The rodent vibrissal system as a model to study motor cortex function

Abstract

The function of mammalian motor cortex was one of the first problems studied in neuroscience. But until today, the major principles of the workings of motor cortex have remained conjectural. It is clear that motor cortex holds a topographic map of body parts. However, does that necessarily imply that motor cortex itself undertakes the challenging task of converting movement plans (i.e. intended trajectories and effects of actions) into low level motor commands appropriate for driving the muscles? Many decades of research on motor function has shown that this is not entirely true by revealing the existence of dedicated networks, the socalled central pattern generators (CPGs). Many, if not all of them, are located subcortically, and are likely to take over this task. Unfortunately the detailed circuitry and cellular elements of CPGs are only vaguely known. More recent work has elucidated continuous as well as discontinuous (discrete) mapping of motor cortex to movement. In the quest to understand motor cortex-CPG interactions, discontinuities are important because they allow us to dissect how neighboring motor cortex sites connect to different CPGs for different purposes-driving the very same muscles. The rodent whisker motor system is a decidedly modular system. Neighboring cortical areas drive very distinct whisker movements used by the animals in different contexts. We review the state of art in this system and argue that the modularity of the whisker system together with its great accessibility makes it a promising candidate for a model system for the investigation of motor cortex-CPG interactions on the cellular and network level-a highly valuable tool for the subsequent understanding of the more complex and continuously organized motor cortex of the arm/hand/finger system in primates.