Integration of reflex activity and central pattern generation in intact Aplysia

This paper reviews analog and digital studies of the population of siphon motoneurons involved in a defensive withdrawal reflex that is activated by a tactile stimulus to the siphon of Aplysia californica. Multiunit activity was monitored during normal behavior by implanting noninvasive cuff electrodes on the siphon nerve of intact animals. The siphon motoneuron population integrates activity originating in distributed reflex pathways and in a local oscillatory circuit called the Interneuron II (INT II) network, a central pattern generator (CPG) that spontaneously produces large siphon and gill contractions. In intact animals (but not in reduced preparations) a novel tactile stimulus to the siphon phase advances the spontaneously occurring burst of activity in the CPG (on about 60% of trials), thereby transforming the reflex from one that is graded with stimulus intensity to one that is nearly maximal even with weak stimuli. When a series of stimuli are given, reflex decrement (habituation) results, in large part, from a failure to stably entrain the CPG component. When a noxious stimulus is presented across days, long-term sensitization results in large and very prolonged withdrawal responses. The increase in reflex amplitude is due, in part, to heightened entrainment of the CPG (about 90% of trials). Digital spike train analysis revealed an additional factor associated with the prolonged responses of sensitized animals; increased tonic background firing in the motoneuron population that could last for several minutes.