Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, C46
Motor unit rotation in a variety of human muscles
P. Bawa1, C. Murnaghan1
1. Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, British Columbia, Canada.
To study the phenomena of substitution and rotation among motor units of a muscle, motoneurone pools of seven different muscles were investigated. Intramuscular motor unit activity and surface EMG (electromyogram) were recorded from one of the following muscles: abductor digiti minimi, first dorsal interosseous, extensor digitorum communis, flexor and extensor carpi radialis, tibialis anterior and soleus. The subject was asked to discharge a discernible motor unit at a comfortable constant or rhythmically modulated rate with audio and visual feedback. Results are reported from a total of 42 sets of motor units from all seven muscles. We observed that when a subject fired a motor unit for a long period, frequently an additional motor unit started to discharge after a few minutes. When the subject was asked to keep activity down to one unit, very often it was Unit 1 that dropped and Unit 2 continued to fire. While unit 2 had fired for a few minutes, Unit 1 resumed firing without any conscious effort by the subject. If the subject was then asked to retain just one unit, it was Unit 2 that dropped. Rhythmic modulation of firing rate of a tonically firing unit showed that while the threshold of this unit increased, the threshold of a phasically discharging unit decreased substantially, and rotation occurred. Quantification of firing times of lower and higher threshold units showed that the total discharge time was significantly longer for lower threshold than for higher threshold units. This finding indicates that even when lower threshold units do rest, they recover faster to contribute for a longer time to the total contraction. The increase in threshold of a tonically discharging unit is suggested to arise from inactivation of Na+ and Ca++ channels and the decrease in threshold of higher threshold units is suggested to arise from an increase in persistent inwards currents that may occur during prolonged contractions. Whether a unit stops or starts to fire is suggested to depend on a balance between the strength of the net excitatory motor command, persistent inward currents, and inactivation of voltage gated channels. Rotations among low threshold motoneurones would ensure sustained contractions in small as well as larger muscles.
Where applicable, experiments conform with Society ethical requirements