Spontaneous motor unit behavior in human thenar muscles after spinal cord injury

Our first aim was to characterize spontaneous motor unit activity in thenar muscles influenced by chronic cervical spinal cord injury. Thenar surface electromyography (EMG), intramuscular EMG, and abduction and flexion forces were recorded. Subjects were instructed to relax for 2 min. Units still firing after 10 s were considered spontaneously active. Two distinct patterns of spontaneous unit activity were recorded. Units either fired tonically at a mean frequency of 6.1 HZ or were active sporadically (2.2 HZ). Stimuli (e.g., light touch of nearby skin) were then used to influence tonic spontaneous unit activity. Most stimuli produced a change in firing frequency, usually a temporary increase, but sometimes unit frequency decreased or new activity was initiated. Inputs to these motoneurons clearly make important contributions to changes in unit activity. However, the difficulty that subjects had in stopping unit activity, and the initiation of activity when subjects relaxed, suggest that the source of spontaneity may be the motoneuron itself.