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

Key points • Muscles below a human spinal cord injury start to contract involuntarily a few weeks after trauma. The basis for this spontaneous activity is still unclear. • We show motor units in these muscles exhibited regular or irregular activity for several minutes. The regularly firing motor units had longer estimated afterhyperpolarisation potentials, higher mean firing rates and generated weaker forces. • These observations suggest that regularly firing motor units are more excitable than those that fire irregularly. • Spontaneous firing of motor units at regular rates may reflect active properties (such as persistent currents) within motoneurones, visible here in the absence of voluntary drive. • This basic motor unit firing behaviour may underlie the firing patterns typically recorded during voluntary contractions. 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, CV, n= 19 units) or irregular intervals (CV > 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 (∼20 intervals in 3 s). 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 irregularl