Journal of Applied Physiology, Vol 50, Issue 2 348-357, Copyright © 1981 by American Physiological Society
Single motor unit activity in the diaphragm of cat during pressure breathing
B. Bishop, S. Settle and J. Hirsch
In this study we analyzed the breath-by-breath activity of single motor
units in the diaphragm slip of allobarbital-anesthetized cats during quiet
breathing and during continuous positive- and negative-pressure breathing.
Our objective was to determine whether single motor units, on the basis of
their activities, can be separated into discrete subpopulations or whether
they fall on a continuum analogous to that of motor units of hindlimb
muscles. The firing profiles of each unit were characterized for each
pressure level by the onset and peak firing frequencies, onset latency,
duration of firing, number of impulses per breath, and minimal frequency,
when appropriate. Units with shorter onset latencies had higher peak
frequencies, longer firing durations, and increased firing frequencies than
did units with longer onset latencies. These comparative relationships
persisted even though the activity of every motor unit was altered during
pressure breathing. During positive-pressure breathing onset latencies were
lengthened, and durations of firing were shortened with little change in
onset or peak frequencies. Late units might be silenced. During
negative-pressure breathing onset latencies were shortened, and durations
of firing were lengthened, sufficiently in some cases to fill the
expiratory pause. In addition, previously inactive units were recruited
late in inspiration for short, relatively high frequency bursts during
inspiration. The results support the concept that the phrenic motoneuron
pool is comprised of three discrete subpopulations.
