A. Schmied et al., HUMAN SPINAL LATERALIZATION ASSESSED FROM MOTONEURON SYNCHRONIZATION - DEPENDENCE ON HANDEDNESS AND METER UNIT TYPE, Journal of physiology, 480, 1994, pp. 369-387
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.