A significant goal in studies on learning and memory is to relate cellular plasticity to the modification of behaviour. The phenomenon of dual-process learning affords an ideal opportunity to explore the complexities inherent in establishing this relationship. Dual-process learning occurs when depression (habituation) and facilitation (sensitization) are expressed simultaneously within a neural network and compete to determine the behavioural outcome. A large body of literature is reviewed to define characteristics which are common across the neural networks that exhibit dual-process learning: depression occurs at loci early in the reflex pathway, upstream of the modulatory system necessary for the induction of facilitation. Consequently, depression not only competes directly with facilitation for the determination, of behavioural change (by serial and/or parallel expression), but depression also precludes the ongoing development and maintenance of sensitization (by serial induction). A mathematical model is presented to formally describe the nature of this competition and how this competition leads to the kinetics of dual-process learning. The tentacle withdrawal reflex of the snag Helix aspersa exhibits dual-process learning and was further investigated in this study. The neural circuit mediating tentacle withdrawal is described along with the nature and the location of plasticity which occurs within that circuit. In turn, plasticity at the cellular level is related, via the network level, to plasticity at the behavioural level. The data demonstrate the importance of localizing the sites of plasticity within a neural network in order to explain (1) how plasticity at a particular locus influences plasticity occurring elsewhere in the network and (2) how plasticity at different loci affect different aspects of behaviour.

No description available.

The morphology of C3, a motoneuron mediating the tentacle withdrawal reflex, was investigated in the snail Helix aspersa by intracellular injections of the tracers Neurobiotin and biocytin. Axonal projections were identified in the optic nerve, the olfactory nerve, the internal peritentacular nerve, the external peritentacular nerve, the cerebral-pedal connective and the cerebral commissure. A rare characteristic of the cell was the multibranching of axons in the neuropil and the exiting of this bundle of fibres into the cerebral-pedal connective. Dendritic arborizations were observed branching from the cell body, the axon hillock and the dorsal main axon. In addition, tufts of dendrites were seen to branch from the ventral axon. Based on its morphology, C3 is probably a central component in the avoidance behaviour, receiving sensory input at extensive dendritic sites and sending axons to a number of key effector sites to co-ordinate the chain of reactions that constitutes the snail's avoidance behaviour.

No description available.