Generation of Variants of a Motor Act in a Modular and Hierarchical Motor Network

BACKGROUND Most motor systems can generate a variety of behaviors, including categorically different behaviors and variants of a single motor act within the same behavioral category. Previous work indicated that many pattern-generating interneuronal networks may have a modular organization and that distinct categories of behaviors can be generated through flexible combinations of a small number of modules or building blocks. However, it is unclear whether and how a small number of modules could possibly generate a large number of variants of one behavior. RESULTS We show that the modular feeding motor network of Aplysia mediates variations in protraction duration in biting-like programs. Two descending commands are active during biting behavior and trigger biting-like responses in a semiintact preparation. In the isolated CNS, when activated alone, the two commands produce biting-like programs of either long or short protraction duration by acting specifically on two modules that have opposite effects on protraction duration. More importantly, when coactivated at different frequencies, the two commands produce biting programs with an intermediate protraction duration. CONCLUSIONS It was previously hypothesized that behavioral variants may be produced by combining different activity levels of multiple descending commands. Our data provide direct evidence for such a scheme and show how it is implemented in a modularly organized network. Thus, within a modular and hierarchical architecture, in addition to generating different categories of behavior, a small number of modules also efficiently implements variants of a single behavior.