Key points The processes whereby various excitatory and inhibitory inputs are integrated in spinal motoneurones during naturally occurring motor acts are not well understood, largely because there are amplifying mechanisms within the motoneurone that can control the effective strengths of the inputs. Knowledge of these processes is important in understanding conditions such as motoneurone disease, or the spasticity that can follow spinal cord injury or stroke Respiration is a natural motor act that continues normally under experimental conditions, and this study investigated, for the first time, the likely amplifying processes at work in respiratory motoneurones. In phrenic motoneurones, which control the most important respiratory muscle, the diaphragm, we found that the mechanism most favoured by investigations in other motoneurones, the activation of persistent inward currents via calcium channels, appears to make a very small contribution. Instead, modulation of synaptic currents (through NMDA channels) appears to be more important.