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.