| A fundamental goal in the fields of psychology and neuroscience is an understanding of the neural mechanisms of learning and memory. The marine mollusc Aplysia californica has been a valuable model system for exploring the mechanisms of simple forms of learning, such as sensitization and classical conditioning, that are expressed in defensive withdrawal reflexes. Forms of learning that involve non-reflexive behaviors are important as well, but have received comparatively little attention in Aplysia. This dissertation examines learned modulation of head waving (a spontaneously occurring exploratory behavior of Aplysia) produced by aversive stimulation of the anterior tentacles.; Behavioral experiments on intact, freely behaving animals demonstrated that training with electric shock as an aversive unconditioned stimulus (US), delivered unilaterally to the anterior tentacles on the head of the animal, produces a learned shift in head waving away from the side on which shock was applied. This behavioral change represents a novel form of learning, which is influenced by the topographic location of an aversive stimulus.; Cellular experiments, conducted in reduced preparations, examined plasticity in cerebral ganglion sensory neurons (the J/K cluster neurons), which constitute an afferent pathway for the aversive US (anterior tentacle stimulation). Plasticity in these neurons, particularly reflecting side-specific or site-specific alterations, could be involved in mediating aspects of the learning induced by tentacle shock. The results of these cellular experiments show that these neurons exhibit several forms of plasticity: (1) side-specific spike broadening, (2) post-tetanic potentiation (PTP), (3) heterosynaptic facilitation, and (4) activity-dependent synaptic facilitation.; Moreover, by activating sensory neurons directly to produce PTP, and producing heterosynaptic facilitation by stimuli to the tentacles that either do or do not activate particular sensory neurons, it was possible to dissect and analyze the differential contribution of intrinsic activity and heterosynaptic modulation in activity-dependent facilitation induced by behaviorally relevant stimuli. Collectively, these data raise the possibility that plasticity in primary afferent sensory neurons may be involved in US processing during learning induced by tentacle stimulation. |
