2 TYPES OF PLASTICITY IN THE TENTACLE WITHDRAWAL REFLEX OF HELIX-ASPERSA ARE DISSOCIATED BY TISSUE LOCATION AND RESPONSE MEASURE

The tentacle withdrawal reflex of the terrestrial snail Helix aspersa was studied in vitro. The reflex is evoked by mechanical stimulation o f the nose. Lesion experiments showed that 45% to 75% of the response amplitude is attributable to peripheral pathways alone. The central co ntribution increases with increasing stimulus intensity. Repeated stim ulation produced pure habituation at low stimulus strengths, and habit uation mixed with intrinsic sensitization (warm-up effect) at high sti mulus strengths. The simultaneous occurrence of habituation and sensit ization is consistent with the dual process theory of plasticity. Addi tional results differentiate the two processes. Habituation can occur without the CNS, whereas intrinsic sensitization requires the CNS. Als o, the two processes are differentially effective in their influences on response amplitude and duration: habituation is more effective in d etermining response amplitude, while sensitization is more effective i n determining response duration. Although the establishment of sensiti zation requires the CNS, 81% of the memory for intrinsic sensitization was localized to the periphery, by lesion experiments. Extrinsic sens itization, caused by stimulation of the medial lip nerve, had similar behavioural effects and a similar memory locus. Both types of sensitiz ation appear to be caused by neuromuscular facilitation mediated by a central pathway.