Learning by the Aplysia Model System: Lack of Correlation Between Gill and Gill Motor Neurone Responses

A semi-intact preparation of Aplysia californica was used to monitor simultaneously behavioural and motor neurone responses during classical conditioning of the gill withdrawal reflex. Gill motor neurone responses and gill withdrawal responses were both capable of enhancement in response to the conditioned stimulus after associative training. The neuronal and behavioural responses did not, however, correlate. In 32% of the conditioned (paired) preparations and 27% of the control (unpaired) preparations, the neuronal response was facilitated whereas the gill withdrawal response did not change, or decreased. In addition, amongst those preparations that showed behavioural enhancement, the acquisition of learning of gill withdrawal followed a different pattern from that displayed by the central neurones. This suggests that facilitation of the central sensory-motor neurone synapses is not primarily responsible for conditioning of the gill withdrawal reflex. The gill withdrawal response elicited by direct depolarization of the central motor neurones decreased following the unpaired (control) presentations of the conditioned and unconditioned stimuli, and remained unchanged following paired presentations, suggesting that there is a site of neuronal plasticity in the gill.