Control of rhythmic firing in normal and axotomized cat spinal motoneurons

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Abstract
The rhythmic firing of normal and axotomized cat spinal motoneurons to intracellular current injection was studied. The gain for 17 normal motoneurons was constant over 2 regions for all frequency-current (f-I) plots based on the instantaneous frequencies of the first interspike intervals. Plots based on the instantaneous frequencies of the second interspike interval and on adapted rates showed the same pattern. Mean slopes were 1.9 .+-. 0.4 SD impulses s-1 nA-1 for lower primary ranges and 6.9 .+-. 1.0 SD impulses s-1 nA-1 for secondary ranges. In axotomized motoneurons, where the threshold for dendritic spikes was lowered, f-I plots had only 1 region of constant gain. The mean slopes of these regions of constant gain (e.g., 7.2 .+-. 1.4 SD impulses s-1 nA-1 for adapted firing rates) were similar to responses in the secondary range of normal motoneurons. The patterns of trajectory changes in axotomized motoneurons were very similar to those of normal motoneurons firing in the secondary range and quite unlike trajectories associated with primary-range firing in normal motoneurons. The presence of high gain f-I plots may indicate dendritic spiking in normal motoneurons.