. No widespread hyperalgesia to the ankle and other leg (tested at 10 cm below the patella and ankle) was observed suggesting a predominant peripheral origin of the experimentally induced hyperalgesic stage.

Adult↗

PubMed2000

Quantitative analysis of reflex inhibition in single motor units in human masseter muscle: effects of stimulus intensity.

Inhibitory reflex responses to electrical stimulation of the mental nerve were studied in recordings from single motor units (SMU) in the human masseter muscle. A new analytical technique for spike train data was used. Electrical stimuli were delivered to the mental nerve when the SMU fired with two consecutive inter-spike intervals (ISIs) within the range of 90 ms to 110 ms. Stimuli were delivered with increasing stimulus delays after the preceding SMU action potential (AP). Sham, non-painful, and painful stimulus intensities were applied. The post-stimulus firing probability of the SMU was progressively decreased among the three conditions. Analysis of the relation between stimulus delays and ISI for the first post-stimulus APs revealed a linear relation which was shifted upward, and the slope was increased with increasing stimulus intensity. This may be explained by a differential effect of the increasing stimulus intensity on the duration and amplitude of the inhibitory post-synaptic potential. The methods used in the present study provide a useful means of quantifying the effects of motoneuron excitability in detail.

Adult↗

PubMed2000

Inhibition of motor unit firing during experimental muscle pain in humans.

Electromyographic activity was recorded in the masseter muscle to investigate whether the firing characteristics of single motor units (SMUs) were affected by muscle pain. Capsaicin was injected into the masseter to induce pain. The interspike interval (ISI) and recruitment threshold of SMUs were measured while subjects performed isometric contractions at 5, 7.5, 10, 15, and 20% of maximum voluntary contraction. All subjects were able to maintain a stable isometric force during pain, but the mean ISI was significantly increased without changing the recruitment threshold. In all the experimental conditions, the firing frequency increased with increasing force, and SMUs recruited at low force fired at higher rates for all force levels. These results suggest that acute stimulation of nociceptive muscle afferents inhibits SMU activity without changing recruitment order in the homonymous muscle.

Action Potentials↗