Respiratory muscle activation by limb muscle afferent stimulation in anesthetized dogs

In 10 chloralose anaesthetized and spontaneously breathing dogs, we assessed the effect of limb muscle afferents on the peak integrated EMG activities of the genioglossus, alae nasi, costal diaphragm, parasternal intercostal, triangularis sterni, and transverse abdominis muscles. The influence of vagal and baroreceptor afferents were eliminated by vagotomy and perfusion of carotid sinuses at a constant pressure. Muscle afferents were activated by stimulating the central end of the gastrocnemius nerve for 1 min at 40 Hz and at different voltages. Stimulation at voltages equal to 5, 10 and 20 times twitch-threshold increased minute ventilation to 165, 216 and 250% of pre-stimulation values, respectively, which was achieved by increasing breathing frequency (shortening of the inspiratory and expiratory times) and tidal volume. The activity of the parasternal intercostal and alae nasi muscles increased by a similar degree to that of the diaphragm while the activities of the genioglossus and transverse abdominis were augmented to a greater degree than that of the diaphragm. On the other hand, the motor drive to triangularis sterni increased significantly only at 20 times twitch-threshold and to a lesser degree than that to the diaphragm. These results suggest that upper airway, inspiratory and expiratory rib cage and abdominal muscles may be independently regulated. Differences in the sensitivity of these muscles to the activation of limb muscle afferents can be explained by a complex pattern of central projections of these afferents on the central respiratory controllers or by intrinsic properties of the motor output of these controllers.