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  • 1
    ISSN: 1432-1106
    Keywords: Key words Long-latency reflexes ; Magnetic brain stimulation ; Sensory feedback
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Stimulation of cutaneous foot afferents has been shown to evoke a facilitation of the tibialis anterior (TA) EMG-activity at a latency of 70–95 ms in the early and middle swing phase of human walking. The present study investigated the underlying mechanism for this facilitation. In those subjects in whom it was possible to elicit a reflex during tonic dorsiflexion while seated (6 out of 17 tested), the facilitation in the TA EMG evoked by stimulation of the sural nerve (3 shocks, 3-ms interval, 2.0–2.5× perception threshold) was found to have the same latency in the swing phase of walking. The facilitation observed during tonic dorsiflexion has been suggested to be – at least partly – mediated by a transcortical pathway. To investigate whether a similar mechanism contributes to the facilitation observed during walking, magnetic stimulation of the motor cortex (1.2× motor threshold) was applied in the early swing phase at different intervals in relation to the cutaneous stimulation in 17 subjects. In 13 of the subjects, the motor potentials evoked by the magnetic stimulation (MEPs) were more facilitated by prior sural-nerve stimulation (conditioning-test intervals of 50–80 ms) than the algebraic sum of the control MEP and the cutaneous facilitation in the EMG when evoked separately. In four of these subjects, a tibialis anterior H-reflex could also be evoked during walking. In none of the subjects was an increase of the H-reflex similar to that for the MEP observed. In five experiments on four subjects, MEPs evoked by magnetic and electrical cortical stimulation were compared. In four of these experiments, only the magnetically induced MEPs were facilitated by prior stimulation of the sural nerve. We suggest that a transcortical pathway may also contribute to late cutaneous reflexes during walking.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-1106
    Keywords: Key words H-reflex ; Motor units ; Motor evoked potentials ; Motoneuron recruitment ; Transcranial magnetic stimulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  The responses of 34 extensor-carpi-radialis motor units to graded transcranial magnetic stimulation (TMS) and electrical stimulation of the radial nerve were investigated in six human subjects. Simultaneously with the recording of the single motor-unit discharges, motor-evoked potentials (MEPs) and H-reflexes evoked by the two types of stimulation were recorded by surface electrodes and expressed as a percentage of the maximal motor response (Mmax). Ten motor units were activated in the H-reflex when it was less than 5% of Mmax, but not in the MEP even when it was 15% of Mmax. The opposite was observed for three motor units. Eleven motor units were recruited by both stimuli, but with significantly different recruitment thresholds. Only ten motor units had a threshold similar to TMS and radial nerve stimulation. From these observations, we suggest that caution should be taken when making conclusions regarding motor cortical excitability based on changes in the size of MEPs, even when it is ensured that there are no similar changes in background EMG-activity or H-reflexes.
    Type of Medium: Electronic Resource
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