Relationship between parasternal and external intercostal muscle length and load compensatory responses in dogs

1. The effects of tracheal occlusion on peak parasternal (PA) and external intercostal (EI) (3rd interspace) EMG activities were examined at different end-expiratory lung volumes both above and below functional reserve capacity (FCR) in anaesthetized, vagotomized and spontaneously breathing dogs. 2. Parasternal (PA) and external intercostal (EI) muscle lengths were monitored in situ. The difference in peak EMG activity between free and occluded breaths (test breaths) was related to the coincident peak change in intercostal muscle length (delta L) for each muscle, respectively. 3. At FRC, tracheal occlusion resulted in compensatory augmentation of peak EI, but little change in peak PA EMG activities. At lung volumes below FRC, airway occlusion resulted in augmentation of both PA and EI activities. Responses to airway occlusion at lung volumes above FRC were variable. The magnitude and duration of these changes in EMG, however, could be linearly related to the value of delta L. With delta L = 0, there was no change in peak EI or PA EMG; for values of delta L less than 0, there was attenuation of EI and PA EMG; for delta L greater than 0, there was enhancement of EI and PA EMG activation. 4. The magnitude of the changes in EMG activity in response to tracheal occlusion was more prominent for the EI muscle compared to the PA, the latter of which are known to have much fewer muscle spindles than EI muscle. 5. Our results suggest that a difference in end-inspiratory muscle length between the control and occluded breaths is a stimulus for the intercostal response to applied loads implicating muscle spindles as the predominant receptor moderating these responses. We hypothesize that when delta L = 0, no change in EMG occurs since the spindles sense no change in muscle length. When delta L less than 0 (i.e. peak muscle length during the occluded breath is shorter than control) muscle spindles would be disengaged, resulting in a disfacilitation of EMG activity. Where delta L greater than 0 (i.e. peak muscle length during the occluded breath is longer than control), muscle spindles are stimulated, resulting in enhancement of EMG activity. 6. Additional doses of Nembutal (20 mg), which produced significant changes in breathing pattern, did not affect the magnitude of the load compensatory responses.