Spike trains from single motor units in human parasternal intercostal muscles

Recordings of single motor unit activity were obtained from parasternal intercostal muscles of normal humans during quiet breathing. Spike trains from 52 individual motor units were analyzed. All these units were low threshold ones, recruited at low inspired volumes and therefore at low tension thresholds. Mean frequency of firing at onset was 7.8 Hz and mean increase in frequency through the breath was 3.6 Hz. Onset and peak frequencies were positively correlated with inspiratory flow rate. Alternation of interspike intervals between long and short was found in the spike trains of 6 of 13 units tested and this occurred at frequencies of 6-12/s. Doublet discharges at the beginnings of spike trains were seen during voluntary neck flexion but never in quiet breathing or voluntary deep breaths. The pattern of activity in these human intercostal motor units was similar to that reported for low threshold, slow twitch units in other mammalian skeletal muscles, including respiratory muscles.