1. Motoneurone synchronization as a means of investigating synaptic connectivity was studied in the extensor carpi radialis muscles of the preferred and non-preferred arms of healthy right-and left-handed human subjects. The activities of pairs of motor units recorded during voluntary isometric contractions were analysed by cross-correlation to detect any synchronous motor unit firing in the form of central peaks in the crosscorrelation histograms. 2. The synchronization peaks were compared first in the case of 273 motor unit pairs tested in the preferred and non-preferred arms of two left-handed subjects and two righthanded subjects. The percentage of synchronized motor unit pairs was found to be significantly higher in the preferred arm with synchronization peaks significantly larger and broader than in the non-preferred arm. The narrow peaks (< 7 5 ms) likely to reflect the activity of common inputs to motoneurones were also found to be significantly larger in the preferred arm of all four subjects. 3. The handedness-related differences in synchronization were definitely confirmed in a total of 275 pairs of motor units tested in the left extensor carpi radialis muscles of fourteen right-handed subjects using their non-preferred arm and six left-handed subjects using their preferred arm. In order to determine whether the differences in synchronization were dependent on the motor unit type, each motor unit was characterized on the basis of its recruitment threshold and on the basis of the contraction time of its twitch extracted from the overall muscle force using the spike-triggered averaging method. Two populations of motor units were distinguished, namely the 'slow' motor units (recruitment thresholds < 0 4 N, contraction times > 40 ms) and the 'fast' motor units (recruitment thresholds > 0-6 N, contraction times < 40 ms). 4. In the non-preferred arm, the synchronization peaks were always fairly narrow, whatever the motor unit's biomechanical properties; whereas in the preferred arm, broad peaks were found to be particularly common among the pairs including one or two fast motor units, which also showed the largest rate of synchronization occurrence. 5. The narrow peaks (< 7-5 ms) were found to be consistently larger in the preferred than the non-preferred arm whatever the categories of motor unit pairs. In both arms, however, the amplitude of the narrow peaks tended to increase as the recruitment threshold of the motor unit decreased and as their contraction time increased. This suggests the involvement of common inputs acting on the various types of motoneurones in much the same way as shown in the case of the muscle spindle primary afferents in animals. 6. The idea is put forward that the general enhancement of the motor unit discharge synchronization observed in the arm preferentially used by the subjects may reflect, in the first place, an enhancement of the efficiency of the motoneurones' common inputs, particularly in the case of the slow motor units, and, in the second place, an enhancement of the pr...

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Recent experiments on monkeys suggest that attention-related changes in the synchronous activity of neurones occur in the motor cortex. In humans, the clinical data available suggest that the weak synchronization of the motor unit firing observed during voluntary contraction might reflect the activity of the motoneurone cortico-spinal afferents. The present study was therefore designed to investigate how the synchronous motor-unit activity might depend on the attention required in performing a motor task. Twenty-five motor unit pairs were tested in the right extensor carpi radialis muscles of five right-handed subjects, who were instructed to keep both motor units firing tonically while trying to maintain the extension force as constant as possible using visual feedback set either at low or high gain. Fifteen motor unit pairs (60%) showed a greater amount of synchronous activity, without any consistent changes in the motor unit firing rates, when the subjects were monitoring the force at high gain. In addition, the mean value and the steadiness of the force did not differ significantly between the two tasks. The amplitude of the motor units' contractile force extracted by spike-triggered averaging did not change consistently despite the slight increase in the synchronous impulse probability observed under the high-gain conditions. Changes in synchrony affected slowly contracting, low-threshold motor units as well as fast-contracting, high-threshold motor units. The most noteworthy finding was that these changes actually focused on a narrow component of the synchronous activity (within a period of less than 4 ms) in keeping with the short-term synchronization process. This suggests that common inputs, possibly of cortical origin, may have contributed more greatly to controlling the motoneurone firing pattern in the motor task which required the subjects to pay more attention.