Tonically driven and self-sustaining activity in the lamprey hemicord: When can they co-exist?

In lamprey hemisegmental preparations, two types of rhythmic activity are found: slower tonically driven activity which varies according to the external drive, and faster, more stereotypic activity that arises after a transient electrical stimulus. We present a simple conceptual model where a bistable excitable system can exhibit the two states. We then show that a neuronal network model can display the desired characteristics, given that synaptic dynamics—facilitation and saturation—are included. The model behaviour and its dependence on key parameters are illustrated. We discuss the relevance of our model to the lamprey locomotor system.