Restoration of function in external intercostal motoneurones of the cat following partial central deafferentation

The activity of external intercostal motoneurones in the cat was studied under anaesthesia and paralysis before and after partial central deafferentation caused by single or double ipsilateral hemisections of the thoracic spinal cord. The normal efferent inspiratory discharges recorded from external intercostal nerve filaments caudal to the upper lesion were greatly reduced acutely, but activity of approximately normal intensity and phase at eupneoic levels of CO2 was restored within a few days and remained at similar levels for up to two years. The patterns of the restored activity were abnormal, with more discharges of alpha-motoneutrones during expiration than normal and a stronger modulation of the discharges by the respiratory pump than normal. A common abnormal component of the restored activity was a tonic discharge in hypocapnic apnoea, often modulated by the respiratory pump. This activity was never seen in normal animals or in those with acute lesions under similar conditions of anaesthesia. Synchronization of the discharges of alpha-motoneurones caudal to the upper lesion was studied by constructing cross-correlation histograms between paired groups of motoneurones, each group being represented by the discharges in one filament. Synchronization was stronger than normal, usually extending over a time course of +/- 20 to +/- 50 ms (broad-peak synchronization). This synchronization was particularly strong for the discharges in hypocapnic apnoea. We conclude that the restored activity was derived in large part from abnormal tonic (non-respiratory-phased) inputs, partly proprioceptive in origin, probably involving spinal cord interneurones with abnormally synchronized discharges. This conclusion is supported by intracellular measurements including respiratory drive potentials, synaptic noise and average common excitation potentials.