From snail to man, one of the hallmarks of the brain is the ease with which behavioral variants are generated--for example, humans can easily walk with different stride lengths or different speeds. By studying how a relatively simple motor network of the marine snail Aplysia produces variants of a particular feeding behavior, researchers have found that the ability to generate a large number of behavioral variants stems from the elegant hierarchical architecture of the brains motor network.
Credit: Timothy Kang, Jin-sheng Wu and Jian Jing
Most motor systems are organized into a hierarchy of at least two layers of neurons, with higher-order neurons acting on lower-order neurons, which form a small number of building blocks or modules that produce a variety of behaviors. However, it was not clear how variants of a single motor act are generated in such a hierarchical system.
In the new work, Jian Jing and Klaudiusz Weiss of the Mount Sinai School of Medicine in New York studied the feeding network of Aplysia, which exhibits a biting behavior in response to the presence of food. The researchers showed that within the feeding network, two higher-order neurons that are active during biting behavior employ a combinatorial mechanism to produce variations in one particular movement parameter of the biting behavior. The researchers showed that, tellingly, these higher-order neurons accomplish their roles through their specific actions on two groups of lower-order interneurons that directly influence the particular biting-behavior movement parameter. Therefore, in this system, and likely others, the generation of large numbers of behavioral variants is characterized by higher-order neurons that flexibly combine an "alphabet system" of outputs that are generated by lower-order modules within the brains motor network.
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