Central respiratory drive and recruitment order of phrenic and inspiratory laryngeal motoneurones.

Central respiratory drive and recruitment order of phrenic motoneurones and inspiratory laryngeal motoneurones were studied in anaesthetized, paralyzed and artificially ventilated cats. Unitary activities of pairs of motoneurones originating from the same population were recorded on thin filaments. Differences of recruitment during inspiration enabled distinction to be made between early (E) and late (L) recruited motoneurones. 'On-line' cross-correlation analyses were performed on either homogeneous pairs (2E or 2L) or heterogeneous pairs (1E and 1L). In 14/30 pairs of phrenic motoneurones and 26/32 pairs of inspiratory laryngeal motoneurones cross-correlation analysis revealed a bell-shaped increase of probability of firing whose characteristics indicate synchronization by central shared excitatory inputs originating from central respiratory drive. For phrenic motoneurones synchronization appeared mainly in homogeneous pairs (2E:7/11; 2L:5/9) whereas motoneurones of heterogeneous pairs rarely shared the same central respiratory drive (2/10). For inspiratory laryngeal motoneurones, cases of synchronization were equally obtained in homogeneous (2E:8/10; 2L:14/16) and heterogeneous (4/6) pairs. These results suggest that differences of recruitment order during inspiration are related: (i) for phrenic motoneurones, partly to differences of excitability but mainly to a dual central respiratory drive which is assumed to divide the population into two components; (ii) for inspiratory laryngeal motoneurones to differences of excitability since all the motoneurones share the same drive.