Modeling the lowering of motoneuron voltage threshold during fictive locomotion.

The excitability of lumbar motoneurons in the cat is increased during fictive locomotion induced by stimulation of the mesencephalic locomotor region (MLR). This change in excitability has previously been demonstrated experimentally. During fictive locomotion: (1) the afterhyperpolarization (AHP) is reduced and (2) the voltage threshold for production of an action potential is lowered. The excitability of motoneurons is determined by intrinsic properties of the cells that are modulated by the premotoneuronal locomotor network. In this computer simulation study we focus on the lowering of the voltage threshold for an action potential and the accompanying reduction of the AHP. The purpose of this paper is to explore the potential ionic conductances responsible for the increase in motoneuron excitability during fictive locomotion.