Response of ventral respiratory group inspiratory neurons to mechanical loading

The bursting patterns of 32 ventral respiratory group (VRG) medullary inspiratory neurons were studied in response to selected mechanical loads in 68 allobarbital-urethan-anesthetized cats. Mechanical loads consisted of three levels of resistive loads, one elastic load, and tracheal occlusion (TO). The application of each load was manipulated to oppose either inspiration or expiration. Loads were applied for only one inspiration or expiration to prevent changes in chemical drive. All loaded breath unit responses were analyzed and compared with control values for inspiratory time (TI), expiratory time (TE), spikes per burst (SPB), and average firing rate (FR). Inspiratory mechanical loads resulted in statistically significant increases in TI and SPB but only small nonsignificant increases in FR and TE. Expiratory resistive loading produced its predominant effect on TE. The higher expiratory mechanical loads also caused significant increases in the subsequent unloaded TI and SPB. In contrast to expiratory loading, large inspiratory loads did not significantly affect the next unloaded TE. Bilateral cervical vagotomy eliminated the observed neural responses to loading for both inspiratory and expiratory loads. All of the 71 neurons tested with lung inflations and TO at end inspiration showed inhibition (Ra type) rather than facilitation (R beta).