HUMAN SPINAL LATERALIZATION ASSESSED FROM MOTONEURON SYNCHRONIZATION - DEPENDENCE ON HANDEDNESS AND METER UNIT TYPE

1. Motoneurone synchronization as a means of investigating synaptic co nnectivity was studied in the extensor carpi radialis muscles of the p referred and non-preferred arms of healthy right- and left-handed huma n subjects. The activities of pairs of motor units recorded during vol untary isometric contractions were analysed by cross-correlation to de tect any synchronous motor unit firing in the form of central peaks in the crosscorrelation histograms. 2. The synchronization peaks were co mpared first in the case of 273 motor unit pairs tested in the preferr ed and non-preferred arms of two left-handed subjects and two right-ha nded subjects. The percentage of synchronized motor unit pairs was fou nd to be significantly higher in the preferred arm with synchronizatio n 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 diff erences in synchronization were definitely confirmed in a total of 275 pairs of motor units tested in the left extensor carpi radialis muscl es of fourteen right-handed subjects using their non-preferred arm and six left-handed subjects using their preferred arm. In order to deter mine 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-trigge red averaging method. Two populations of motor units were distinguishe d, namely the 'slow' motor units (recruitment thresholds < 0.4 N, cont raction times > 40 ms) and the 'fast' motor units (recruitment thresho lds > 0.6 N, contraction times less than or equal to 40 ms). 4. In the non-preferred arm, the synchronization peaks were always fairly narro w, 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 n on-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 co ntraction time increased. This suggests the involvement of common inpu ts 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 mag reflect, in the first place, an enhancement of ti le efficiency of the motoneurones' common inputs, particularly in the case of the slow motor units, and, in the second place, an enhancement of the presynaptic synchronization of the motoneurone inputs, particu larly in the case of the fast motor units, which require a strong driv e to be recruited.