Journal of Applied Physiology, Vol 62, Issue 1 187-193, Copyright © 1987 by American Physiological Society
Voluntary hyperventilation changes recruitment order of parasternal intercostal motor units
T. W. Watson and W. A. Whitelaw
The order of recruitment of single-motor units in parasternal intercostal
muscles during inspiration was studied in normal human subjects during
quiet breathing and voluntary hyperventilation. Electromyograms were
recorded from the second and third intercostal spaces by means of bipolar
fine wire electrodes. Flow at the mouth, volume, end-expired CO2, and rib
cage and abdominal anterior-posterior diameters were monitored.
Single-motor units were identified using criteria of amplitude and shape,
and the time of first appearance of each unit in each inspiration was
noted. Hyperventilation was performed with visual feedback of the display
of rib cage and abdomen excursions, keeping the ratio of rib cage to
abdominal expansion. Subjects were normocapnic in quiet breathing and
developed hypocapnia during hyperventilation. Recruitment order was stable
in quiet breathing, but in some cases was altered during voluntary
hyperventilation. Some low threshold units that fired early in the breath
in quiet breathing fired earlier at the beginning of a period of voluntary
hyperventilation but progressively later in the breath as hyperventilation
went on, whereas later firing units moved progressively toward the early
part of inspiration. This suggests that different groups of motoneurons in
the pool supplying parasternal intercostal muscles receive different
patterns of synaptic input.
