Limits...
Motor unit firing rates during spasms in thenar muscles of spinal cord injured subjects.

Zijdewind I, Bakels R, Thomas CK - Front Hum Neurosci (2014)

Bottom Line: Mean recruitment frequency (7.1 Hz, 3.2 SD) was significantly higher than derecruitment frequency (5.4 Hz, 2.4 SD).Later recruited units had higher or lower maximal firing rates than lower threshold units.Further, thenar motoneurons can still fire at high rates in response to various peripheral inputs after SCI, supporting the idea that low maximal voluntary firing rates and forces in thenar muscles result from reduced descending drive.

View Article: PubMed Central - PubMed

Affiliation: Department Neuroscience, Medical Physiology, University Medical Center Groningen, University of Groningen Groningen, Netherlands.

ABSTRACT
Involuntary contractions of paralyzed muscles (spasms) commonly disrupt daily activities and rehabilitation after human spinal cord injury (SCI). Our aim was to examine the recruitment, firing rate modulation, and derecruitment of motor units that underlie spasms of thenar muscles after cervical SCI. Intramuscular electromyographic activity (EMG), surface EMG, and force were recorded during thenar muscle spasms that occurred spontaneously or that were triggered by movement of a shoulder or leg. Most spasms were submaximal (mean: 39%, SD: 33 of the force evoked by median nerve stimulation at 50 Hz) with strong relationships between EMG and force (R (2) > 0.69). Unit recruitment occurred over a wide force range (0.2-103% of 50 Hz force). Significant unit rate modulation occurred during spasms (frequency at 25% maximal force: 8.8 Hz, 3.3 SD; at maximal force: 16.1 Hz, 4.1 SD). Mean recruitment frequency (7.1 Hz, 3.2 SD) was significantly higher than derecruitment frequency (5.4 Hz, 2.4 SD). Coactive unit pairs that fired for more than 4 s showed high (R (2) > 0.7, n = 4) or low (R (2):0.3-0.7, n = 12) rate-rate correlations, and derecruitment reversals (21 pairs, 29%). Later recruited units had higher or lower maximal firing rates than lower threshold units. These discrepant data show that coactive motoneurons are drive both by common inputs and by synaptic inputs from different sources during muscle spasms. Further, thenar motoneurons can still fire at high rates in response to various peripheral inputs after SCI, supporting the idea that low maximal voluntary firing rates and forces in thenar muscles result from reduced descending drive.

No MeSH data available.


Related in: MedlinePlus

Typical muscle spasms showing two motor units that are recruited, increase their firing rate during the force rise, decrease firing rate as the force declines, and are derecruited (A,B) or continue to fire after the contraction (B). Overlays of the marked potentials show accurate identification of each motor unit (right).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4231945&req=5

Figure 1: Typical muscle spasms showing two motor units that are recruited, increase their firing rate during the force rise, decrease firing rate as the force declines, and are derecruited (A,B) or continue to fire after the contraction (B). Overlays of the marked potentials show accurate identification of each motor unit (right).

Mentions: Most thenar muscle spasms occurred spontaneously or spasms were evoked by movement of a shoulder or leg. The mean increase in spasm force lasted 4.17 s (SD, 1.91) while the force decrease lasted 6.50 s (SD, 5.69 s, n = 37 spasms). Two other spasms lasted up to 24 s because they had double peaks. Possibly two spasms had occurred in close succession. The rate of change for EMG and force was similar during the upward phase of a spasm (1.89 N/s, SD 3.02) and during the downward phase (1.59 N/s, SD 2.95, F = 6.635, P = 0.58, Figure 1). In general, mean EMG and force were strongly associated during the spasms (Figures 2A,B; up: 32/39 spasms, R2 > 0.69; down: 32/39 spasms, R2 > 0.52). Some motor units were not derecruited following spasms but rather continued to fire for several minutes at low firing rates (Figure 1B).


Motor unit firing rates during spasms in thenar muscles of spinal cord injured subjects.

Zijdewind I, Bakels R, Thomas CK - Front Hum Neurosci (2014)

Typical muscle spasms showing two motor units that are recruited, increase their firing rate during the force rise, decrease firing rate as the force declines, and are derecruited (A,B) or continue to fire after the contraction (B). Overlays of the marked potentials show accurate identification of each motor unit (right).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4231945&req=5

Figure 1: Typical muscle spasms showing two motor units that are recruited, increase their firing rate during the force rise, decrease firing rate as the force declines, and are derecruited (A,B) or continue to fire after the contraction (B). Overlays of the marked potentials show accurate identification of each motor unit (right).
Mentions: Most thenar muscle spasms occurred spontaneously or spasms were evoked by movement of a shoulder or leg. The mean increase in spasm force lasted 4.17 s (SD, 1.91) while the force decrease lasted 6.50 s (SD, 5.69 s, n = 37 spasms). Two other spasms lasted up to 24 s because they had double peaks. Possibly two spasms had occurred in close succession. The rate of change for EMG and force was similar during the upward phase of a spasm (1.89 N/s, SD 3.02) and during the downward phase (1.59 N/s, SD 2.95, F = 6.635, P = 0.58, Figure 1). In general, mean EMG and force were strongly associated during the spasms (Figures 2A,B; up: 32/39 spasms, R2 > 0.69; down: 32/39 spasms, R2 > 0.52). Some motor units were not derecruited following spasms but rather continued to fire for several minutes at low firing rates (Figure 1B).

Bottom Line: Mean recruitment frequency (7.1 Hz, 3.2 SD) was significantly higher than derecruitment frequency (5.4 Hz, 2.4 SD).Later recruited units had higher or lower maximal firing rates than lower threshold units.Further, thenar motoneurons can still fire at high rates in response to various peripheral inputs after SCI, supporting the idea that low maximal voluntary firing rates and forces in thenar muscles result from reduced descending drive.

View Article: PubMed Central - PubMed

Affiliation: Department Neuroscience, Medical Physiology, University Medical Center Groningen, University of Groningen Groningen, Netherlands.

ABSTRACT
Involuntary contractions of paralyzed muscles (spasms) commonly disrupt daily activities and rehabilitation after human spinal cord injury (SCI). Our aim was to examine the recruitment, firing rate modulation, and derecruitment of motor units that underlie spasms of thenar muscles after cervical SCI. Intramuscular electromyographic activity (EMG), surface EMG, and force were recorded during thenar muscle spasms that occurred spontaneously or that were triggered by movement of a shoulder or leg. Most spasms were submaximal (mean: 39%, SD: 33 of the force evoked by median nerve stimulation at 50 Hz) with strong relationships between EMG and force (R (2) > 0.69). Unit recruitment occurred over a wide force range (0.2-103% of 50 Hz force). Significant unit rate modulation occurred during spasms (frequency at 25% maximal force: 8.8 Hz, 3.3 SD; at maximal force: 16.1 Hz, 4.1 SD). Mean recruitment frequency (7.1 Hz, 3.2 SD) was significantly higher than derecruitment frequency (5.4 Hz, 2.4 SD). Coactive unit pairs that fired for more than 4 s showed high (R (2) > 0.7, n = 4) or low (R (2):0.3-0.7, n = 12) rate-rate correlations, and derecruitment reversals (21 pairs, 29%). Later recruited units had higher or lower maximal firing rates than lower threshold units. These discrepant data show that coactive motoneurons are drive both by common inputs and by synaptic inputs from different sources during muscle spasms. Further, thenar motoneurons can still fire at high rates in response to various peripheral inputs after SCI, supporting the idea that low maximal voluntary firing rates and forces in thenar muscles result from reduced descending drive.

No MeSH data available.


Related in: MedlinePlus