Intracortical inhibition in the human trigeminal motor system.

Objective: To investigate the presence and features of short-interval intracortical inhibition (SICI) in the human trigeminal motor system.

Methods: Surface electromyogram (EMG) was recorded from left and right digastric muscles in 7 subjects, along with additional experiments with intramuscular EMG in 2 subjects. Focal transcranial magnetic stimulation (TMS) was used to activate the motor cortex of one hemisphere and elicit motor evoked potentials (MEPs) in digastric muscles on each side, at rest and while subjects activated the muscles at 10% maximal EMG.

Experimental muscle pain does not affect fine motor control of the human hand.

Conditions known to cause hand pain, such as arthritis, are often accompanied by impaired dexterity. The aim of this study was to determine whether this association is coincident, or whether pain affects dexterity directly. In the first part of the study, several tests of dexterity based on pegboard skills were compared with a precision-grip-lift task: the correlations between the results of any of these tests were not significant at the 0.

Interhemispheric coupling of corticospinal excitability is suppressed during voluntary muscle activation.

Motor-evoked potentials (MEPs) after transcranial magnetic stimulation (TMS) show a trial-to-trial variation in size at rest that is positively correlated for muscles of the same, and opposite, upper limbs. To investigate the mechanisms responsible for this we have examined the effect of voluntary activation on the correlated fluctuations of MEP size. In 8 subjects TMS was concurrently applied to the motor cortex of each hemisphere using 2 figure-8 coils.

Differential modulation of intracortical inhibition in human motor cortex during selective activation of an intrinsic hand muscle.

Paired-pulse transcranial magnetic stimulation (TMS) was used to assess the effectiveness of intracortical inhibition (ICI) acting on corticospinal neurons controlling three intrinsic hand muscles in humans. We hypothesised that the suppression of ICI with selective activation of a muscle would be restricted to corticospinal neurons controlling the muscle targeted for activation. Surface EMG was recorded from abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles of the left hand.

Is the long-latency stretch reflex in human masseter transcortical?

A long-latency stretch reflex (LLSR) has been described in the human masseter muscle, but its pathway remains uncertain. To investigate this, the excitability of corticomotoneuronal (CM) cells projecting to masseter motoneurons during the LLSR was assessed with transcranial magnetic stimulation (TMS). A facilitated response to TMS would be evidence of a LLSR pathway that traverses the motor cortex.

Responses of single motor units in human masseter to transcranial magnetic stimulation of either hemisphere.

The corticobulbar inputs to single masseter motoneurons from the contra- and ipsilateral motor cortex were examined using focal transcranial magnetic stimulation (TMS) with a figure-of-eight stimulating coil. Fine-wire electrodes were inserted into the masseter muscle of six subjects, and the responses of 30 motor units were examined. All were tested with contralateral TMS, and 87 % showed a short-latency excitation in the peristimulus time histogram at 7.