Motor control on the move: from insights in insects to general mechanisms

This review discusses how the nervous system controls the complex body movements keeping animals up and running. In particular, we revisit how research in insects has shed light on motor control principles that govern movements across the animal kingdom. Starting with the organization and evolution of the insect nervous system, we discuss insights into the neuronal control of behaviors varying in complexity, including escape, flight, crawling, walking, grooming, and courtship. These behaviors share specific control features. For instance, central pattern-generating circuits (CPG), which reside in proximity to the motor neurons and muscles, support the generation of rhythmic motor activity. The number of CPGs involved depends on the complexity of the motor apparatus controlled, such as wing pairs for flight or six pairs of multisegmented legs for walking. The different control architectures are introduced with respect to their organization, topology, and operation. Sensory feedback plays a pivotal role in shaping CPG activity into a functional, well-coordinated motor output. The activity of motor circuits is orchestrated by descending neurons connecting the brain to the ventral nerve cord or spinal cord, which initiate, maintain, modulate, and terminate different actions. We therefore discuss the current understanding of descending control and the contributions of individual, command-like descending neurons and population codes. To highlight how insights in insects help discover fundamental motor control principles, we cross-reference findings from other animals, particularly vertebrates. In addition, we discuss methodological advances that enabled breakthroughs in motor control research, including neurogenetics and connectomics, and discuss key open questions.