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Firing patterns of spontaneously active motor units in spinal cord-injured subjects

Zijdewind, I. & Thomas, C. K., Apr-2012, In : Journal of physiology-London. 590, 7, p. 1683-1697 15 p.

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Involuntary motor unit activity at low rates is common in hand muscles paralysed by spinal cord injury. Our aim was to describe these patterns of motor unit behaviour in relation to motoneurone and motor unit properties. Intramuscular electromyographic activity (EMG), surface EMG and force were recorded for 30 min from thenar muscles of nine men with chronic cervical SCI. Motor units fired for sustained periods (>10 min) at regular (coefficient of variation 0.15, n = 14). Regularly firing units started and stopped firing independently suggesting that intrinsic motoneurone properties were important for recruitment and derecruitment. Recruitment (3.6 Hz, SD 1.2), maximal (10.2 Hz, SD 2.3, range: 7.5-15.4 Hz) and derecruitment frequencies were low (3.3 Hz, SD 1.6), as were firing rate increases after recruitment (similar to 20 intervals in 3s). Once active, firing often covaried, promoting the idea that units received common inputs. Half of the regularly firing units showed a very slow decline (> 40 s) in discharge before derecruitment and had interspike intervals longer than their estimated afterhyperpolarisation potential (AHP) duration (estimated by death rate and breakpoint analyses). The other units were derecruited more abruptly and had shorter estimated AHP durations. Overall, regularly firing units had longer estimated AHP durations and were weaker than irregularly firing units, suggesting they were lower threshold units. Sustained firing of units at regular rates may reflect activation of persistent inward currents, visible here in the absence of voluntary drive, whereas irregularly firing units may only respond to synaptic noise.

Original languageEnglish
Pages (from-to)1683-1697
Number of pages15
JournalJournal of physiology-London
Volume590
Issue number7
Publication statusPublished - Apr-2012

    Keywords

  • HUMAN THENAR MUSCLES, PERSISTENT INWARD CURRENTS, CONTRACTILE PROPERTIES, MOTONEURON AFTERHYPERPOLARIZATION, TIME-COURSE, DISCHARGE VARIABILITY, MEDIAL GASTROCNEMIUS, STROKE SURVIVORS, COMMON DRIVE, CAT

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