Intracellular recordings were made from motoneurones with axons in the intercostal nerves of T9 or T10 in adult rats, with neuromuscular blockade and artificial ventilation, under hypercapnia and under either anaesthesia or decerebration. In nearly all motoneurones, central respiratory drive potentials (CRDPs) were seen, which included an excitatory wave in inspiration, in expiration, or in both of these. This was the case both for motoneurones with axons in the internal intercostal nerve (n = 81) and for those with axons in the external intercostal nerve (n = 5). In the decerebrates, motoneurones with purely inspiratory CRDPs were rare (1/44), but those excited in both phases (showing biphasic CRDPs) were common (22/44). For about one-third of biphasic CRDPs (11/30), the inspiratory depolarization was seen to reverse to a hyperpolarization when the motoneurone was depolarized, which was interpreted as indicating concurrent inhibition and excitation during this phase. A few motoneurones were seen where depolarization revealed signs of inhibition in both phases. The results confirm the novel observations of biphasic excitation in individual intercostal nerve branches, EMG sites and motor units reported in a companion paper. They also provide new insights into the functional roles of inhibition in motoneurones physiologically activated in natural rhythmic behaviours.
