Proceedings of The Physiological Society

Physiology 2015 (Cardiff, UK) (2015) Proc Physiol Soc 34, C70

Oral Communications

Recovery of human motoneurones after prolonged voluntary contractions

S. Gandevia1,2, M. Heroux1,2, A. A. Butler1,2, J. Taylor1,2, J. Butler1,2

1. Neuroscience Research Australia, Randwick,Sydney, New South Wales, Australia. 2. University of New South Wales, Sydney, New South Wales, Australia.

Motoneurones often fire repetitively and for long periods. In sustained voluntary contractions the excitability of motoneurones declines (e.g. McNeil et al. 2009; Manning et al. 2010). To understand this better, we mapped the time course of recovery of motoneurones in human volunteers who gave written informed consent to the study. We recorded the discharge of single motor units (n=30) with intramuscular wire electrodes inserted into triceps brachii during weak isometric contractions. Recordings lasted 40-60 min. Subjects (n=15) discharged single motor units at a constant frequency (~10 Hz) with visual feedback for prolonged durations (3-7 minutes) until the rectified surface electromyogram (EMG) of triceps brachii increased by ~100%. After rest intervals which varied from ~2- 240 s, subjects resumed the constant-frequency contraction (with 3-4 intervals being tested per motor unit). Compared to baseline, the level of EMG required to re-recruit the target motoneurone at the test frequency increased after the sustained contraction (for rest intervals from 2-60 s, p = 0.001-0.038). This increased EMG indicates that greater excitatory drive was needed to discharge the motoneurone at the test rate. The increased EMG returned to baseline levels with an exponential time course (time-constant 28 sec) and was fully recovered after a rest period of ~240 s. We suggest that the decline in motoneurone excitability in a sustained weak contraction depends on repetitive activation of the motoneurones. The lack of apparent muscle fatigue with weak contractions indicates that the CNS compensates for this decreased excitability.

Where applicable, experiments conform with Society ethical requirements