Memory formation in the motor cortex ipsilateral to a training hand.

Cortical reorganization within the primary motor cortex (M1) contralateral to a practicing hand has been extensively investigated. The extent to which the ipsilateral M1 participates in these plastic changes is not known. Here, we evaluated the influence of unilateral hand practice on the organization of the M1 ipsilateral and contralateral to the practicing hand in healthy human subjects. Index finger movements elicited by single-pulse transcranial magnetic stimulation (TMS) delivered to each M1 were evaluated before and after practice of unilateral voluntary index finger abduction motions. Practice increased the proportion and acceleration of TMS-evoked movements in the trained direction and the amplitude of motor-evoked potentials (MEPs) in the abduction agonist first dorsal interosseous (FDI) muscle in the practicing hand and decreased the proportion and acceleration of TMS-evoked abduction movements and MEP amplitudes in the abduction agonist FDI in the opposite resting hand. Our findings indicate that unilateral hand practice specifically weakened the representation of the practiced movement in the ipsilateral M1 to an extent proportional to the strengthening effect in the contralateral M1, a result that varied with the practicing hand's position. These results suggest a more prominent involvement of interacting bilateral motor networks in motor memory formation and probably acquisition of unimanual motor skills than previously thought.

Changes in the recruitment curve of the soleus H-reflex associated with chronic low back pain.

OBJECTIVE: We investigated whether patients with chronic low back pain (CLBP) manifest changes in the excitability of the soleus H-reflex. METHODS: H-reflex stimulus-response curve was studied in 14 CLBP patients and 14 age-matched healthy subjects. H-threshold, H-maximum size, H-steepness and H-latency were determined for both legs. Homosynaptic depression (HD), following a train of H-reflexes, and presynaptic inhibition (PI) from flexor afferents onto soleus Ia afferents were also evaluated. RESULTS: H-threshold was significantly increased, H-size as a function of stimulus intensity was significantly different, and H-recruitment curve steepness was significantly lower in CLBP patients compared to healthy subjects. No significant difference in the amount of HD and PI of the H-reflex was found between the two groups. H-latency and Hmax/Mmax ratio was comparable between the subjects groups. CONCLUSIONS: In CLBP there is a reduced excitability of group Ia afferent fibres from the soleus muscle to which presynaptic factors do not seem to contribute and that presumably depend on changes in the peripheral sensory input. SIGNIFICANCE: Changes in H-reflex excitability may underlie a decrease in the gain of a peripheral sensor in CLBP. Estimation of soleus H-threshold and H-recruitment curve may contribute to the diagnostic evaluation of CLBP and may be used to monitor the efficacy of treatment.

Motor unit synchronous firing as revealed by determinism of surface myoelectric signal.

Information on motor strategies can be extracted from the surface electromyogram (EMG) by non-linear methods. The percentage of determinism (%DET) obtained from recurrence quantification analysis (RQA) may be a sensitive variable to detect synchronous motor unit behaviour. The purpose of the present study was to validate this methodology by comparing it with an established technique estimating the degree of synchronization of pairs of voluntary activated motor units from the correlation of their firing in the time-domain. Single motor unit activity was recorded in extensor carpi radialis (ECR) muscle by pairs of tungsten microelectrodes inserted into the muscle belly. Cross-correlation analysis was performed in order to determine synchronization peak area by computing synchronous impulse probability. Surface EMG activity was recorded in parallel by electrodes placed over the skin of the same muscle and %DET was used as a measure of synchronous activity. The %DET appeared to be a valid measure of synchronization yielding results comparable to those obtained with cross-correlation analysis. Increases in %DET (t = 64.59, P < 0.0001) highly correlated (r2 = 0.70, P = 0.0013) with pharmacologically induced increases in the synchronization activity of pairs of ECR motor units (t = 8.71, P < 0.0001). RQA may be used as an alternative methodology for testing synchronous motor unit behaviour from surface EMG under physiological and pathological conditions.

Changes in coordination of postural control during dynamic stance in chronic low back pain patients.

The human postural system operates on the basis of integrated information from three independent sources: vestibular, visual and somatosensory. It is conceivable that a derangement of any of these systems will influence the overall output of the postural system. The peripheral proprioceptive system or the central processing of proprioceptive information may be altered in chronic low back pain (CLBP). We therefore investigated whether patients with CLBP exhibited an altered postural control during quiet standing. Dynamic posturography was performed by 12 CLBP patients and 12 age-matched controls. Subject's task was to stand quietly on a computer-controlled movable platform under six sensory conditions that altered the available visual and proprioceptive information. While the control of balance was comparable between the two groups across stabilized support surface conditions (1-3), CLBP patients oscillated much more than controls in the anterior-posterior (AP) direction in platform sway-referenced conditions (4-6). Control experiments ruled out that increased sway was due to pain interference. In CLBP patients, postural stability under challenging conditions is maintained by an increased sway in AP direction. This change in postural strategy may underlie a dysfunction of the peripheral proprioceptive system or the central integration of proprioceptive information.

Cortical projection of putative group Ib afferent fibres from the human forearm.

The possibility was investigated that impulses in group Ib afferents from fore-arm flexors have access to cerebral cortex in man. A long-lasting increase in the threshold (Th) of group Ia afferent fibres from flexor carpi radialis muscle (Fcr) was obtained after prolonged (100 Hz for 20 min) tendon vibration at the wrist. It was assumed that under this condition a weak electrical stimulation of the median nerve at the elbow, insufficient to reactivate Ia fibres because of the rise in their threshold (as verified by the method of homonymous Ia facilitation of Fcr H reflex), engaged Ib fibres only. Peripheral volleys at Erb's point and cerebral cortical potentials to median nerve stimulation at the elbow were evoked before and after prolonged vibration of Fcr. During 10-30 min after the end of vibration, in which homonymous facilitation of Fcr H reflex was abolished, both the ongoing peripheral volley and the cortical responses were markedly reduced with respect to their control values. Recovery to pre-vibration control amplitudes coincided with recovery of Fcr H reflex homonymous facilitation. In order to verify if activity in afferents other than group Ib fibres might contribute to the cortical response after vibration, specially designed experiments were also performed. It is concluded that the cortical wave recorded after 20 min vibration represents the arrival of Ib impulses from flexor carpi radialis to the human cerebral cortex.

Effect of chemically activated fine muscle afferents on spinal recurrent inhibition in humans.

OBJECTIVE: To test the hypothesis that 'metabolites released during fatiguing muscle contractions excite group III-IV muscle nociceptive afferents, inhibiting homonymous motoneurones via Renshaw cells,' by recording changes in recurrent inhibition of soleus motoneurones when high-threshold, small-diameter afferents (group III-IV fibres) from the same muscle were tonically activated. METHODS: Experiments were performed in 7 healthy subjects at rest and during weak isometric voluntary contraction of the soleus muscle. Muscle nociceptive afferents were activated by local standardized injection of levo-ascorbic acid. Renshaw cells were orthodromically activated by a conditioning H reflex and the resulting recurrent inhibition of the soleus motoneurones was assessed by a subsequent test H reflex. An additional H reflex of the same size as the test reflex was used to assess motoneurone excitability. RESULTS: At rest, muscle nociceptive stimulation produced transient facilitation of both test H and reference H reflexes. Under weak voluntary contraction, muscle nociceptive stimulation produced long-lasting extra-inhibition and extra-facilitation of the test reflex and reference reflex respectively, the time course of which closely resembled that of the subjective muscle pain curve. CONCLUSIONS: Discharge of putative group III-IV muscle afferents facilitated homonymous recurrent inhibition. The filtering property of recurrent inhibition may contribute to limit motoneurone activity during muscle pain and/or adapt motoneurone firing rate to the modified contractile properties of motor units as muscle fatigue developed.

Further evidence for Renshaw inhibition in man. A combined electrophysiological and pharmacological approach.

Experiments involving the acute administration of a central cholinergic substance (L-acetylcarnitine) were performed on 6 healthy subjects to obtain additional evidence that the electrophysiological method developed by Bussel and Pierrot-Deseilligny in 1977 does actually assess recurrent inhibition in man. In all the subjects, the drug further decreased the amplitude of a test H reflex (H') following a conditioning H reflex (H1). The amount of this supplementary inhibition was found to be related to the size of H1 reflex. Experimental evidence is also presented that H' reflex supplementary depression is not contaminated by homonymous Ib effects. It is concluded that the method effectively tests the excitability of Renshaw cells in man.

Changes in Ia reciprocal inhibition from the peroneal nerve to the soleus alpha-motoneurons with different static body positions in man.

Experiments were conducted in man to evaluate the changes in Ia inhibitory interneurons activated from the anterior tibial muscle and projecting to the soleus alpha-motoneurons in relation to different static body positions. Subjects were fixed to a tilting chair and the effects of body rotation were evaluated at 80 degrees (normal sitting position) and 40 degrees of backward inclination (head supine, nose-up). A test H-reflex was used to assess changes in excitability of the soleus alpha-motoneurons after a conditioning stimulus applied to the deep peroneal nerve. In 5 out of 6 subjects, we observed a significant increase in the reciprocal inhibition after backward inclination of the body (40 degrees) with respect to the control position (80 degrees). Such increase was attributed to facilitation of the Ia inhibitory interneurons projecting to the soleus motoneurons. We consider the possibility that the observed increment in reciprocal inhibition after backward inclination be sustained by variations of tonic vestibular activity.

Group Ia non-reciprocal inhibition from wrist extensor to flexor motoneurones in humans.

Interneurones mediating disynaptic inhibition from extensor to flexor carpi radialis muscles were characterized by pharmacological stimulation of Renshaw cells. It is, indeed, known that only Ia interneurones are blocked by recurrent inhibition. Renshaw cell potentiation, induced by intravenous administration of 2 g levo-acetylcarnitine, blocked Ia reciprocal inhibition from triceps to biceps muscles but not disynaptic inhibition from extensor to flexor carpi radialis muscles. It is concluded that the interneurones mediating this latter inhibition are not Ia interneurones. This kind of inhibition could be an example of a Ia non-reciprocal inhibitory pathway.

A method for potentiating Renshaw cell activity in humans.

Owing to the introduction of a special electrophysiological method [3] it has been possible to study spinal recurrent inhibition in humans. The method, however, is indirect and, being based on an H reflex technique, can only be tested in motor nuclei from which a large monosynaptic response can be obtained. We have developed a complementary method by which pharmacological stimulation of Renshaw cells is obtained [6]. It exploits the central cholinergic properties of L-acetylcarnitine [18], a substance which most likely acts potentiating the synaptic drive of the motoneurone collaterals [7], known to be the main source of excitation of Renshaw cells [15,20]. The use of L-acetylcarnitine has allowed to establish the validity of the original methodology [6] and to confirm the presence of recurrent inhibition, tested either by the H reflex technique [11-13] or, if not possible, by the PSTH technique and/or rectified averaged EMG analysis [1,4], in many limb motor nuclei. Thus, it can be expected that L-acetylcarnitine may be used as an independent means for identifying changes in motoneuronal activity related or attributed to the influence of Renshaw cells.

Aspecific inflammatory lesion (histiocytosis?) simulating intramedullary astrocytoma. Case report.

We report an exceptional case of aspecific inflammatory lesion of the thoracic spinal cord simulating an intramedullary glial tumor. Patient history was characterized by progressive spastic paraparesis with urinary incontinence; MR imaging (T4-T5) showed an enhancing intrinsic mass lesion. Myelotomy enabled partial resection of grayish astrocytoma-like tissue. Only light microscope examination was possible and disclosed aspecific inflammatory tissue composed of eosinophils, lymphocytes and histiocytes. After the operation, the patient improved promptly and no further therapy was administered. Control MRI after four months, one year and two years showed complete disappearance of the intramedullary mass lesion and the patient remained clinically stable with no other signs of disease. Since electron microscopic as well as immunohistochemical studies were not available, a definitive histological diagnosis was not possible. However on the basis of some clinical similarities with cases of isolated histiocytosis X of the CNS reported in the literature, we suggest that a diagnosis of isolated intramedullary eosinophilic granuloma could be reasonable.

Recruitment curve of the soleus H-reflex in chronic back pain and lumbosacral radiculopathy.

BACKGROUND: Needle EMG may be negative in mild or predominantly sensory lumbosacral radiculopathies. In such cases, an increase in the latency of the soleus H-reflex is a useful diagnostic criterion for establishing sensory fiber compromise at the S1 root level. However, if clinical signs of radicular involvement are lacking, the latency of the H-reflex is normal. We therefore studied the recruitment curve of the soleus H-reflex to investigate whether a change in the electrical threshold for eliciting the H-reflex might be a more sensitive criterion for detecting subclinical S1 root dysfunction. METHODS: Clinical and electrophysiological findings from 26 patients with chronic back pain and radiculopathy were compared with data obtained from 40 healthy subjects. RESULTS: An increase in the mean H-reflex threshold was the only abnormal electrophysiological finding in patients with no clinical sign of root injury (58%). A decrease in the mean H-reflex amplitude and a prolongation of H-reflex latency was observed in patients with radicular signs (42%). In both patients groups, F-wave and needle EMG studies were normal. No radiological evidence of S1 root compression was found. CONCLUSIONS: The study of the recruitment curve of the soleus H-reflex may be usefully associated to F-wave and needle EMG studies to detect possible S1 root dysfunction in mild lumbosacral radiculopathies. An increase in H-threshold may be the earliest abnormality in the absence of focal neurological signs.

Motoneurone recurrent inhibition is enhanced by L-acetylcarnitine in humans.

Renshaw cells, which mediate the recurrent inhibition of spinal a-motoneurones, are activated by acetylcholine, both through motoneurone collaterals and the reticulo-spinal system. Since it is known that L-acetylcarnitine (L-AC) has central cholinergic effects, we tested in ten normal subjects and three spastic patients the ability of L-AC to induce changes in excitability of the Renshaw cells. These were activated by a conditioning monosynaptic reflex of the soleus muscle, evoked by electrical stimulation of the tibial nerve, and the resulting recurrent inhibition of the motoneurones was assessed by a subsequent monosynaptic reflex (H'). Recurrent inhibition was tested prior to, during and after an intravenous administration of a solution containing 2000 mg of L-AC. L-AC administration proved to be able to induce in all subjects a decrease in the H'-reflex. This effect ensued approximately 30 min after onset of L-AC administration, reached the peak after 40 min and vanished in about one hour. The extent of the decrease in H' varied among subjects, being on the average 22% of the control values. A relationship was found between duration of L-AC administration and time for reaching the maximal effect. These results show that L-AC is able to decrease a-motoneurone excitability by increasing Renshaw cell activity, both in normal and spastic subjects.

Relationship between the modified Rankin Scale and the Barthel Index in the process of functional recovery after stroke.

OBJECTIVE: The modified Rankin Scale (mRS) and the Barthel Index (BI) are the most common clinimetrical instruments for measuring disability after stroke. This study investigated the relationship between the BI and the mRS at multiple time points after stroke. The BI, which is a widely used instrument for longitudinal follow-up post-stroke, was used as reference to determine the effect of time on the sensitivity of the mRS in differentiating functional recovery. METHODS: Ninety-two patients with first stroke and hemispheric brain lesion were evaluated using the BI and mRS at 10 days, 3 and 6 months. The Kruskal-Wallis test was applied to examine median differences in BI among the mRS levels at 10 days, 3 and 6 months with Dunn's correction for multigroup comparison. The Mann and Whitney test was used to compare median differences in BI scores between two aggregations of mRS grades (mRS=0-2, mRS=3-5) at the same time periods after stroke. RESULTS: BI score distribution amongst mRS grades overlapped at 10 days, differentiating only between extreme grades (no disability vs severe disability). At 3 months, independent patients with slight disability could be distinguished from dependent patients with marked disability. At 6 months, grade 2 and 3 overlapped no more, differentiating independence (class 0-2) from dependence (class 3-5). The largest transition to an independent functional status occurred from grade 4, at 3 months. CONCLUSION: Maximum sensitivity of mRS in differentiating functional recovery is reached at six months post-stroke.

Anticipatory control of impending postural perturbation in elite springboard divers.

Among athletes, elite springboard divers (ED) should develop an optimal anticipatory control of postural stability, as a result of specific training. Postural strategies of ED and healthy subjects (HS) while expecting an impending perturbation were compared. The mean center of pressure (COP) position was analyzed during control quiet stance (cQS) and during anticipatory quiet stance (aQS(1-4)), i.e., in expectation of four backward translations of the support surface. During cQS, COP position in ED was not significantly different as compared to HS. During aQS(1-4,) a significant increase in the mean COP position was observed in both groups with ED adopting a more forward inclined vertical alignment than HS. In ED specific training may have resulted in a reference frame offset in a more anterior direction while expecting an impending perturbation. We suggest that leaning more forward may represent a more reliable way of coping with predictable perturbations of postural stability.