Motor Unit Firing During and After Voluntary Contractions of Human Thenar Muscles Weakened by Spinal Cord Injury

2003 ◽  
Vol 89 (4) ◽  
pp. 2065-2071 ◽  
Author(s):  
Inge Zijdewind ◽  
Christine K. Thomas
Keyword(s):  
Spinal Cord ◽  
Spinal Cord Injury ◽  
Motor Unit ◽  
Cervical Spinal Cord ◽  
Motor Units ◽  
Firing Rates ◽  
Rate Modulation ◽  
Motor Unit Firing ◽  
Cord Injury ◽  
Voluntary Contractions

Spinal cord injury may change both the distribution and the strength of the synaptic input within a motoneuron pool and therefore alter force gradation. Here, we have studied the relative contributions of motor unit recruitment and rate modulation to force gradation during voluntary contractions of thenar muscles performed by five individuals with chronic (>1 yr) cervical spinal cord injury. Mean ± SD thenar unit firing rates were low during both steady-level 25% (8.3 ± 2.2 Hz, n = 27 units) and 100% maximal voluntary contractions (MVCs, 9.2 ± 3.1 Hz, n = 23 units). Thus modest rate modulation, or a lack of it in some units, was seen despite an average fourfold increase in integrated surface electromyographic activity and force. During ramp contractions, units were recruited at 5.7 ± 2.5 Hz, but still only reached maximal firing rates of 12.8 ± 4.9 Hz. Motor units were recruited up to 85% of the maximal force achieved (14.6 ± 5.6 N). In contrast, unit recruitment in control hand muscles is largely complete by 30% MVC. Thus, during voluntary contractions of thenar muscles weakened by cervical spinal cord injury, motor unit rate modulation was limited and recruitment occurred over a wider than usual force range. Those motor units that were stopped voluntarily had significantly lower derecruitment versus recruitment thresholds. However, 8 units (24%) continued to fire long after the signal to end the voluntary contraction at a mean frequency of 5.9 ± 0.8 Hz. The forces generated by this prolonged unit activity ranged from 0.3 to 7.2% maximum. Subjects were unable to stop this involuntary unit activity even with the help of feedback. The mechanisms that underlie this prolonged motor unit firing need to be explored further.

Distinct Patterns of Motor Unit Behavior During Muscle Spasms in Spinal Cord Injured Subjects

1997 ◽  
Vol 77 (5) ◽  
pp. 2847-2850 ◽  
Author(s):  
C. K. Thomas ◽  
B. H. Ross
Keyword(s):  
Spinal Cord ◽  
Spinal Cord Injury ◽  
Motor Unit ◽  
Motor Units ◽  
Triceps Surae ◽  
Firing Rates ◽  
Rate Modulation ◽  
Cord Injury ◽  
Muscle Spasms ◽  
Spinal Cord Injured

Thomas, C. K. and B. H. Ross. Distinct patterns of motor unit behavior during muscle spasms in spinal cord injured subjects. J. Neurophysiol. 77: 2847–2850, 1997. Surface electromyograms (EMG) and force were recorded during repeated involuntary spasms of paralyzed triceps surae muscles of four men with chronic cervical spinal cord injury. The firing rates of 78 medial gastrocnemius (MG) motor units also were recorded intramuscularly with tungsten microelectrodes. Spasms typically involved a relatively rapid rise, then a more gradual fall in triceps surae EMG and torque. Motor unit firing rates either increased and then decreased with the spasm intensity (54%) or were relatively constant (26%), firing mainly at 2–10 Hz. The remaining units (20%) produced trains that included one or several doublets. Mean peak spasm firing rates were 18 ± 9 Hz (mean ± SD) for rate modulated units and 11 ± 10 Hz for units with little or no rate modulation. Some motor units fired at rates comparable with those recorded previously during maximum voluntary contractions performed by intact subjects. Others fired at rates below the minimum usually seen when normal units are first recruited (<6 Hz). Doublets (interspike interval <10 ms) often repeated every 123–333 ms, or were interspersed in trains firing at low steady rates (<11 Hz). This study shows that rate coding for many motor units appears to be similar whether descending motor input is intact or whether it has been reduced severely by spinal cord injury. In contrast, rate modulation in other units appears to depend mainly on voluntary motor commands.

scholarly journals Do Additional Inputs Change Maximal Voluntary Motor Unit Firing Rates After Spinal Cord Injury?

2011 ◽  
Vol 26 (1) ◽  
pp. 58-67 ◽  
Author(s):  
Inge Zijdewind ◽  
Katie Gant ◽  
Rob Bakels ◽  
Christine K. Thomas
Keyword(s):  
Spinal Cord ◽  
Spinal Cord Injury ◽  
Motor Unit ◽  
Firing Rates ◽  
Voluntary Motor ◽  
Motor Unit Firing ◽  
Cord Injury

scholarly journals Sensing and decoding the neural drive to paralyzed muscles during attempted movements of a person with tetraplegia using a sleeve array

2021 ◽  
Author(s):  
Jordyn E Ting ◽  
Alessandro Del Vecchio ◽  
Devapratim Sarma ◽  
Nikhil Verma ◽  
Samuel C Colachis ◽  
...  
Keyword(s):  
Motor Unit ◽  
Motor Neurons ◽  
Cervical Spinal Cord ◽  
Hand Movement ◽  
Motor Units ◽  
Single Digit ◽  
Firing Rates ◽  
Motor Unit Firing ◽  
Cord Injury ◽  
Test Feasibility

Motor neurons convey information about motor intent that can be extracted and interpreted to control assistive devices. However, most methods for measuring the firing activity of single neurons rely on implanted microelectrodes. Although intracortical brain-computer interfaces (BCIs) have been shown to be safe and effective, the requirement for surgery poses a barrier to widespread use that can be mitigated by instead using noninvasive interfaces. The objective of this study was to evaluate the feasibility of deriving motor control signals from a wearable sensor that can detect residual motor unit activity in paralyzed muscles after chronic cervical spinal cord injury (SCI). Despite generating no observable hand movement, volitional recruitment of motor units below the level of injury was observed across attempted movements of individual fingers and overt wrist and elbow movements. Subgroups of motor units were coactive during flexion or extension phases of the task. Single digit movement intentions were classified offline from the EMG power (RMS) or motor unit firing rates with median classification accuracies >75% in both cases. Simulated online control of a virtual hand was performed with a binary classifier to test feasibility of real-time extraction and decoding of motor units. The online decomposition algorithm extracted motor units in 1.2 ms, and the firing rates predicted the correct digit motion 88 ± 24% of the time. This study provides the first demonstration of a wearable interface for recording and decoding firing rates of motor units below the level of injury in a person with motor complete SCI.

Twitch and Tetanic Properties of Human Thenar Motor Units Paralyzed by Chronic Spinal Cord Injury

2006 ◽  
Vol 96 (1) ◽  
pp. 165-174 ◽  
Author(s):  
C. K. Häger-Ross ◽  
C. S. Klein ◽  
C. K. Thomas
Keyword(s):  
Spinal Cord ◽  
Spinal Cord Injury ◽  
Cervical Spinal Cord ◽  
Motor Units ◽  
Total Force ◽  
Force Output ◽  
Relative Contribution ◽  
Control Units ◽  
Cord Injury ◽  
Contraction Times

Little is known about how human motor units respond to chronic paralysis. Our aim was to record surface electromyographic (EMG) signals, twitch forces, and tetanic forces from paralyzed motor units in the thenar muscles of individuals ( n = 12) with chronic (1.5–19 yr) cervical spinal cord injury (SCI). Each motor unit was activated by intraneural stimulation of its motor axon using single pulses and trains of pulses at frequencies between 5 and 100 Hz. Paralyzed motor units ( n = 48) had small EMGs and weak tetanic forces ( n = 32 units) but strong twitch forces, resulting in half-maximal force being achieved at a median of only 8 Hz. The distributions for cumulative twitch and tetanic forces also separated less for paralyzed units than for control units, indicating that increases in stimulation frequency made a smaller relative contribution to the total force output in paralyzed muscles. Paralysis also induced slowing of conduction velocities, twitch contraction times and EMG durations. However, the elevated ratios between the twitch and the tetanic forces, but not contractile speed, correlated significantly with the extent to which unit force summated in response to different frequencies of stimulation. Despite changes in the absolute values of many electrical and mechanical properties of paralyzed motor units, most of the distributions shifted uniformly relative to those of thenar units obtained from control subjects. Thus human thenar muscles paralyzed by SCI retain a population of motor units with heterogeneous contractile properties because chronic paralysis influenced all of the motor units similarly.

scholarly journals Increases in human motoneuron excitability after cervical spinal cord injury depend on the level of injury

2017 ◽  
Vol 117 (2) ◽  
pp. 684-691 ◽  
Author(s):  
Christine K. Thomas ◽  
Charlotte K. Häger ◽  
Cliff S. Klein
Keyword(s):  
Spinal Cord ◽  
Spinal Cord Injury ◽  
Conduction Velocity ◽  
Spinal Injury ◽  
Cervical Spinal Cord ◽  
Motor Units ◽  
M Wave ◽  
Human Spinal Cord ◽  
Motoneuron Excitability ◽  
Cord Injury

After human spinal cord injury (SCI), motoneuron recruitment and firing rate during voluntary and involuntary contractions may be altered by changes in motoneuron excitability. Our aim was to compare F waves in single thenar motor units paralyzed by cervical SCI to those in uninjured controls because at the single-unit level F waves primarily reflect the intrinsic properties of the motoneuron and its initial segment. With intraneural motor axon stimulation, F waves were evident in all 4 participants with C4-level SCI, absent in 8 with C5 or C6 injury, and present in 6 of 12 Uninjured participants ( P < 0.001). The percentage of units that generated F waves differed across groups (C4: 30%, C5 or C6: 0%, Uninjured: 16%; P < 0.001). Mean (±SD) proximal axon conduction velocity was slower after C4 SCI [64 ± 4 m/s ( n = 6 units), Uninjured: 73 ± 8 m/s ( n = 7 units); P = 0.037]. Mean distal axon conduction velocity differed by group [C4: 40 ± 8 m/s ( n = 20 units), C5 or C6: 49 ± 9 m/s ( n = 28), Uninjured: 60 ± 7 m/s ( n = 45); P < 0.001]. Motor unit properties (EMG amplitude, twitch force) only differed after SCI ( P ≤ 0.004), not by injury level. Motor units with F waves had distal conduction velocities, M-wave amplitudes, and twitch forces that spanned the respective group range, indicating that units with heterogeneous properties produced F waves. Recording unitary F waves has shown that thenar motoneurons closer to the SCI (C5 or C6) have reduced excitability whereas those further away (C4) have increased excitability, which may exacerbate muscle spasms. This difference in motoneuron excitability may be related to the extent of membrane depolarization following SCI. NEW & NOTEWORTHY Unitary F waves were common in paralyzed thenar muscles of people who had a chronic spinal cord injury (SCI) at the C4 level compared with uninjured people, but F waves did not occur in people that had SCI at the C5 or C6 level. These results highlight that intrinsic motoneuron excitability depends, in part, on how close the motoneurons are to the site of the spinal injury, which could alter the generation and strength of voluntary and involuntary muscle contractions.

scholarly journals Effects of baclofen on motor units paralysed by chronic cervical spinal cord injury

Brain ◽  
2009 ◽  
Vol 133 (1) ◽  
pp. 117-125 ◽  
Author(s):  
C. K. Thomas ◽  
C. K. Hager-Ross ◽  
C. S. Klein
Keyword(s):  
Spinal Cord ◽  
Spinal Cord Injury ◽  
Cervical Spinal Cord ◽  
Motor Units ◽  
Cervical Spinal Cord Injury ◽  
Cord Injury
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