Bistable characteristics of motoneurone activity during DOPA induced fictive locomotion in spinal cats.

Bistable motoneuronal firing characteristics were investigated in anaemically decapitated paralysed high spinal cats during DOPA induced fictive locomotion. The activity of alpha-motoneurons was recorded intracellularly and/or from identified ventral root filaments or peripheral nerves. During stable fictive locomotor rhythmic activity the long lasting motoneuronal activity during the active phase of a motoneurone pool revealed indications for the development of bistable properties: (1) motoneuronal membrane potentials switched between two levels. Depolarization during active phases was characterised by stable fixed levels and could partly anticipatedly be initiated by excitatory inputs and terminated by inhibitor inputs. (2) Discharge patterns of motoneurones resembled patterns occurring during bistable firing behaviour. (3) Occasionally motoneuronal activity was switched 'on' during a certain phase of a step cycle but was not switched 'off' at the end of that phase but at the end of a corresponding phase several step cycles later. It is assumed, that the bistable firing pattern of the motoneurones during DOPA induced fictive locomotion is generated by plateau potentials. This mechanism would reduce the need for synaptic drive to motoneurones during locomotion. Motoneurones would not require sustained excitatory input from the rhythm generator during their active phase but a short-lasting initial excitation and a terminating inhibition.