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date: 25 June 2017

Lymnaea Respiratory Control

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Neuroscience. Please check back later for the full article.

The first behavioral choice that Lymnaea (the Great Pond snail) makes on hatching from its egg is to move (negative geotaxis) to the surface to perform aerial respiration. At the surface, the snail opens its pneumostome (breathing tube) to perform aerial respiration. That is, it can now take atmospheric air into its lung to help satisfy its respiratory needs. These snails also satisfy their respiratory needs via cutaneous respiration. Dissolved oxygen in pond water is able to move across the snail’s skin. If pond water has sufficient dissolved oxygen the snail does not have to perform aerial respiration. However, Linnaeus, in the 1750s, named this snail Lymnaea stagnalis, because it could easily survive in stagnant water (i.e., low dissolved oxygen levels), because it possesses the ability to perform aerial respiration. This behavior is very obvious as the snails must surface and then open the pneumostome. In fact, there is an audible pop as the snail opens its pneumostome and expels air before it is replaced with atmospheric levels of oxygen. Two exciting facts now have to be addressed. The first is that the neural circuit that drives this behavior has been elucidated. It has been experimentally demonstrated that there is a three-neuron network that mediates this behavior. The three neurons have been shown to be both necessary and sufficient for this behavior. The second finding is that this behavior is modifiable in two manners. One, the frequency of “breathing” changes as the levels of oxygen decrease in the water (i.e., as the pond water becomes hypoxic, aerial respiratory increases). Two, this behavior can be operantly conditioned (a form of associative learning). Moreover, since the neural circuit underlying this behavior has been delineated, it can be shown that changes in the activity of the neurons are causative for the ability to learn and form long-lasting memory. It has been further shown that the ability of snails to learn and form memory is altered by environmentally relevant stressors. Thus, we have a model system in which to study the causative mechanisms of learning and memory and how this is altered by stress.