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

• Published in: Journal of neurophysiology Vol. 96; no. 1; pp. 165 - 174
• Main Authors: Hager-Ross, C. K, Klein, C. S, Thomas, C. K
• Format: Journal Article
• Language: English
• Published: United States Am Phys Soc 01.07.2006
• Subjects: Adult; Chronic Disease; Electric Stimulation; Electromyography; Female; Hand - innervation; Hand - physiology; Hand/innervation/physiology; Humans; Male; Motor Neurons - physiology; Muscle Contraction - physiology; Muscle, Skeletal - physiology; Neural Conduction - physiology; Physiotherapy; sjukgymnastik; Spinal Cord Injuries - physiopathology; Skeletal/physiology; Muscle
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.01339.2005

1 The Miami Project to Cure Paralysis, Department of Neurological Surgery and 2 Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, Florida; and 3 Department of Community Medicine and Rehabilitation, Umeå University, Umeå, Sweden Submitted 20 December 2005; accepted in final form 6 April 2006 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. Address for reprint requests and other correspondence: C. K. Thomas, The Miami Project to Cure Paralysis, University of Miami Miller School of Medicine, Lois Pope LIFE Center, 1095 NW 14 Terrace, R48, Miami, FL 33136-2104 (E-mail: cthomas{at}miami.edu )