INTRINSIC AND EXTRINSIC FACTORS AFFECTING PHRENIC MOTONEURONAL EXCITABILITY IN NEONATAL RATS

We examined intrinsic and extrinsic factors affecting phrenic motoneur on (PMN) excitability in neonatal rats. Using an in vitro brainstem-sp inal cord en bloc, 127 PMNs were recorded under whole-cell patch-clamp conditions. Inspiratory synaptic drives and passive membrane properti es, including whole-cell membrane capacitance (C-m), input resistance (R-n), and time constant (tau), were measured with either voltage- or current-clamp techniques. On the basis of firing behavior during Inspi ration, two types of PMNs could be distinguished: active (107/127 = 84 %) and silent PMNs (20/127 = 160io). Active PMNs always produced multi ple spikes during inspiration, while silent PMNs remained silent for m ost inspiratory cycles. Compared to silent PMNs, active PMNs had signi ficantly higher R-n, inspiratory drive potential, and more depolarized resting membrane potential (RMP). With respect to inspiratory drive c urrent, no significant difference was observed between the two types o f PMN. Analysis of action potential waveforms did not show a significa nt difference between their threshold levels. Our results suggest that in addition to size-related properties, RMP determines the recruitmen t of PMNs and consequently, of motor units in lire diaphragm. (C) 1997 Elsevier Science B.V.