Recruitment and discharge frequency of phrenic motoneurones during inspiration

The discharges of 107 phrenic motor axons were recorded from cats under chloralose-urethane anaesthesia with spinal cords transected at T1 or with intact neuraxis. During inspiratory occlusions in spinal cats, each motoneurone was recruited at a mouth pressure constant at a given end tidal CO2; no motoneurone was recruited at a pressure greater than 70% of maximum. In eupnoea (32.3 torr CO2) 73% of motoneurones were recruited during the first 30% of inspiration; during CO2 rebreathing (60.8 torr CO2), 89% were recruited in the first 30% of inspiration. Neurones recruited earlier in inspiration had a lower onset frequency than later recruited units; all increased instantaneous frequency in a linear relation to pressure. Early recruited units showed a smaller increase in frequency per unit change in pressure than did later recruited units. During CO2 rebreathing, mean and peak frequencies increased on average 0.92 and 1.78 spikes.sec(-1) (%CO2)(-1), respectively, these increases being significantly less for early than for late recruited neurones. The data show that a stable order of recruitment of phrenic motoneurones exists during inspiration, the excitability of each motoneurone likely determining its time of recruitment. Above threshold, later recruited motoneurones are more 'sensitive' to a change in input. Recruitment of motoneurones is responsible for pressure generation at the start of inspiration and increase in discharge frequency (rate coding) is the dominant mechanism in the second half of inspiration.