Central nervous pathways underlying synchronization of human motor unit firing studied during voluntary contractions.

Abstract

SUMMARY1. Motor unit firing has been studied during weak voluntary isometric contractions with pairs of needle electrodes in normal human subjects.2. Pre-and post-stimulus time histograms of the firing time of firing of one event unit before and after the time of firing of another reference (stimulus) unit showed a clear central peak, indicative of synchronization.3. Synchronization was seen in all the muscles studied. The mean strength of synchronization, expressed as the number of concomitant discharges of the two units as a proportion of the number of stimulus unit discharges, was 0 095 extra event unit spikes/reference unit spike (range 0 042-0{28) for first dorsal interosseous muscle, 0-016 extra event unit spikes per reference unit spike (range 0-4043) for medial gastrocnemius and 0-056 extra event unit spikes per reference unit spike (range 0-016-0-079) for tibialis anterior.4. The mean duration of synchronization was 11-3 ms (range 5-0-21-0 ms) for first dorsal interosseous, 10-3 ms (range 3-5-21-7 ms) for medial gastrocnemious and 13-5 ms (range 3 0-25 0) for tibialis anterior.5. Seven patients with radiographically and clinically identified central strokes were studied while they made weak voluntary isometric contractions. The duration of synchronization was significantly prolonged compared to that found in normal subjects. In these stroke patients the mean duration of synchronization on the affected side was longer than that seen in the normal subjects, and in first dorsal interosseous muscle was 35-4 ms (range 12-0-65-0 ms), in medial gastrocnemius was 21-3 ms (range 4 0-43 0 ms) and in tibialis anterior was 28-8 ms (range 14-0-49-0 ms).6. The mean strength of synchronization of motor unit discharge was found to be greater in the stroke patients than that seen in the normal subjects for first dorsal interosseous muscle (0 161 extra event unit spikes per reference unit spike, range 0-017-04391) and for medial gastrocnemius (0'030 extra event unit spikes per reference unit spike) but only significantly so when pooled data was compared. There was no difference in the strength of motor unit synchronization in tibialis anterior between stroke patients and normal subjects. * To whom correspondence should be addressed. 8073A. K. DATTA, S F. FARMER AND J. A. STEPHENS 7. Broad duration synchronization among first dorsal interosseous motor units was also found in a pateint with a rostral cervical spine lesion (total duration range 43-46 ms; n = 2), but not in a patient with a caudal (thoracic) spinal lesion. Broad duration 'synchronization was also found in a patient with untreated idiopathic Parkinson's disease (range 37-45 ms; n = 2) and in a patient with a cerebellar haematoma (range 19-28 ms; n = 2) but not in a patient with Huntington's chorea (range 9-20 ms; n = 2).8. Normal subjects were asked to isometrically co-contract two muscles; the firing of a motor unit in each muscle was studied. Synchronization of motor unit firing was always found between muscles defined as anatomical antagonists. The strengt...

Neuromuscular activation and motor-unit firing characteristics in cerebral palsy

“…A similar result has been found by other investigators in participants with CP and stroke (Datta et al 1991, Farmer et al 1993, Gibbs et al 1999). The reduction is thought to reflect a substantial decrease in the number of direct cortico-motoneuronal connections, which would be consistent with a reduction in strength and dexterity.…”

Simulation system of spinal cord motor nuclei and associated nerves and muscles, in a Web-based architecture

“…Other studies in the literature have dealt with the synchronization between motor-units and its effects on muscle force (Datta et al 1991;Yao et al 2000;Mochizuki et al 2005). In the present simulator, motor unit synchronization may be obtained naturally (not algorithmically) by choosing appropriately the descending drive connectivity so that a sufficient amount of input divergence guarantees the required level of motor unit synchronization.…”

Common Input across Motor Nuclei Mediating Precision Grip in Humans

“…Nevertheless, the magnitude of the synchronous peak provides a noisy representation of the proportion of common (including synchronized presynaptic inputs) to noncommon inputs to the two neurons (Kirkwood and Sears, 1978). Because short-term synchrony appears to be minimally affected by lesions of the dorsal roots (Kirkwood et al, 1982) yet is markedly diminished with section of descending pathways (Kirkwood et al, 1982;Datta et al, 1991), the source of such synchrony is thought to be primarily central rather than peripheral.…”