Changes in the distribution of synaptic potentials from bulbospinal neurones following axotomy in cat thoracic spinal cord

1. Plasticity in functional connections of expiratory bulbospinal neurones was investigated by measurement of terminal potentials (TPs) and focal syna ptic potentials (FSPs), recorded with spike-triggered averaging in the thor acic spinal cord of anaesthetized, paralysed cats. These measurements were made in normal cats and in those which had previously been subjected to spi nal cord lesions that transected the axons of the bulbospinal neurones in t he segment below that under investigation, either about 2 weeks or about 16 weeks previously. 2. In both groups of operated animals bulbospinal neurones with firing prop erties and conduction velocities similar to normal were present. The extrac ellular recordings that were averaged to reveal TPs and FSPs were made on t wo standard grids, each consisting of eight sites spaced 0.25 mm apart on t wo electrode tracks. One grid was positioned at a rostral and one at a caud al location within one segment (T7-T9). 3. In the normal animals TPs and FSPs were larger and/or more common at ros tral sites than at caudal sites, by a factor of about 1.7. In both 2 week a nd 16 week animals, TPs and FSPs were observed, both showing normal time co urses and latencies. At rostral sites in both the 2 week and 16 week animal s the amplitudes and/or the frequency of occurrence of TPs and FSPs were si milar to normal, as was the case for caudal sites in the 2 week animals. Ho wever, at caudal sites in the 16 week animals the FSPs were more common and /or significantly larger than normal, with the increase particularly marked on the lateral track, being equivalent to a factor of about 2. A correspon ding increase in the amplitude and/or frequency of occurrence of TPs at cau dal lateral sites was also seen, but was not significant. 4. The results are interpreted as evidence for short-range sprouting of the bulbospinal axons and the formation by them of new connections in the caud al parts of the segments concerned.