Human spinal cord injury: Motor unit properties and behaviour

C. K. Thomas, R. Bakels, C. S. Klein, I. Zijdewind

Research output: Contribution to journalReview article

21 Scopus citations

Abstract

Spinal cord injury (SCI) results in widespread variation in muscle function. Review of motor unit data shows that changes in the amount and balance of excitatory and inhibitory inputs after SCI alter management of motoneurons. Not only are units recruited up to higher than usual relative forces when SCI leaves few units under voluntary control, the force contribution from recruitment increases due to elevation of twitch/tetanic force ratios. Force gradation and precision are also coarser with reduced unit numbers. Maximal unit firing rates are low in hand muscles, limiting voluntary strength, but are low, normal or high in limb muscles. Unit firing rates during spasms can exceed voluntary rates, emphasizing that deficits in descending drive limit force production. SCI also changes muscle properties. Motor unit weakness and fatigability seem universal across muscles and species, increasing the muscle weakness that arises from paralysis of units, motoneuron death and sensory impairment. Motor axon conduction velocity decreases after human SCI. Muscle contractile speed is also reduced, which lowers the stimulation frequencies needed to grade force when paralysed muscles are activated with patterned electrical stimulation. This slowing does not necessarily occur in hind limb muscles after cord transection in cats and rats. The nature, duration and level of SCI underlie some of these species differences, as do variations in muscle function, daily usage, tract control and fibre-type composition. Exploring this diversity is important to promote recovery of the hand, bowel, bladder and locomotor function most wanted by people with SCI.

Original languageEnglish (US)
Pages (from-to)5-19
Number of pages15
JournalActa Physiologica
Volume210
Issue number1
DOIs
StatePublished - Jan 1 2014

Keywords

  • Axon conduction velocity
  • Maximal voluntary force
  • Motor unit firing frequency
  • Motor unit recruitment
  • Muscle spasms

ASJC Scopus subject areas

  • Physiology

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    Thomas, C. K., Bakels, R., Klein, C. S., & Zijdewind, I. (2014). Human spinal cord injury: Motor unit properties and behaviour. Acta Physiologica, 210(1), 5-19. https://doi.org/10.1111/apha.12153