Intrinsic and extrinsic factors affecting phrenic motoneuronal excitability in neonatal rats

We examined intrinsic and extrinsic factors affecting phrenic motoneuron (PMN) excitability in neonatal rats. Using an in vitro brainstem-spinal cord en bloc, 127 PMNs were recorded under whole-cell patch-clamp conditions. Inspiratory synaptic drives and passive membrane properties, including whole-cell membrane capacitance (Cm), input resistance (Rn), and time constant (tau), were measured with either voltage- or current-clamp techniques. On the basis of firing behavior during inspiration, two types of PMNs could be distinguished: active (107/127 = 84%) and silent PMNs (20/127 = 16%). Active PMNs always produced multiple spikes during inspiration, while silent PMNs remained silent for most inspiratory cycles. Compared to silent PMNs, active PMNs had significantly higher Rn, inspiratory drive potential, and more depolarized resting membrane potential (RMP). With respect to inspiratory drive current, no significant difference was observed between the two types of PMN. Analysis of action potential waveforms did not show a significant difference between their threshold levels. Our results suggest that in addition to size-related properties, RMP determines the recruitment of PMNs and consequently, of motor units in the diaphragm.