The functional organization of motor cortex: a multilevel analysis

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Early researchers studying the function of motor cortex initially thought it was a homogenous structure. But my laboratory and others have shown this is not the case. To begin, we asked, "what is the function of motor cortex"? The traditional response is that motor cortex serves to make individual muscles twitch. Yet this long-standing view was based upon experimentation with short-duration stimulation trains (40 ms) that are only capable of eliciting individual muscle twitches. My laboratory recently adopted long-duration stimulation trains that last for the same amount of time it takes to make a purposeful movement. We discovered that there are subsets of highly reproducible and robust reaching movement patterns across the surface of motor cortex. We have named these forelimb reaching areas: elevate, advance, grasp and retract. We then observed that temporarily inactivating the "grasp" region results in deficits in grasping behaviour. This is important because our evidence supports the view that the motor cortex is organized to make purposeful multi-joint movements rather than simple muscle twitches. Thus, our general hypothesis is that the motor cortex is primarily organized around purposeful multi-joint movements. I will advance our knowledge of motor learning and memory by examining 4 objectives; Objective 1 will derive the motor map of multi-joint movements during development and then determine the effects of rearing environment and skilled learning on the movement representations. Objective 2 will determine the role of various sub-areas of multi-joint forelimb movements (elevate, advance, retract) on motor performance and motor learning using cortical cooling in adult rats. Objective 3 will determine the intrinsic and extrinsic anatomical connections underpinning the various multi-joint movement categories in the adult rat. Objective 4 will determine the contribution of the transient receptor potential (TRP) channels on motor learning, the expression of motor representations and layer V pyramidal cell excitability. Thus, I propose to analyze the functioning of motor cortex at four inter-related levels of analysis; a) behaviour of the forelimb, b) motor representations (motor maps) of the forelimb primarily using long-duration intracortical microstimulation which gives rise to multi-joint movement motor maps, c) intrinsic and extrinsic anatomy and d) synaptic ligands and channels using an in vitro slice preparation that my laboratory developed. Significance: The motor cortex is tractable for understanding how the brain regulates complex behaviour. Delineating the network, synaptic and channel properties that underlie the organization of motor cortex will improve our understanding of the fundamental processes that regulate behaviour.