Central determination of recruitment order: intracellular study of phrenic motoneurons.

Simultaneous recordings were made intracellularly from phrenic motoneurons (PMs) and extracellularly from their central drivers, the inspiratory bulbospinal neurons (IBSNs) of the dorsal respiratory nucleus. On the basis of their order of recruitment from the beginning of inspiration (as estimated by the phrenic discharge). PMs and IBSNs were classified as early (E) or late (L) units. Unitary excitatory postsynaptic potentials (EPSPs) evoked in the PM by the IBSN were frequently observed between IBSNs and PMs with a similar recruitment, both E, 11/28 or both L, 10/13, and were scarce between IBSNs and PMs with different recruitment. E and L, 8/28, or L and E, 2/11. Since measured membrane resistances of E and L PMs were not statistically different, the recruitment order of PMs may be considered as mainly determined by the timing (E or L) of the central drive that they received.