We have used a combined behavioral and cellular neurophysiological analysis to examine the relationship of sensitization to dishabituation of the gill-withdrawal reflex in Aplysia. The reflex withdrawal of the gill to tactile stimulation of the siphon or the purple gland (at the edge of the mantle shelf) shows habituation, dishabituation and sensitization. We have found that the purple gland and siphon provide independent afferent pathways each capable of eliciting the gill-withdrawal reflex. Habituation of one pathway did not affect the other, but a common 'dishabituatory' stimulus produced dishabituation of the habituated pathway as well as sensitization of the non-habituated pathway. These findings support the idea that dishabituation is not due to the removal of habituation but is an independent facilitation superimposed upon habituation. Our neurophysiological analysis showed that, on the cellular level, the neural correlates of sensitization and dishabituation are different reflections of a common heterosynaptic facilitatory process involving an incteased effectiveness of excitatory synaptic transmission at the synapse between sensory and motor neurons.
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