Neuroethological studies of reflex plasticity in intact Aplysia.

Multi-unit recording of siphon nerve activity in intact Aplysia california with chronically implanted cuff electrodes provided a monitor of activity in a central pattern generator, the Interneuron II (Int II) network, which produces large siphon and gill contractions both spontaneously and after tactile stimulation of the siphon. The phase-response curve of the Int II oscillator for single stimuli at different phases of the cycle showed a "refractory" period early in the cycle after which most stimuli phase advanced the oscillator and caused a short-latency Int II burst and a large contraction. The amplitude of gill withdrawal and the duration of siphon withdrawal in response to different stimulus intensities depended on whether an Int II burst was triggered. Activation of the Int II oscillator transformed the reflex from one that was graded smoothly with stimulus intensity to one in which nearly maximal responses were elicited even by weak stimuli. Entrainment and habituation training both involved monotonous repetition of a stimulus at specific intervals. With repeated siphon stimuli, nearly maximal reflex responses were maintained in intact animals as long as the Int II oscillator was entrained, whereas habituation was associated primarily with failure to entrain the oscillator. Long-term sensitization of the reflex was characterized by large and prolonged withdrawal responses. Sensitized animals showed significantly more triggered Int II bursts than did controls. In addition, digital spike-train analysis indicated that individual siphon motoneurons showed significantly increased background activity which often persisted for several minutes.