Motor unit rotation in a variety of human muscles.
The phenomena of substitution and rotation among motor units of a muscle were examined in seven different muscles. Intramuscular motor unit activity and surface electromyographic (EMG) activity 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 unit at a comfortable constant or rhythmically (pseudosinusoidally) 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, an additional motor unit frequently 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. Whereas 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 whereas the threshold of this unit increased, the threshold of a phasically discharging unit decreased substantially. The increase in threshold of a tonically discharging unit is suggested to arise from inactivation of Na(+) and Ca(2+) channels and the decrease in threshold of higher-threshold units is suggested to arise from an increase in persistent inward 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 central motor command, persistent inward currents, and inactivation of voltage-gated channels. Such rotations among low-threshold motoneurons would ensure low-level sustained contractions to be viable not only in small hand muscles but also in larger limb muscles.
Bawa
Parveen
P
School of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, British Columbia, Canada. bawa@sfu.ca
Murnaghan
Chantelle
C
D000161:Acoustic Stimulation; D000328:Adult; D000704:Analysis of Variance; D004576:Electromyography; D030141:Feedback, Psychological; D005260:Female; D005542:Forearm / Q000502:physiology*; D006225:Hand / Q000502:physiology*; D006801:Humans; D007866:Leg / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000502:physiology*; D010775:Photic Stimulation; D012399:Rotation; D055815:Young Adult