Motor unit recruitment by intraspinal microstimulation and long-term neuromuscular adaptations

  • Author / Creator
    Bamford, Jeremy, Andrew
  • Spinal cord injury is a devastating neurological disorder partially characterized by a loss of motor function below the lesion. The dramatic loss of activity results in muscle atrophy and slow-to-fast transformation of contractile elements, producing smaller, weaker and more fatiguable muscles. Functional electrical stimulation (FES), has been proposed in order to induce muscular activity and reverse these changes. FES has primarily been applied in the periphery, either at the surface or implanted in or around a nerve or muscle. Although this can excite nervous tissue and produce muscular contractions, these systems often produce reversed recruitment of motor units leading to inappropriate force generation and increased fatigue.
    We applied intraspinal microstimulation (ISMS) through fine microwires implanted into the spinal cord of rats. Electrical stimulation through these microwires caused contractions of the quadriceps muscles in both acute and chronically spinalized animals. We showed that muscle recruitment is significantly more gradual with ISMS in intact rats compared to that produced by a standard nerve cuff. Our results further showed that this was due to preferential activation of fatigue resistant muscle fibers.
    Given this more orderly recruitment of motor units by ISMS, we tested the muscle phenotypes produced by ISMS or nerve cuffs after chronic stimulation. Surprisingly, over a 30 day stimulation period the quadriceps muscles chronically activated by either daily ISMS or nerve cuff stimulation underwent similar fast-to-slow transformations in fiber type and functional properties. This indicates that the recruitment order of motor units does not play the only role in determining the muscle phenotype. Other factors such as the total daily time of activity may be critically important to the phenotypic outcome of skeletal muscle.
    Finally, we demonstrated that quadriceps force recruitment by ISMS was unchanged following the 30 day stimulation period. In addition, 30 days of chronic ISMS did not cause observable damage in the spinal cord beyond that incurred by the implantation of sham microwires. These studies advance our understanding of the force recruitment properties, neuromuscular plasticity and damage incurred by ISMS and move us closer to developing a clinically viable ISMS procedure.

  • Subjects / Keywords
  • Graduation date
    Fall 2009
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.