Starting in the 1960s the answers to intriguing questions such as this began to be worked out at the cellular and biochemical level. Part of this watershed of new insight into the basis of learning and memory came about as a result of the insight to capitalize on easily studied, simple forms of learning in special preparations that lent themselves to experimental investigation at the cellular level. In particular, the work of Eric Kandel and his colleagues allowed enormous progress in our understanding of the cellular basis of behavior, and learning and memory specifically. Kandel, along with Jimmy Schwartz, Vince Castellucci, Jack Byrne, Tom Carew, Bob Hawkins, and many others have used the simple marine mollusk Aplysia californica to great effect to study the behavioral attributes and cellular and molecular mechanisms of learning and memory.
Much (but by no means all) of the work in Aplysia has been geared toward understanding the basis of sensitization in this animal. Aplysia has on its dorsum a respiratory gill and siphon complex, which is normally extended when the animal is in the resting state. If the gill or siphon is lightly touched (or experimentally squirted with a Water-Pic), a defensive withdrawal reflex is elicited to protect the gill from potential damage. This defensive withdrawal reflex can undergo both habituation (by repeated modest stimuli) and sensitiza-tion. Sensitization occurs when the animal receives an aversive stimulus, for example a tail shock. After sensitizing stimulation, the animal exhibits a more robust, longer-lasting gill-withdrawal in response to the identical light touch or water squirt. Acquisition of this sensitization response is graded; repetitive sensitizing stimuli can give sensitization lasting minutes to hours (one to a few shocks), or weeks (repeated training trials over a few days).
Progress in beginning to understand this memory system came by way of mapping certain aspects of the neuronal circuitry underlying the defensive withdrawal reflex and the associated modulatory inputs from the tail. One appeal of the Aplysia experimental system was the relatively simple nervous system in the animal, allowing the tracing of significant parts of the circuitry underlying the behavior using electro-physiology techniques. This circuit tracing was greatly facilitated by the enormous (relatively speaking) size of the neurons in Aplysia, allowing for easy microelectrode recording from specific, identified neurons in the animal's CNS. Ironically, the critical locus for the memory of sensitization resides for the most part in the smallest neurons in the animal.
A greatly simplified diagram of the circuitry underlying sensitization of the gill- and siphon-withdrawal reflex in Aplysia is given in Figure 5. The touch to the gill and siphon complex stimulates siphon sensory neurons, which make direct and indirect (via interneurons) connections to gill motor neurons. The gill motor neurons stimulate muscles in the gill-and-siphon complex that mediate the defensive
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