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

Abstract

. Twitch and tetanic properties of human thenar motor units paralyzed by chronic spinal cord injury. J Neurophysiol 96: 165-174, 2006. First published April 12, 2006 doi:10.1152/jn.01339.2005. 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 ϭ 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 ϭ 48) had small EMGs and weak tetanic forces (n ϭ 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.

Identification and classification of involuntary leg muscle contractions in electromyographic records from individuals with spinal cord injury

“…The records were rectified, and integrated every 10 ms, which corresponds to the average duration of motor unit potentials, the smallest signal we expect to record (Häger-Ross et al, 2006; Thomas et al, 2006). EMG signals were distinguished from baseline noise by calculating a threshold.…”

Motoneuron Death after Human Spinal Cord Injury

“…55 Thinner than usual myelin and shorter internodal distances with remyelination will diminish impulse propagation, which is consistent with the general slowing of single thenar motor axon conduction velocities measured 1.5-19 years after human SCI. 56 These changes in axon conduction, as well as chronic motor unit firing at low rates, may induce further adaptations since thenar motor units have low recruitment and maximal firing rates during involuntary and voluntary contractions, and are weak and slow after chronic SCI. 18,[57][58][59] Defining the cause of muscle paralysis is important for planning rehabilitation Muscle denervation is routinely overlooked after human SCI because muscle function is assessed manually in the clinic.…”

Decomposition of motor unit tetanic contractions of rat soleus muscle: Differences between males and females

“…The applied mathematical method also has limitations -the decomposition can be made only for unfused tetanic contractions, when the beginning of relaxation following a response to each stimulus is visible. Such unfused tetanic contractions can be evoked for isolated MUs only, and for humans this method was only used in experiments on small distal muscles of the limbs (Thomas et al, 2005;Häger-Ross et al, 2006). From these reasons one cannot simply generalize the obtained results to humans, neither with respect to high variability of responses to individual stimuli nor to sex differences in the process of force generation for slow soleus MUs.…”