Left Abductor Pollicis Brevis Research Articles

Cortisol Inhibits Neuroplasticity Induction in Human Motor Cortex

We investigated whether plasticity of human motor cortex (M1) is influenced by time of day, and whether changes in circulating levels of cortisol contribute to this effect. Neuroplasticity was induced using paired associative stimulation (PAS), involving electrical stimulation of left median nerve, paired with transcranial magnetic stimulation over the right M1 25 ms later (90 pairs at 0.05 Hz). Surface EMG was recorded from the left abductor pollicis brevis (APB) and first dorsal interosseous muscle. Cortisol levels were assessed from saliva. Time-of-day modulation of PAS effectiveness was assessed in 25 subjects who were tested twice, at 8:00 A.M. and 8:00 P.M. on separate days. In a second double-blind study, 17 subjects were tested with PAS at 8:00 P.M. on two occasions after administration of oral hydrocortisone (24 mg) or placebo. The motor-evoked potential (MEP) in resting APB increased significantly after PAS in the evening (when endogenous cortisol levels were low), but not in the morning. Oral hydrocortisone prevented facilitation of the APB MEP after PAS, and in the drug study, mean salivary cortisol levels were negatively associated with PAS effectiveness. The GABA(B)-mediated cortical silent period for APB was longer in the morning than in the evening, and was lengthened by PAS and oral hydrocortisone. We conclude that neuroplasticity in human M1 and GABA(B)-dependent intracortical inhibitory systems are influenced by time of day and modified by circulating levels of cortisol.

Factors influencing the magnitude and reproducibility of corticomotor excitability changes induced by paired associative stimulation

Several paired-associative stimulation (PAS) protocols induce neuroplastic changes in human motor cortex (M1). To understand better the inherent variability of responses to PAS, we investigated the effectiveness and reproducibility of two PAS paradigms, and neurophysiological and experimental variables that may influence this. Motor evoked potentials (MEPs) were elicited by transcranial magnetic stimulation (TMS) of right M1, and recorded from surface EMG of left abductor pollicis brevis (APB) and first dorsal interosseous before and after PAS. PAS consisted of electrical stimulation of left median nerve paired with TMS over right M1 25 ms later. Twenty subjects were given one of two PAS protocols: short (132 paired stimuli at 0.2 Hz) or long (90 paired stimuli at 0.05 Hz), and were re-tested with the same protocol on 3 separate occasions, with 11 subjects tested in the morning and 9 in the afternoon. Neurophysiological variables assessed included MEP amplitude, resting and active motor threshold, short-interval intracortical inhibition, intracortical facilitation and cortical silent period duration. The short PAS protocol produced greater APB MEP facilitation (51%) than the long protocol (11%), and this did not differ between sessions. The neurophysiological variables did not consistently predict responses to PAS. Both PAS protocols induced more APB MEP facilitation, and greater reproducibility between sessions, in experiments conducted in the afternoon. The mechanism for this is unclear, but circadian rhythms in hormones and neuromodulators known to influence neuroplasticity warrant investigation. Future studies involving PAS should be conducted at a fixed time of day, preferably in the afternoon, to maximise neuroplasticity and reduce variability.

Nitrous oxide can enhance the hypnotic effect, but not the suppression of spinal motor neuron excitability by propofol in humans

We investigated whether nitrous oxide can enhance the suppressive effect of propofol on spinal motor neuron excitability in humans. Sixteen adult patients were prospectively randomly assigned to be given either propofol alone (group P; n = 8) or a supplement of 66% nitrous oxide with propofol (group PN; n = 8) for intraoperative sedation. Propofol was administered by a target-controlled infusion system to maintain sequentially increasing plasma propofol concentrations (Cpt) of 0.5, 0.8, 1.0, 1.3, 1.5 and 1.8 microg x ml(-1) in all patients. Assessment of the patient's level of sedation in both groups was performed with the Wilson Sedation Scale (WSS). F-wave analysis on the left abductor pollicis brevis muscle was carried out for the assessment of spinal motor neuron excitability at each plasma propofol concentration. Significant differences in the WSS scores between group P and group PN were observed at 0.8, 1.0, 1.3, and 1.5 microg x ml(-1) of Cpt (group P < group PN; P < 0.01). Cpt greater than 1.0 microg x ml(-1) significantly reduced F-wave persistence in a concentration-dependent manner, and the ICpt 50 and ICpt 95 values for plasma propofol concentration (plasma propofol concentrations that produced 50% and 95% inhibition of the baseline, respectively) were 1.05 and 1.95 microg x ml(-1) in group P, and 1.07 and 2.14 microg x ml(-1) in group PN, respectively. These results suggest that nitrous oxide can enhance the hypnotic effect, but not the suppression of spinal motoneuron excitability by propofol in humans at clinical levels of Cpt.

Altered plasticity of the human motor cortex in Parkinson's disease

Interventional paired associative stimulation (IPAS) to the contralateral peripheral nerve and cerebral cortex can enhance the primary motor cortex (M1) excitability with two synchronously arriving inputs. This study investigated whether dopamine contributed to the associative long-term potentiation-like effect in the M1 in Parkinson's disease (PD) patients. Eighteen right-handed PD patients and 11 right-handed age-matched healthy volunteers were studied. All patients were studied after 12 hours off medication with levodopa replacement (PD-off). Ten patients were also evaluated after medication (PD-on). The IPAS comprised a single electric stimulus to the right median nerve at the wrist and subsequent transcranial magnetic stimulation of the left M1 with an interstimulus interval of 25 milliseconds (240 paired stimuli every 5 seconds for 20 minutes). The motor-evoked potential amplitude in the right abductor pollicis brevis muscle was increased by IPAS in healthy volunteers, but not in PD patients. IPAS did not affect the motor-evoked potential amplitude in the left abductor pollicis brevis. The ratio of the motor-evoked potential amplitude before and after IPAS in PD-off patients increased after dopamine replacement. Thus, dopamine might modulate cortical plasticity in the human M1, which could be related to higher order motor control, including motor learning.

Efficacy of a half dose of oral pyridostigmine in the treatment of chronic fatigue syndrome: three case reports

Chronic fatigue syndrome (CFS) is characterized by persistent mental and physical fatigue for at least 6 months. Its pathophysiology is unknown and there is no proven effective treatment. We describe three cases who fulfill the criteria of CFS, in whom a defect of neuromuscular transmission and dysautonomia are present and who respond to acetylcholine-esterase inhibition. Case 1: 18-year-old female with a 3-year history of CFS. Response of compound-muscle-action potential, recorded using surface recording electrode, over left abductor pollicis brevis muscle, to repetitive nerve stimulation (RNS) at a rate of 10 Hz showed a 42% incremental response. Composite autonomic scoring system (CASS) showed mild cholinergic impairment (cardiovagal score: 1; sudomotor score: 2). Serological tests for Epstein-Barr virus (EBV) revealed positive antiviral capsid antigens (anti-VCA) immunoglobulins G (IgG). Oral pyridostigmine therapy (30 mg) resulted in marked improvement in symptoms. Case 2: 28-year-old female with 10-year history of CFS. RNS, using identical protocol, showed a 60% incremental response over the same muscle. CASS showed mild cholinergic impairment (cardiovagal score: 1; sudomotor score: 2) and this patient was also positive for EBV. This patient responded dramatically to 10-mg pyridostigmine. Case 3: 29-year-old female with a history of CFS for longer than 15 years. Repetitive stimulation, using identical paradigm to left abductor pollicis brevis muscle, showed a 42% incremental response. CASS showed mildly cholinergic impairment (cardiovagal score: 2; sudomotor score: 1). EBV antibody titers were positive. Patient responded to 30-mg pyridostigmine with an improvement in her fatigue. These three cases generate the hypothesis that the fatigue in some patients with clinical CFS might be due to a combination of mild neuromuscular transmission defect combined with cholinergic dysautonomia. Support for this thesis derives from the improvement with cholinesterase inhibition.

Elevated 14-3-3 protein and axonal loss in immunoglobulin-responsive, idiopathic acute transverse myelitis

To report the elevation of the 14-3-3 protein and the complete denervation of hand muscles in idiopathic acute transverse myelitis (IATM) of the cervical cord. In a 29-year-old woman with a 2-week history of neck pain and repeated attenuated flus, subacute quadriplegia, hypaesthesia of both arms, a T3 sensory level, and urinary dysfunction occurred. Based upon the clinical findings, the cervical MRIs, and an elevated 14-3-3 protein in the CSF, IATM C4-C7 was diagnosed. Ten, 17, 28 and 61 days after onset, nerve conduction studies revealed complete denervation of the right abductor pollicis brevis and abductor digiti minimi muscles but gradual improvement of the compound muscle action potential of the left abductor pollicis brevis muscle. F-waves of the right median nerve were absent. Tibial somatosensory evoked potentials showed a prolonged central conduction time. Transcranial magnetic stimulation evoked a response in the left but not the right abductor digiti minimi muscle. IATM may cause elevation of the 14-3-3 protein and loss of motor axons originating from affected anterior horn cells.

Event-Related Functional MRI Study on Central Representation of Acute Muscle Pain Induced by Electrical Stimulation

Although pathological muscle pain involves a significantly larger population than any other pain condition, the central mechanisms are less explored than those of cutaneous pain. The aims of the study were to establish the pain matrix for muscle pain in the full head volume and, further, to explore the possibility of a functional segregation to nonpainful and painful stimuli within the area of the parasylvian cortex corresponding to the secondary somatosensory area. Additionally, we speculate that a randomization of nonpainful and painful stimuli may target specific structures related to stimulus salience. We used event-related functional magnetic resonance imaging (MRI) and the high sensitivity of the 3-T MRI scanner to study the central processing of acute muscle pain induced by intramuscular electrostimulation. Brief nonpainful and painful stimuli (1-ms duration, interstimulus interval = 12 s) were randomly applied to the left abductor pollicis brevis of 10 subjects. The data disclose a pain matrix for muscle pain similar to that for cutaneous pain. Individual analysis suggests separate representations within the area bounded by the upper bank of the Sylvian fissure (SF) and the circular sulcus of insula (CSI). Nonpainful stimulation activated the superficial parietal operculum adjoining the SF, while the painful condition additionally targeted the deeper parietal operculum bordering the CSI. Randomization of stimuli of different intensities likely introduces cognitive components that engage neural substrates servicing the appreciation of stimulus salience in the context of affect-laden pain imposition.

Changes in motor cortex excitability during ipsilateral hand muscle activation in humans

To test whether unilateral hand muscle activation involves changes in ipsilateral primary motor cortex (M1) excitability. Single- and paired-pulse transcranial magnetic stimulation (TMS) of the right hemisphere was used to evoke motor evoked potentials (MEPs) from the resting left abductor pollicis brevis (APB) in 9 normal volunteers. We monitored changes in motor threshold (MT), MEP recruitment, intracortical inhibition (ICI) and intracortical facilitation (ICF) while the ipsilateral right APB was either at rest or voluntarily activated. Spinal motoneuron excitability was assessed using F-wave recording procedures. Voluntary muscle activation of the ipsilateral APB significantly facilitated the MEPs and F-waves recorded from the contralateral APB. Facilitation was observed with muscle activation >50% of the maximum voluntary force and with stimulus intensities >20% above the individual resting motor threshold. Intracortical inhibition significantly decreased in the ipsilateral M , while there was no significant change in intracortical facilitation during this maneuver. Unilateral hand muscle activation changes the excitability of homotopic hand muscle representations in both the ipsilateral M1 and the contralateral spinal cord. While the large proportion of MEP facilitation most likely occurred at a spinal level, involvement of the ipsilateral hemisphere may have contributed to the enlargement of magnetic responses.

Hand preference and transcranial magnetic stimulation asymmetry of cortical motor representation

Human handedness may be associated with asymmetry in the corticospinal motor system. Previous studies measuring the threshold for eliciting motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) have provided evidence consistent with this hypothesis. However, TMS asymmetry observed in previous studies may have reflected cortical or spinal differences. We therefore undertook this investigation to test the hypothesis that handedness is associated with asymmetry in cortical motor representations. We used TMS to map contralateral cortical motor representations of the right and left abductor pollicis brevis (APB) and flexor carpi radialis (FCR) muscles in nine normal subjects (three left-handed). Using focal stimulation with a figure-of-8 shaped magnetic coil, we found no differences in MEP threshold or MEP size between the preferred and the nonpreferred hand. However, we observed that the number of scalp stimulation sites eliciting MEPs was statistically greater for APB and FCR muscles of the preferred limb. We found significant asymmetry between right-handed and left-handed subjects, such that in right-handers, the representation of the right APB was larger than that of the left APB, but in left-handers the representation of right APB was smaller than that of the left APB. These results suggest that handedness is associated with asymmetry in cortical motor representation.

Age-dependent changes in physiological threshold asymmetries for the motor evoked potential and silent period following transcranial magnetic stimulation

To study the effect of age on the physiological threshold asymmetries for the motor evoked potential (MEP) and silent period (SP) following transcranial magnetic stimulation. We studied 63 right-handed subjects and 13 young left-handed subjects (19-39 years). The right-handers were classified into three age groups; 22 young (20-38 years), 20 middle-aged (40-58 years) and 21 old (61-82 years) subjects. We measured the MEP thresholds at rest and during voluntary contraction (VC), and the SP thresholds from the right and left abductor pollicis brevis (APB) muscles. We also measured the side to side differences of the F wave persistency and the F wave/M wave amplitude ratio from the same muscles. Among young subjects, all of the MEP and SP thresholds for the right APB were significantly lower than those for the left APB in the right-handers, and the reverse was true in the left-handers. The results in the middle-aged right-handers were similar to those in the young right-handers, but in the old right-handers, none of the thresholds were different between the two sides. We did not find any asymmetries of the F wave in the subjects of any age group. We speculate that the age-dependent threshold asymmetries are preferentially related to functional asymmetries at the cortical level.

Physiological motor asymmetry in human handedness: evidence from transcranial magnetic stimulation

We hypothesized that human handedness might be associated with measurable differences in the excitability of the motor system. We compared the thresholds for electromyographic activation of the left and right abductor pollicis brevis (APB) and biceps muscles in 30 left-handers and 30 right-handers, by varying the direction of a brief monophasic pulse in a circular electromagnetic coil centered over the vertex of the scalp. In right-handers, we found that the threshold for activation of muscles in the right arm was lower than the threshold for activation of corresponding muscles in the left arm. In left-handers, the reverse was true. Threshold asymmetry was influenced significantly by the consistency with which each subject used the writing hand to perform other motor tasks, and was not significant between non-consistent left-handers and right-handers. Our results indicate that human handedness, and in particular, consistency of hand preference, are associated with lateralized differences in the excitability of motor system projections activated by transcranial magnetic stimulation. Our findings might reflect physiological differences in corticospinal tract function or cortical motor representation.

Optimal Focal Transcranial Magnetic Activation of the Human Motor Cortex

We studied the effects of coil orientation, stimulus intensity, and shape of the induced current pulse on the amplitudes of motor evoked potentials in the left abductor pollicis brevis of 10 normal adults who had transcranial magnetic stimulation. The optimal direction of currents induced in the brain is approximately perpendicular to the central sulcus, flowing diagonally from back to front. The most effective coil orientation depends on the shape of the induced current pulse and, when the first and second phases of the pulse are of similar size, also on the intensity of stimulation. Optimal mapping of the human motor cortex with magnetic stimulation requires knowledge of the influences of all these factors.

A noninvasive technique to assess completeness of spinal cord lesions in humans

The effect of scalp stimulation delivered through electrodes overlying the motor cortex was evaluated in five healthy subjects and six patients with traumatic spinal cord injury. The latency to the onset of the electromyographic response was measured in the biceps brachii and abductor pollicis brevis muscles. In all the patients, latencies to the muscle (biceps brachii) whose innervation originated above the lesion were in the normal range; whereas, latencies to the muscle (abductor pollicis brevis) whose innervation originated below the lesion were prolonged. Electromyographic signals were recorded in muscles which showed no voluntary motor activity. No lateral differences in latencies were found in healthy subjects; however, in the patients, significant differences were obtained between the right and left abductor pollicis brevis muscles. The results of this study demonstrate that the spinal cord of patients with a lesion deemed to be clinically complete, contains nerve fibers which descend through the lesion and are capable of conveying impulses leading to muscle contraction.