Active electrophysiological properties of spinal motoneurons in aged cats following axotomy

The present study was designed to examine the effects of the aging process on the response of motoneurons to axotomy. Accordingly, in aged cats using intracellular recording techniques, the electrophysiological properties of axotomized lumbar spinal cord motoneurons were compared with those of control (nonaxotomized) motoneurons. In motoneurons that were subjected to axotomy, there was a reduction in axonal conduction velocity compared to that exhibited by control motoneurons. In addition, there were a number of changes in the configuration of the action potential following axotomy. The amplitude of the spike and the overshoot increased as did the slope of the soma-dendritic spike, whereas the delay between the initial segment and the soma-dendritic spikes decreased. The duration of the action potential's afterhyperpolarization increased; its amplitude remained unaffected although the calculated afterhyperpolarization current decreased. Following the spike, most of the axotomized motoneurons exhibited hyperpolarization undershoots and delayed depolarizations. Axotomized motoneurons exhibited a small decrease in the membrane potential and a reduction in the rheobasic current compared to control cells. The changes in the frequency distribution of axonal conduction velocity, afterhyperpolarization duration, afterhyperpolarization current and rheobase measurements suggest that aged motoneurons dedifferentiated following axotomy. These results indicate that axotomy, in aged motoneurons, results in the disruption of a variety of electrophysiological parameters and that the specific patterns of the responses that occur in axotomized motoneurons of adult cats also emerge in axotomized motoneurons of aged animals.