## For studying the molecular basis of memory one needs an organism that has large neurons and easily measurable indicator of memory formation

This requirement is met by the sea slug Aplysia californica, which was chosen by the American neurobiologist of Austrian origin Eric Kandel as a model organism for the study of biochemical events that accompany the creation of a memory (Figure 20.7).

Aplysia reacts to mechanical irritation by retracting its gills and the siphon, an organ through which it sucks water into the body. The retraction is a way for the sea slug to protect vital organs from a possible attack by a predator. When the irritation is repeated without a real predator attack, Aplysia stops reacting to it (it remembers that the irritation is not associated with danger). How is this memory stored at the neuronal level? Kandel studied what happens at the level of the connection (synapse) between the neuron and the muscle cell that is responsible for retracting the siphon during memory retention. He used the fact that this reflex can also be studied outside the animal's body and that the studied cells are relatively large, which facilitates experimental work with them. He described a series of biochemical reactions that are triggered in response to repeated stimuli and that "mark" the corresponding synapse, thereby changing its properties and the way it communicates with the target muscle cell. It turned out that these reactions are responsible for the creation of memory traces in other animals, including humans. It was therefore not surprising that Kandel won the Nobel Prize in Physiology or Medicine in 2000.