Subgroups of phrenic motoneurons and their reactions to stimulation of vagal input

In experiments with rabbits anaesthetized with halothane, paralyzed and artificially ventilated the homogeneity of phrenic motoneurons was investigated. It has been shown that there are two main subgroups: 1) "early" motoneurons whose activity continues throughout the whole inspiration at relatively low frequency and amplitude of action potentials. The pattern of firing and the responses of these motoneurons to stimulation of vagal input correspond closely to those of the whole phrenic nerve; 2) the second subgroup is formed by "late" motoneurons firing usually in the later two-thirds of the inspiration at high frequency and amplitude of action potentials, which are highly sensitive to vagal input inhibiting their activity. This subgroup is, presumably, responsible for the shape of the late part of the inspiratory discharge.