Figure 5. Firing frequencies of motor units in the dorsal and ventral portions of the external intercostal muscle in the third interspace.

Histograms of the inspiratory modulation of the discharge frequencies of all motor units identified in the dorsal (top) and ventral (bottom) portions of the muscle in the three subjects who had phasic inspiratory activity in both portions during quiet breathing (subjects 2, 4 and 5 in Fig. 3). Bin width, 1.0 Hz. Mean values are indicated on each histogram. For units which fired tonically throughout expiration, the firing frequency is taken as the difference in discharge rate between inspiration and expiration (see Methods).