Human spinal lateralization assessed from motoneurone synchronization: dependence on handedness and motor unit type.

Abstract

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...

Coherence at 16-32 Hz Can Be Caused by Short-Term Synchrony of Motor Units

“…This result is similar to experimental observations of an inverse relation between the amount of motor-unit synchronization and the time to peak force in a twitch response, which was used as an index of recruitment threshold; slow-twitch motor units were more synchronized with one another than fast-twitch units (Schmied et al 1993(Schmied et al , 1994. Furthermore, the width of the peak in the cross-correlation histogram was modestly related to recruitment threshold in the dominant arm but not the nondominant arm (Schmied et al 1994). The current simulations produced a small decrease in peak width across the range of simulated forces.…”

“…After a discharge by the reference unit was randomly selected, another motor unit with a similar recruitment threshold was randomly chosen using a Gaussian distribution with a SD of 15 units, but within the range of Ϯ45 units (threshold based synchrony) (see Datta and Stephens 1990;Huesler et al 2000;Schmied et al 1994). If a discharge time for this motor unit occurred within Ϯ30 ms of the discharge by the reference unit ("adjustment time limit"), this discharge time was aligned with the reference unit with a Gaussian distribution that had a SD of 1.67 ms centered on the reference unit.…”

“…Synchrony can be produced by both branched common input near the motor neurons or by independent inputs that are synchronized at a presynaptic level in the motor cortex (Farmer et al 1997;Schmied et al 1994). For example, efferent neurons in the motor cortex are often synchronized (Baker et al 2001;Smith and Fetz 1989), and this may result in broadpeak synchrony of motor units due to the greater number of interposed synapses.…”

Common Input across Motor Nuclei Mediating Precision Grip in Humans

“…For example, Schmied et al (1994) found greater motor unit synchrony in extensor carpi radialis on the dominant side for both right-handed and lefthanded subjects, whereas Semmler and Nordstrom (1995) found greater synchrony in first dorsal interosseus on the nondominant side of right-handed subjects and no difference in synchrony across hands of left-handed subjects. To our knowledge, motor unit synchrony across muscles in the dominant and nondominant arms has not been examined previously.…”

Neural Adjustment in the Activation of the Lower Leg Muscles through Daily Physical Exercises in Community-Based Elderly Persons

“…Furthermore, it is task-dependent (Mochizuki et al 2005;Mottram et al 2005). Studies have indicated that the levels of synchronization are adaptive and the magnitude of synchronization is affected by such factors as learning (Schmied et al 1993), handedness (Schmied et al 1994;Semmler and Nordstrom 1995), and recovery from a lesion in the central nervous system (Farmer et al 1993). It is also reported that synchronization of motor units are modulated in the process of muscle strengthening (Moritani 1993).…”