THE BASIS FOR REPETITIVE ACTIVITY IN PHRENIC MOTONEURONS

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Abstract
The changes in excitability in phrenic motoneurons of the cat which result from activity were studied by quantifying the electrical response of the nerve to testing stimuli delivered at intervals after preliminary conditioning stimuli. Conditioning and testing stimuli were variously applied to the inspiratory center in the medulla oblongata, the spinal respiratory pathways and antidromically to the phrenic nerve. The electrical response of the nerve was recorded oscillographically. A single shock, applied to the inspiratory center in the medulla oblongata led to the discharge of impulses over spinal respiratory pathways for periods of 30 m.sec. or more. If the stimulus was weak and applied during expiration, it caused few phrenic neurons to respond, but facilitated those neurons to subsequent shocks if they followed the first at intervals of less than 30 m.sec. A strong stimulus produced this same facilitation, but since it caused large numbers of phrenic neurons to respond, it initiated sub-normality in those neurons. For an initial period of 20 m.sec, the more short-lived facilitation outweighed sub-normality, but the latter dominated the picture during the succeeding 100 m.sec. Facilitation largely resulted from the continued delivery of impulses from center to moto-neuron as a result of delay pathways or reentry circuits within the center. Spinal interneuron repetitive activity plays a much less prominent role. Subnormality is mainly resident within the phrenic motoneurons. The repetitive discharge of phrenic neurons which characterizes normal inspiratory activity may be explained in terms of a balance between the degree of excitation of those neurons and their rates of recovery of excitability.

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