Changes in segmental and motor cortical output with contralateral muscle contractions and altered sensory inputs in humans

Depression of involuntary activity in muscles paralyzed by spinal cord injury

Involuntary muscle contractions are common after spinal cord injury (SCI). Increased sensitivity to Ia muscle afferent input may contribute to the development of these spasms. Since tendon vibration results in a period of postactivation depression of the Ia synapse, we sought to determine whether Achilles tendon vibration (80 HZ for 2 s) altered involuntary contractions evoked by superficial peroneal nerve (SPN) stimulation (5 pulses at 300 HZ) in paralyzed leg muscles of subjects with chronic (>1 year) SCI. Responses to SPN stimulation that were conditioned by vibration were reduced in 66% of trials (by 33+/-12% in tibialis anterior and 40+/-16% in soleus). These reductions in electromyographic activity are unlikely to be mediated by changes at the Ia synapse or motoneuron because vibration did not alter the magnitude of the soleus H reflex. The electromyographic reductions may involve long-lasting neuromodulatory effects on spinal inhibitory interneurons or synapses involved in the flexor reflex pathway. Vibration-evoked depression of electromyographic activity may be clinically useful in controlling involuntary muscle contractions after SCI.

Modulation of short-latency intracortical inhibition in human primary motor cortex during synchronised versus syncopated finger movements

Rhythmic movements are inherently more stable and easier to perform when they are synchronised with a periodic stimulus, as opposed to syncopated between the beats of a pacing stimulus. Although this behavioural phenomenon is well documented, its neurophysiological basis is poorly understood. In a first experiment, we demonstrated that all healthy subjects (N=8) performing index finger abduction in time with an auditory metronome exhibited transitions from syncopation to synchronisation when the metronome tempo was scaled from 0.8 to 2.0 Hz. Subjects' mean transition frequency was 1.7+/-0.2 Hz. In a second experiment, we used paired-pulse transcranial magnetic stimulation to examine short-latency intracortical inhibition (sICI) directed towards the first dorsal interosseous (FDI) muscle in healthy subjects (N=9) who made synchronised and syncopated phasic finger movements in time with metronome pacing of 1.0 Hz. Despite the equivalence between the patterns in terms of task performance and corticospinal excitability of FDI at this movement frequency, there was significantly greater sICI during syncopation than during synchronisation. From this result, we infer that the stability of movement patterns may be contingent upon excitability of inhibitory networks within primary motor cortex.

Effects of Swedish massage on blood pressure

Swedish massage technique includes mechanically activated muscular tissue and also skin, tendons, fascias, and connected tissue, which indirectly regulates the tonus of the autonomous nervous system. This study set out to examine the effects of Swedish massage on blood pressure. Healthy males were given massage treatment at the Karolinska Hospital, Stockholm, Sweden. Treatment was over a 12-week period divided into three parts, each consisting of 4 weeks. Two treatment periods contained massage treatment either on back, neck and chest (BNC), or leg, arm and face (LAF), with an in between washout period. The first treatment period with massage decreased systolic blood pressure directly after treatment (BNC: P<0.005, LAF: P<0.01), but no significant changes were seen in diastolic blood pressure. In the second period, BNC massage decreased systolic (P<0.005) and diastolic (P<0.005) blood pressure whereas LAF massage (P<0.05) increased systolic blood pressure. Swedish massage on the BNC resulted in a minor decrease in blood pressure possibly due to sympathetic inhibition. It may be suggested that massage may be tried as a complementary therapy in patients suffering from increased blood pressure due to stress.

Training with unilateral resistance exercise increases contralateral strength

Evidence that unilateral training increases contralateral strength is inconsistent, possibly because existing studies have design limitations such as lack of control groups, lack of randomization, and insufficient statistical power. This study sought to determine whether unilateral resistance training increases contralateral strength. Subjects ( n = 115) were randomly assigned to a control group or one of the following four training groups that performed supervised elbow flexion contractions: 1) one set at high speed, 2) one set at low speed, 3) three sets at high speed, or 4) three sets at low speed. Training was 3 times/wk for 6 wk with a six- to eight-repetition maximum load. Control subjects attended sessions but did not exercise. Elbow flexor strength was measured with a one-repetition maximum arm curl before and after training. Training with one set at slow speed did not produce an increase in contralateral strength (mean effect of −1% or −0.07 kg; 95% confidence interval: −0.42–0.28 kg; P = 0.68). However, three sets increased strength of the untrained arm by a mean of 7% of initial strength (additional mean effect of 0.41 kg; 95% confidence interval: 0.06–0.75 kg; P = 0.022). There was a tendency for training with fast contractions to produce a greater increase in contralateral strength than slow training (additional mean effect of 5% or 0.31 kg; 95% confidence interval: −0.03–0.66 kg; P = 0.08), but there was no interaction between the number of sets and training speed. We conclude that three sets of unilateral resistance exercise produce small contralateral increases in strength.

Neural pathways mediating bilateral interactions between the upper limbs

The ease with which we perform tasks such as opening the lid of a jar, in which the two hands execute quite different actions, belies the fact that there is a strong tendency for the movements of the upper limbs to be drawn systematically towards one another. Mirror movements, involuntary contractions during intended unilateral engagement of the opposite limb, are considered pathological, as they occur in association with specific disorders of the CNS. Yet they are also observed frequently in normally developing children, and motor irradiation, an increase in the excitability of the (opposite) homologous motor pathways when unimanual movements are performed, is a robust feature of the mature motor system. The systematic nature of the interactions that occur between the upper limbs has also given rise to the expectation that functional improvements in the control of a paretic limb may occur when movements are performed in a bimanual context. In spite of the ubiquitous nature of these phenomena, there is remarkably little consensus concerning the neural basis of their mediation. In the present review, consideration is given to the putative roles of uncrossed corticofugal fibers, branched bilateral corticomotoroneuronal projections, and segmental networks. The potential for bilateral interactions to occur in various brain regions including the primary motor cortex, the supplementary motor area, non-primary motor areas, the basal ganglia, and the cerebellum is also explored. This information may provide principled bases upon which to evaluate and develop task and deficit-specific programs of movement rehabilitation and therapy.

Cross-Education of Arm Muscular Strength Is Unidirectional in Right-Handed Individuals

PURPOSE

Cross-education of strength is a neural adaptation defined as the increase in strength of the untrained contralateral limb after unilateral training. The purpose was to determine the effect of the direction of transfer on cross-education in right-handed individuals.

METHODS

Thirty-nine strongly right-handed females were randomized into a left-hand training (LEFT), right-hand training (RIGHT), or nontraining control (CON) group. Strength training was 6 wk of maximal isometric ulnar deviation, 4x wk(-1). Peak torque, muscle thickness (ultrasound), and EMG activity were assessed before and after training in both limbs.

RESULTS

The change in strength in the untrained limb was greatest in the RIGHT group (39.2%; P < 0.01), whereas no significant changes in strength were observed for the untrained limb of the LEFT group (9.3%) or for either of the CON group limbs (10.4 and 12.2%). Strength training also increased trained limb strength in the LEFT (41.9%, P < 0.01) and the RIGHT (25.9%; P < 0.01) groups. Training groups increased trained limb muscle thickness (RIGHT and LEFT combined: 4.1%) compared to CON (-4.0%) (P < 0.01). There were no changes in muscle thickness of untrained limbs compared to CON. Trained limb agonist EMG activation increased with training (P < 0.05) with no change for the antagonist. Changes in untrained limb EMG were not different compared to CON.

CONCLUSIONS

Cross-education with hand strength training occurs only in the right-to-left direction of transfer in right-handed individuals. We conclude that cross-education of arm muscular strength is most pronounced to the nondominant arm.

Stimulation at the cervicomedullary junction in human subjects

In awake human subjects, corticospinal axons can be activated at the level of the cervicomedullary junction by electrical or magnetic stimulation. Such stimuli evoke single descending volleys which activate motoneurones and elicit responses in muscles of the upper limb. These responses (cervicomedullary motor evoked potentials, CMEPs) have a large monosynaptic component and can be used to test motoneurone excitability in a variety of tasks. CMEPs can be elicited in resting muscle and during all strengths of voluntary contraction. Examination of CMEPs during and after voluntary contractions reveals changes in motoneurone excitability but also suggests activity-dependent changes in the efficacy of the corticospinal pathway. Because they test the same subcortical pathway as transcranial magnetic stimulation, but are unaffected by altered excitability at a cortical level, CMEPs often offer the most appropriate comparison to allow interpretation of changes in motor evoked potentials. The advantages and disadvantages of stimulation at the cervicomedullary junction as a test of motoneurone excitability are reviewed.

Mirrored EMG activity during unimanual rhythmic movements

We studied instances of mirror movements--in the form of coherent EMG activity of the muscles in the arm not intended to move--during the performance of a unimanual rhythmic task in healthy adults. Epochs of involuntary muscle activity were detected and analyzed using time-resolved spectral methods. The observed frequency and phase locking between EMG patterns derived from homologous extensor muscles indicated the presence of neural cross-talk, which is relevant to the study of interlimb coordination.

Finger extensor variability in TMS parameters among chronic stroke patients

BACKGROUND

This study determined the reliability of topographic motor cortical maps and MEP characteristics in the extensor digitorum communis (EDC) evoked by single-pulse TMS among patients with chronic stroke.

METHODS

Each of ten patients was studied on three occasions. Measures included location of the EDC hotspot and center of gravity (COG), threshold of activation and average amplitude of the hotspot, number of active sites, map volume, and recruitment curve (RC) slope.

RESULTS

Consistent intrahemispheric measurements were obtained for the three TMS mapping sessions for all measured variables. No statistically significant difference was observed between hemispheres for the number of active sites, COG distance or the RC slope. The magnitude and range of COG movement between sessions were similar to those reported previously with this muscle in able-bodied individuals. The average COG movement over three sessions in both hemispheres was 0.90 cm. The average COG movement in the affected hemisphere was 1.13 (+/- 0.08) cm, and 0.68 (+/- 0.04) cm) for the less affected hemisphere. However, significant interhemispheric variability was seen for the average MEP amplitude, normalized map volume, and resting motor threshold.

CONCLUSION

The physiologic variability in some TMS measurements of EDC suggest that interpretation of TMS mapping data derived from hemiparetic patients in the chronic stage following stroke should be undertaken cautiously. Irrespective of the muscle, potential causes of variability should be resolved to accurately assess the impact of pharmacological or physical interventions on cortical organization as measured by TMS among patients with stroke.

Excitability changes in human forearm corticospinal projections and spinal reflex pathways during rhythmic voluntary movement of the opposite limb

Rhythmic movements brought about by the contraction of muscles on one side of the body give rise to phase-locked changes in the excitability of the homologous motor pathways of the opposite limb. Such crossed facilitation should favour patterns of bimanual coordination in which homologous muscles are engaged simultaneously, and disrupt those in which the muscles are activated in an alternating fashion. In order to examine these issues, we obtained responses to transcranial magnetic stimulation (TMS), to stimulation of the cervicomedullary junction (cervicomedullary-evoked potentials, CMEPs), to peripheral nerve stimulation (H-reflexes and f-waves), and elicited stretch reflexes in the relaxed right flexor carpi radialis (FCR) muscle during rhythmic (2 Hz) flexion and extension movements of the opposite (left) wrist. The potentials evoked by TMS in right FCR were potentiated during the phases of movement in which the left FCR was most strongly engaged. In contrast, CMEPs were unaffected by the movements of the opposite limb. These results suggest that there was systematic variation of the excitability of the motor cortex ipsilateral to the moving limb. H-reflexes and stretch reflexes recorded in right FCR were modulated in phase with the activation of left FCR. As the f-waves did not vary in corresponding fashion, it appears that the phasic modulation of the H-reflex was mediated by presynaptic inhibition of Ia afferents. The observation that both H-reflexes and f-waves were depressed markedly during movements of the opposite indicates that there may also have been postsynaptic inhibition or disfacilitation of the largest motor units. Our findings indicate that the patterned modulation of excitability in motor pathways that occurs during rhythmic movements of the opposite limb is mediated primarily by interhemispheric interactions between cortical motor areas.

Noninvasive stimulation of the human corticospinal tract

Spinal tracts can be stimulated noninvasively in human subjects by passing a high-voltage stimulus between the mastoids or by magnetic stimulation over the back of the head. The stimulus probably activates the corticospinal tract at the cervicomedullary junction (pyramidal decussation) and evokes large, short-latency motor responses in the arm muscles. These responses have a large monosynaptic component. Responses in leg muscles can be elicited by cervicomedullary junction stimulation or by stimulation over the cervical or thoracic spine. Because nerve roots are more easily activated than spinal tracts, stimulus spread to motor axons can occur. Facilitation of responses by voluntary activity confirms that the responses are evoked synaptically. Stimulation of the corticospinal tract is useful in studies of central conduction and studies of the behavior of motoneurons during different tasks. It also provides an important comparison to allow interpretation of changes in responses to stimulation of the motor cortex. The major drawback to the use of electrical stimulation of the corticospinal tract is that each stimulus is transiently painful.