TY - JOUR
T1 - Motor unit firing rates during spasms in thenar muscles of spinal cord injured subjects
AU - Zijdewind, Inge
AU - Bakels, Robert
AU - Thomas, Christine K.
PY - 2014/11/14
Y1 - 2014/11/14
N2 - 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.
AB - 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.
KW - motor unit recruitment
KW - motor unit derecruitment
KW - motor unit firing rate modulation
KW - afferent input
KW - motoneuron
KW - persistent inward current
KW - PERSISTENT INWARD CURRENTS
KW - PLATEAU POTENTIALS
KW - RECEPTIVE-FIELDS
KW - MOTONEURONS
KW - RECRUITMENT
KW - PATTERNS
KW - BEHAVIOR
KW - EXCITABILITY
KW - CONTRACTIONS
KW - STIMULATION
U2 - 10.3389/fnhum.2014.00922
DO - 10.3389/fnhum.2014.00922
M3 - Article
C2 - 25452723
SN - 1662-5161
VL - 8
JO - Frontiers in Human Neuroscience
JF - Frontiers in Human Neuroscience
M1 - 922
ER -