-
[show abstract]
[hide abstract]
ABSTRACT: The reflex mechanism of the short-latency inhibitory reflex to transient loading of human inspiratory muscles is unresolved. Muscle afferents mediate this reflex, but they may act via pontomedullary inspiratory centers, other bulbar networks, or spinal circuits. We hypothesized that altered chemical drive to breathe would alter the initial inhibitory reflex if the neural pathways involve inspiratory medullary centers. Inspiration was transiently loaded in 11 subjects during spontaneous hypercapnic hyperpnea and matched voluntary hyperventilation. Electromyographic activity was recorded bilaterally from scalene muscles with surface electrodes. The latencies of the initial inhibitory response (IR) onset (32 +/- 0.7 and 38 +/- 1 ms for spontaneous and voluntary conditions respectively, P < 0.001) and subsequent excitatory response (ER) onset (80 +/- 2.9 and 78 +/- 2.6 ms, respectively, P = 0.46) and the normalized sizes of IR (65 +/- 2 and 67 +/- 3%, respectively, P = 0.50) and ER (51 +/- 8 and 69 +/- 6%, respectively, P = 0.005) were measured. Mean end-tidal Pco(2) was 43 +/- 1.5 Torr with dead space ventilation and was 14 +/- 0.6 Torr with matched voluntary hyperventilation (P < 0.001). A mean minute volume >30 liters was achieved in both conditions. The absence of significant difference in the size of the IR suggested that the IR reflex arc does not transit the brain stem inspiratory centers and that the reflex may be integrated at a spinal level. In voluntary hyperventilation, an initial excitation occurred more frequently and, consequently, the IR onset latency was significantly longer. The size of the later ER was also greater during voluntary hyperventilation, which is consistent with it being mediated via longer, presumably cortical, pathways, which are influenced by voluntary drive.
Journal of Applied Physiology 07/2010; 109(1):87-94. · 3.75 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A randomized controlled trial.
To determine the effectiveness of electrical stimulation (ES)-evoked muscle contractions superimposed on progressive resistance training (PRT) for increasing voluntary strength in the quadriceps muscles of people with spinal cord injuries (SCI).
Sydney, Australia.
A total of 20 people with established SCI and neurologically induced weakness of the quadriceps muscles participated in the trial. Participants were randomized between experimental and control groups. Volunteers in the experimental group received ES superimposed on PRT to the quadriceps muscles of one leg thrice weekly for 8 weeks. Participants in the control group received no intervention. Assessments occurred at the beginning and at the end of the 8-week period. The four primary outcomes were voluntary strength (Nm) and endurance (fatigue ratio) as well as the performance and satisfaction items of the Canadian Occupational Performance Measure (COPM; points).
The between-group mean differences (95% confidence interval (CI)) for voluntary strength and endurance were 14 Nm (1-27; P=0.034) and 0.1 (-0.1 to 0.3; P=0.221), respectively. The between-group median differences (95% CI) for the performance and satisfaction items of the COPM were 1.7 points (-0.2 to 3.2; P=0.103) and 1.4 points (-0.1 to 4.6; P=0.058), respectively.
ES superimposed on PRT improves voluntary strength, although there is uncertainty about whether the size of the treatment effect is clinically important. The relative effectiveness of ES and PRT is yet to be determined.
Spinal Cord 07/2010; 48(7):570-5. · 1.80 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Stimuli that preferentially activate rapidly adapting cutaneous receptors impair proprioception in the fingers. These experiments assessed potential mechanisms. The ability to detect passive movements about interphalangeal joints of the fingers was measured when vibrotactile stimuli were applied to the moving digit or to an adjacent digit at a high frequency (300 Hz) and low amplitude (50 microm peak-to-peak) that favours activation of Pacinian corpuscle (PC) afferents. Detection of movement was significantly impaired when vibration was applied to either digit. However, vibration applied to an anaesthetized adjacent digit caused no impairment. Impairment of proprioception was still observed when only skin and joint (but not muscle) afferents could contribute to detection, but was not significant with only muscle afferents intact during anaesthesia of the moving digit. We suggest that activation of PC afferents, either in or near the moving digit, impairs movement detection through an interaction predominantly between the classes of cutaneous afferents.
Experimental Brain Research 09/2009; 197(4):347-55. · 2.39 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: There is evidence to suggest that contractile properties of muscles in people with multiple sclerosis change as a consequence of demyelination in central nervous system. However, passive properties of muscles in people with multiple sclerosis have not been previously investigated. The purpose of this study was to characterise passive mechanical properties of gastrocnemius in people with multiple sclerosis and to compare these properties with those of gastrocnemius in neurologically normal people.
Ten people with multiple sclerosis having signs and symptoms of weakness in the legs (Disease step 1-3) and 10 age- and sex-matched healthy people participated in the study. Ultrasound images of muscle fascicles of medial gastrocnemius as well as passive ankle torque and ankle angle data were obtained simultaneously as the ankle was rotated through its full range with the knee in a range of positions. Analysis of ultrasound images and passive ankle torque-angle relations allowed us to derive the slack lengths and maximal strains of whole muscle-tendon units, muscle fascicles and tendons. Paired-samples t-tests were used to compare these variables in the two groups.
There was no difference between subjects with multiple sclerosis and healthy controls in the mean slack lengths and mean maximal strains of the whole muscle-tendon units or of their fascicles or tendons.
These data suggests that typically, in people with multiple sclerosis who have impaired lower limbs but are still ambulatory, the passive mechanical properties of the gastrocnemius muscles are normal.
Clinical biomechanics (Bristol, Avon) 02/2009; 24(3):291-8. · 1.76 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The most obvious impairments associated with spinal cord injury (SCI) are loss of sensation and motor control. However, many subjects with SCI also develop persistent neuropathic pain below the injury which is often severe, debilitating and refractory to treatment. The underlying mechanisms of persistent neuropathic SCI pain remain poorly understood. Reports in amputees describing phantom limb pain demonstrate a positive correlation between pain intensity and the amount of primary somatosensory cortex (S1) reorganization. Of note, this S1 reorganization has also been shown to reverse with pain reduction. It is unknown whether a similar association between S1 reorganization and pain intensity exists in subjects with SCI. The aim of this investigation was to determine whether the degree of S1 reorganization following SCI correlated with on-going neuropathic pain intensity. In 20 complete SCI subjects (10 with neuropathic pain, 10 without neuropathic pain) and 21 control subjects without SCI, the somatosensory cortex was mapped using functional magnetic resonance imaging during light brushing of the right little finger, thumb and lip. S1 reorganization was demonstrated in SCI subjects with the little finger activation point moving medially towards the S1 region that would normally innervate the legs. The amount of S1 reorganization in subjects with SCI significantly correlated with on-going pain intensity levels. This study provides evidence of a link between the degree of cortical reorganization and the intensity of persistent neuropathic pain following SCI. Strategies aimed at reversing somatosensory cortical reorganization may have therapeutic potential in central neuropathic pain.
Pain 12/2008; 141(1-2):52-9. · 5.78 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Activity in both muscle spindle endings and cutaneous stretch receptors contributes to the sensation of joint movement. The present experiments assessed whether muscle pain and subcutaneous pain distort proprioception in humans. The ability to detect the direction of passive movements at the interphalangeal joint of the thumb was measured when pain was induced experimentally in four sites: the flexor pollicis longus (FPL), the subcutaneous tissue overlying this muscle, the flexor carpi radialis (FCR) muscle and the subcutaneous tissue distal to the metacarpophalangeal joint of thumb. Tests were conducted when pain was at a similar subjective intensity. There was no significant difference in the ability to detect flexion or extension under any painful or non-painful condition. The detection of movement was significantly impaired when pain was induced in the FPL muscle, but pain in the FCR, a nearby muscle that does not act on the thumb, had no effect. Subcutaneous pain also significantly impaired movement detection when initiated in skin overlying the thumb, but not in skin overlying the FPL muscle in the forearm. These findings suggest that while both muscle and skin pain can disturb the detection of the direction of movement, the impairment is site-specific and involves regions and tissues that have a proprioceptive role at the joint. Also, pain induced in FPL did not significantly increase the perceived size of the thumb. Proprioceptive mechanisms signalling perceived body size are less disturbed by a relevant muscle nociceptive input than those subserving movement detection. The results highlight the complex relationship between nociceptive inputs and their influence on proprioception and motor control.
The Journal of Physiology 08/2008; 586(13):3183-93. · 4.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A debilitating consequence of complete spinal cord injury (SCI) is the loss of motor control. Although the goal of most SCI treatments is to re-establish neural connections, a potential complication in restoring motor function is that SCI may result in anatomical and functional changes in brain areas controlling motor output. Some animal investigations show cell death in the primary motor cortex following SCI, but similar anatomical changes in humans are not yet established. The aim of this investigation was to use voxel-based morphometry (VBM) and diffusion tensor imaging (DTI) to determine if SCI in humans results in anatomical changes within motor cortices and descending motor pathways. Using VBM, we found significantly lower gray matter volume in complete SCI subjects compared with controls in the primary motor cortex, the medial prefrontal, and adjacent anterior cingulate cortices. DTI analysis revealed structural abnormalities in the same areas with reduced gray matter volume and in the superior cerebellar cortex. In addition, tractography revealed structural abnormalities in the corticospinal and corticopontine tracts of the SCI subjects. In conclusion, human subjects with complete SCI show structural changes in cortical motor regions and descending motor tracts, and these brain anatomical changes may limit motor recovery following SCI.
Cerebral Cortex 06/2008; 19(1):224-32. · 6.54 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The influence of group III and IV muscle afferents on human motor pathways is poorly understood. We used experimental muscle pain to investigate their effects at cortical and spinal levels. In two studies, electromyographic (EMG) responses in elbow flexors and extensors to stimulation of the motor cortex (MEPs) and corticospinal tract (CMEPs) were evoked before, during, and after infusion of hypertonic saline into biceps brachii to evoke deep pain. In study 1, MEPs and CMEPs were evoked in relaxed muscles and during contractions to a constant elbow flexion force. In study 2, responses were evoked during elbow flexion and extension to a constant level of biceps or triceps brachii EMG, respectively. During pain, the size of CMEPs in relaxed biceps and triceps increased (by approximately 47% and approximately 56%, respectively; P < 0.05). MEPs did not change with pain, but relative to CMEPs, they decreased in biceps (by approximately 34%) and triceps (by approximately 43%; P < 0.05). During flexion with constant force, ongoing background EMG and MEPs decreased for biceps during pain (by approximately 14% and 15%; P < 0.05). During flexion with a constant EMG level, CMEPs in biceps and triceps increased during pain (by approximately 30% and approximately 26%, respectively; P < 0.05) and relative to CMEPs, MEPs decreased for both muscles (by approximately 20% and approximately 17%; P < 0.05). For extension, CMEPs in triceps increased during pain (by approximately 22%) whereas MEPs decreased (by approximately 15%; P < 0.05). Activity in group III and IV muscle afferents produced by hypertonic saline facilitates motoneurones innervating elbow flexor and extensor muscles but depresses motor cortical cells projecting to these muscles.
The Journal of Physiology 04/2008; 586(5):1277-89. · 4.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Gender greatly influences pain processing. Not only do females display greater pain sensitivity, many chronic pain conditions affect females more than males. Although gender-based differences in pain sensitivity may be related to cultural and social factors, animal studies also reveal gender differences in pain sensitivity, suggesting that physiological factors may contribute to differences in the processing of pain in males and females. It has been recently reported that noxious cutaneous heat stimuli evoke gender-based differences in activity in some brain regions. Given that most chronic pain conditions, including those with gender bias are of "deep" origin (e.g. arising in muscle, joints or viscera), we investigated whether gender differences also exist in the central processing of muscle pain. In 24 healthy adults we used functional magnetic resonance imaging (fMRI) to measure signal intensity changes during muscle and cutaneous pain induced by intramuscular and subcutaneous injections of hypertonic saline, respectively. In addition to activating the "pain neuromatrix", i.e. cingulate, insular, somatosensory and cerebellar cortices, both muscle pain and cutaneous pain evoked gender-based differences in the mid-cingulate cortex, dorsolateral prefrontal cortex, hippocampus and cerebellar cortex. These differences may reflect differences in emotional processing of noxious information in men and women and may underlie the gender bias that exists in many chronic pain conditions.
NeuroImage 03/2008; 39(4):1867-76. · 5.89 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This study provides the first in vivo measures of the passive length-tension properties of relaxed human muscle fascicles and their tendons. A new method was used to derive passive length-tension properties of human gastrocnemius muscle-tendon units from measures of ankle stiffness obtained at a range of knee angles. Passive length-tension curves of the muscle-tendon unit were then combined with ultrasonographic measures of muscle fascicle length and pennation to determine passive length-tension curves of the muscle fascicles and tendons. Mean slack lengths of the fascicles, tendons and whole muscle-tendon units were 3.3+/-0.5 cm, 39.5+/-1.6 cm and 42.3+/-1.5 cm, respectively (means +/- s.d., N=6). On average, the muscle-tendon units were slack (i.e. their passive tension was zero) over the shortest 2.3+/-1.2 cm of their range. With combined changes of knee and ankle angles, the maximal increase in length of the gastrocnemius muscle-tendon unit above slack length was 6.7+/-1.9 cm, of which 52.4+/-11.7% was due to elongation of the tendon. Muscle fascicles and tendons underwent strains of 86.4+/-26.8% and 9.2+/-4.1%, respectively, across the physiological range of lengths. We conclude that the relaxed human gastrocnemius muscle-tendon unit falls slack over about one-quarter of its in vivo length and that muscle fascicle strains are much greater than tendon strains. Nonetheless, because the tendons are much longer than the muscle fascicles, tendons contribute more than half of the total compliance of the muscle-tendon unit.
Journal of Experimental Biology 01/2008; 210(Pt 23):4159-68. · 3.00 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Noxious stimulation of skeletal muscle evokes pain that is often referred into distal areas. Despite referred pain being of significant clinical importance, the brain regions responsible for the perception of referred pain remain unexplored. The aim of this investigation is to define these regions using functional magnetic resonance imaging. We induced muscle pain by hypertonic saline injections (0.5 ml) into the tibialis anterior (TA) or flexor carpi radialis (FCR) muscle. TA injections evoked pain that was referred to the ankle/foot in 10/17 subjects, whereas FCR injections evoked pain that was projected into the wrist/hand in 6/12 subjects. Regional brain responses were statistically tested by convolving the temporal profile of the subjective pain intensity rating with the hemodynamic response function. For all subjects, signal increased in the region of primary somatosensory cortex (SI), which represents the leg or arm, that is, the area corresponding to the injection site. However, for those subjects who reported referred pain, signal intensity increases also occurred in the SI region representing the foot or hand. Interestingly, differential signal changes also occurred in anterior cingulate, cerebellar, and insular cortices. This is the first study to provide evidence of cortical differentiation in the processing of primary and referred pain.
Cerebral Cortex 10/2007; 17(9):2050-9. · 6.54 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The uncommonly good proprioceptive performance of the long flexor of the thumb, flexor pollicis longus (FPL), may add significantly to human manual dexterity. We investigated the forces produced by FPL single motor units during a weak static grip involving all digits by spike-triggered averaging from single motor units, and by averaging from twitches produced by intramuscular stimulation. Nine adult subjects were studied. The forces produced at each digit were used to assess how forces produced in FPL are distributed to the fingers. Most FPL motor units produced very low forces on the thumb and were positively correlated with the muscle force at recruitment. Activity in FPL motor units commonly loaded the index finger (42/55 units), but less commonly the other fingers (P < 0.001). On average, these motor units produced small but significant loading forces on the index finger ( approximately 5.3% of their force on the thumb) with the same time-to-peak force as the thumb ( approximately 50 ms), but had no significant effect on other fingers. However, intramuscular stimulation within FPL did not produce significant forces in any finger. Coherence at 2-10 Hz between the thumb and index finger force was twice that for the other finger forces and the coherence to the non-index fingers was not altered when the index finger did not participate in the grasp. These results indicate that, within the long-term coordinated forces of all digits during grasping, FPL motor units generate forces highly focused on the thumb with minimal peripheral transfer to the fingers and that there is a small but inflexible neural coupling to the flexors of the index finger.
The Journal of Physiology 09/2007; 583(Pt 3):1145-54. · 4.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: These experiments assessed whether the impairment in proprioceptive acuity in the hand during 'interfering' cutaneous stimulation could be caused by inputs from Pacinian corpuscles. The ability to detect passive movements at the proximal interphalangeal joint of the index finger was measured when vibrotactile stimuli were applied to the adjacent middle finger and thenar eminence at frequencies and amplitudes that favour activation of rapidly adapting cutaneous afferents. Inputs from Pacinian corpuscles are favoured with high-frequency vibration (300 Hz), while those from Meissner corpuscles are favoured by lower frequencies (30 Hz). Detection of movement was significantly impaired when 300 Hz (20 microm peak-to-peak amplitude) complex vibration or 300 Hz (50 microm) sinusoidal vibration was applied to the middle finger and thenar eminence. In contrast, detection of movements was not altered by low-frequency sinusoidal vibration at 30 Hz with an amplitude of 50 microm or with a larger amplitude matched in subjective intensity to the 300 Hz sinusoidal stimulus. Thus it is unlikely that the impairment in detection was due to attention being diverted by vibration of an adjacent digit. In addition, an increase in amplitude of 300 Hz vibration led to a greater impairment of movement detection, so that the impairment was graded with the input. The time taken to nominate the direction of applied movement also increased during 300 Hz but not during 30 Hz sinusoidal vibration. These findings suggest that stimuli which preferentially activate Pacinian, but not Meissner corpuscles, impair proprioceptive acuity in a movement detection task.
The Journal of Physiology 07/2007; 581(Pt 3):971-80. · 4.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Some voluntary drive reaches human upper limb muscles via cervical propriospinal premotoneurones. Stimulation of the superficial radial nerve can inhibit these premotoneurones selectively and the resultant suppression of voluntary drive to motoneurones changes on-going electromyographic (EMG) activity. We investigated whether muscle fatigue changes this cutaneous-induced suppression of propriospinal drive to motoneurones of upper limb muscles. EMG was recorded from the extensors and flexors of the wrist and elbow. In the first study (n = 10 subjects), single stimuli (2 x perception threshold; 2PT) to the superficial radial nerve were delivered during contraction of the wrist extensors, before and after sustained fatiguing contractions of wrist extensors. In the second study (n = 10), similar stimuli were applied during elbow extension, before and during fatigue of elbow extensors. In the final study (n = 10), trains of three stimuli (2PT) were delivered during contractions of wrist extensors, before and while they were fatigued. With fatigue of either the wrist or elbow extensors, EMG suppression to single cutaneous stimuli increased significantly (by approximately 75%) for the fatigued muscle (P < 0.05). Conversely, in the other muscles, which were coactivated but not principally involved in the task, inhibition decreased or facilitation increased. Trains of stimuli produced greater suppression of on-going wrist extensor EMG than single stimuli and this difference persisted with fatigue. A control study of the H reflex in extensor carpi radialis showed that the mechanism responsible for the altered EMG suppression in fatigue was not at a motoneurone level. The findings suggest that the proportion of descending drive mediated via the disynaptic propriospinal pathway or the excitability of inhibitory interneurones projecting to propriospinal neurones increases substantially to fatigued muscles, but decreases to other active muscles. This pattern of changes may maintain coordination during multimuscle movements when one group of muscles is fatigued.
The Journal of Physiology 04/2007; 580(Pt 1):211-23. · 4.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The insula is involved in processing noxious information. It is consistently activated by acute noxious stimuli, can elicit pain on stimulation, and lesions encompassing the insula can alter pain perception. Anatomical tracing, electrophysiological and functional brain imaging investigations have suggested that the insula is somatotopically organized with respect to noxious cutaneous inputs. It has also recently been revealed that the anterior insula displays differential activation during cutaneous compared with muscle pain. Given this difference, it is important to determine if an insula somatotopy also exists for muscle pain. Using high-resolution functional magnetic resonance imaging (fMRI) we compared insula activation patterns in 23 subjects during muscle and cutaneous pain induced in the right leg and forearm. Group and frequency analyses revealed somatotopically organized signal increases in the posterior contralateral (left) and ipsilateral (right) anterior insula. Within the posterior contralateral insula, signal increases during both cutaneous and muscle forearm pain were located lateral and anterior to those evoked by leg pain, whereas in the ipsilateral anterior insula the pattern was reversed. Furthermore, within the ipsilateral anterior insula, muscle pain activated a region anterior to that activated by cutaneous pain. This somatotopic organization may be crucial for pain localization or other aspects of the pain experience that differ depending on both stimulation site and type of tissue activated. This study reveals that the insula is organized somatopically with respect to muscle and cutaneous pain and that this organization is further separated according to the tissue in which the pain originates.
Pain 04/2007; 128(1-2):20-30. · 5.78 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The excitability of the human motor cortex projecting to hand muscles can be reduced by theta burst transcranial magnetic repetitive stimulation (TBS). This study compared the magnitude and variability of changes evoked by TBS for a distal and proximal arm muscle.
Eight subjects participated in three studies. In each study, electromyographic responses (MEPs) to single-pulse transcranial magnetic stimulation assessed cortical excitability before and after 40s of TBS. In the first two studies, TBS (intensity, 80% active motor threshold) was delivered to the optimal locations for biceps or first dorsal interosseous (FDI). In the final study, weaker intensity TBS was delivered over the biceps representation.
TBS targeting biceps produced highly variable results among subjects. For the group, MEPs were not significantly depressed. Repeat studies in individual subjects highlighted the variability of responses. For FDI, MEPs were significantly depressed 5min after TBS and remained depressed for >30min (p<0.05). No significant changes in biceps MEPs occurred with weaker TBS.
The magnitude and reliability of TBS depends on the region of the cortex targeted.
Results obtained for the hand should not be considered indicative of changes that will occur in other regions of the motor cortex or the brain.
Clinical Neurophysiology 12/2006; 117(12):2684-90. · 3.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: To investigate whether a short-duration reduction of input to the motor cortex affects excitability in the hand region of the motor cortex.
Subjects (n=10) received sets of transcranial magnetic stimulation of the motor cortex (TMS) and peripheral ulnar nerve stimulation. Stimuli were delivered before and after 20 min of inactivity of the test hand. The evoked compound muscle action potentials were recorded in two relaxed intrinsic hand muscles using surface EMG.
Motor evoked potential size (MEP; expressed relative to the maximal M-wave) increased by approximately 30-40 in both hand muscles (P=0.012) following inactivity. The enlarged MEP was not associated with changes in F-wave size, a marker of motoneurone excitability, or changes in intracortical inhibition and facilitation measured with paired-pulse TMS.
MEP growth most likely reflects an increase in motor cortical excitability. The increased excitability appears to be more associated with reduced voluntary drive to and from the motor cortex rather than reduced afferent input from the periphery.
These results have important implications for any investigation of motor cortical excitability in relaxed subjects. The outcome of an experimental intervention is the net result of the intervention itself and alterations in cortical excitability produced by the subjects' inactivity.
Clinical Neurophysiology 12/2006; 117(11):2496-503. · 3.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This study investigated transmission of corticospinal output through motoneurons over a wide range of voluntary contraction strengths in humans. During voluntary contraction of biceps brachii, motor evoked potentials (MEPs) to transcranial magnetic stimulation of the motor cortex grow up to about 50% maximal force and then decrease. To determine whether the decrease reflects events at a cortical or spinal level, responses to stimulation of the cortex and corticospinal tract (cervicomedullary motor evoked potentials, CMEPs) as well as maximal M-waves (M(max)) were recorded during strong contractions at 50 to 100% maximum. In biceps and brachioradialis, MEPs and CMEPs (normalized to M(max)) evoked by strong stimuli decreased during strong elbow flexions. Responses were largest during contractions at 75% maximum and both potentials decreased by about 25% M(max) during maximal efforts (P < 0.001). Reductions were smaller with weaker stimuli, but again similar for MEPs and CMEPs. Thus the reduction in MEPs during strong voluntary contractions can be accounted for by reduced responsiveness of the motoneuron pool to stimulation. During strong contractions of the first dorsal interosseous, a muscle that increases voluntary force largely by frequency modulation, MEPs declined more than in either elbow flexor muscle (35% M(max), P < 0.001). This suggests that motoneuron firing rates are important determinants of evoked output from the motoneuron pool. However, motor cortical output does not appear to be limited at high contraction strengths.
Journal of Neurophysiology 07/2006; 95(6):3512-8. · 3.32 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: All pain is unpleasant, but different perceptual and emotional qualities are characteristic of pain originating in different structures. Pain of superficial (cutaneous) origin usually is sharp and restricted, whereas pain of deep origin (muscle/viscera) generally is dull and diffuse. Despite the differences it has been suggested previously that all pain is mediated by an invariant set ("neuromatrix") of brain structures. However, we report here, using functional magnetic resonance imaging (fMRI), that striking regional differences in brain activation patterns were the rule. Signal differences were found in regions implicated in emotion (perigenual cingulate cortex), stimulus localization and intensity (somatosensory cortex) and motor control (motor cortex, cingulate motor area). Further, most fMRI signal changes matched perceived changes in pain intensity. These findings clearly indicate that distinct neural activity patterns in distinct sets of brain structures are evoked by pain originating from different tissues of the body. Further, we suggest that these differences underlie the different perceptual and emotional reactions evoked by deep versus superficial pain.
Pain 03/2006; 120(3):286-96. · 5.78 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: To assess the contribution of central and peripheral factors to changes in maximum voluntary force and its length dependence after eccentric muscle damage, voluntary and twitch torque were measured across a wide angular range, along with voluntary activation using twitch interpolation. Isometric torque from both maximum voluntary contractions (MVCs) and paired twitches to motor nerve stimulation were measured from 60 to 150 deg elbow flexion in 10 deg increments in eight subjects. Optimal angles were determined by curve fitting. Each subject then performed eccentric contractions until voluntary torque had decreased by approximately 40%. Measurements were repeated at 2 h, 1 day and 8 days post-exercise to follow acute and longer-term changes. Before exercise, the optimal angle was in the mid-range (93+/-10 deg; mean+/-s.d.) for MVCs, and at a more extended elbow angle for the twitch (106+/-6 deg, P < 0.05). Voluntary activation was generally high (> 94%) but depended on elbow angle, with activation being approximately 4% lower at the most flexed compared to the most extended angle. Two hours after exercise, MVCs decreased 40%, while twitch torque declined 70%. All subjects showed a shift in optimal angle to longer muscle lengths for MVCs (17+/-16 deg at 2 h, 14+/-7 deg at day 1, P < 0.05). This shift contributed minimally (approximately 3%) to the reduction in torque at 90 deg, as the torque-angle relation was relatively flat around the optimum. The twitch showed a smaller shift (approximately 4 deg) to longer lengths which was not statistically significant. Voluntary activation was significantly impaired in the early stages after exercise (2 h and day 1, P < 0.05), particularly at short muscle lengths. By 8 days after exercise, the optimal angle had returned to pre-exercise values, but MVC, twitch torque and voluntary activation had not fully recovered. Eccentric exercise causes a short-term shift in the optimal angle for MVCs and produces a length-dependent impairment in voluntary activation. Therefore, it appears that both central and peripheral factors limit muscle performance following eccentric damage, with limits to voluntary drive being especially important at short lengths.
The Journal of Physiology 02/2006; 571(Pt 1):243-52. · 4.72 Impact Factor
