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Pubmed for Handhelds

PUBMED FOR HANDHELDS

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  • Title: Motor unit rotation in a variety of human muscles.
    Author: Bawa P, Murnaghan C.
    Journal: J Neurophysiol; 2009 Oct; 102(4):2265-72. PubMed ID: 19657086.
    Abstract:
    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.
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