Rubrospinal tract of the cat: superposition of antidromic responses and changes in axonal excitability following orthodromic activity

Abstract

Activity was recorded from rubrospinal neurons (RSNs) in anesthetized, paralyzed, artificially ventilated cats. Multiple-unit microelectrodes were used to simultaneously record the activity of neighboring RSNs. When antidromically activated, the RSNs responded forming ‘stacks’ of superimposed spikes. By using appropriate collision tests, it was found that the spikes forming a stack arose from different neurons. In addition, single extracellular and intracellular recordings were obtained from RSNs. The changes in the axonal excitability of rubrospinal axons were tested following synaptically evoked (by contralateral interpositus (IP) stimulation) and/or directly evoked (by injection of current through the intracellular electrode) action potentials at different postspike delays. Subthreshold stimuli for antidromic activation in absence of orthodromic activity were well suprathreshold for most fibers in a wide range of postspike delays. The supernormal axonal periods were longer-lasting when tested after synaptic spikes (up to an average delay of 100.4 ms; range, 10–500 ms) than after directly evoked spikes (mean delay, 78.8 ms; range, 10–296 ms). If synaptic stimulation fires more RSNs than direct stimulation, then the longer-lasting supernormal periods might be due to the activity of adjacent fibers. An additional increase in external potassium concentration in the vicinity of the axon would explain these results.