Ionic Mechanisms Underlying Bistability in Spinal Motoneurons: Insights from a Computational Model

Spinal motoneurons are the final output of spinal circuits that engage skeletal muscles to generate motor behaviors. Many motoneurons exhibit bistable behavior, alternating between a quiescent resting state and a self-sustained firing mode. This bistability is traditionally attributed to plateau potentials, which are driven by persistent inward currents. This intrinsic property is important for normal movement control, but can become dysregulated, causing motor control deficits, like spasticity. Here, we use a conductance-based single-compartment model to investigate the ionic conductances underlying the bistable behaviour of motoneurons. Our simulations demonstrate that the motoneuron bistability and how its emergence is regulated mainly depends on the interplay between several intrinsic ionic mechanisms. In particular, the calcium-activated nonspecific cation current ( I _CAN ), which is amplified by I _CaL and calcium-induced calcium release (CICR), primarily drives the plateau potential to sustain bistability. Additional modulation is provided by the persistent sodium current ( I _NaP ) and the calcium-dependent potassium current ( I _KCa ). This study provides a mechanistic model of motoneuron bistability, offering insights into its disruption in pathological conditions and setting the stage for future research and therapeutic development.