Summation of afterhyperpolarization after a doublet in fast motoneurones of the rat spinal cord.

Abstract

Motoneurones located in lumbar segments (L4-L5) of the rat spinal cord were activated antidromically by stimulation of the sciatic nerve. Records following a single pulse and double stimuli were collected from fast motoneurones and in each case voltage and time parameters of the action potential and afterhyperpolarization (AHP) were compared to respective values calculated for a simulated doublet - obtained by mathematical summation of two individual single potentials. The most prominent differences between experimentally recorded and simulated potentials concerned spike duration and the AHP half-decay times, with significantly lower values obtained for the simulated doublets. Tendencies for a higher AHP amplitude, shorter AHP peak time and higher AHP area were observed in mathematically modeled doublets, though no statistical differences were present in comparison to experimental data. Results indicated a non-linear summation of the AHP after double stimuli running in a short interval. These observations suggest that alterations in the generation of the AHP, mainly based on the sustained activity of K­Na+ channels, influence time-intervals in motoneuronal discharge pattern and this way force development of individual motor units during muscle activity.