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Trainin E, Meir R, Karniel A. Explaining patterns of neural activity in the primary motor cortex using spinal cord and limb biomechanics models. J Neurophysiol 97: 3736 -3750, 2007. First published March 14, 2007; doi:10.1152/jn.01064.2006. What determines the specific pattern of activation of primary motor cortex (M1) neurons in the context of a given motor task? We present a systems level physiological model describing the transformation from the neural activity in M1, through the muscle control signal, into joint torques and down to endpoint forces and movements. The redundancy of the system is resolved by biologically plausible optimization criteria. The model explains neural activity at both the population, and single neuron, levels. Due to the model's relative simplicity and analytic tractability, it provides intuition as to the most salient features of the system as well as a possible causal explanation of how these determine the overall behavior. Moreover, it explains a large number of recent observations, including the temporal patterns of single-neuron and population firing rates during isometric and movement tasks, narrow tuning curves, non cosine tuning curves, changes of preferred directions during a task, and changes of preferred directions due to different experimental conditions. I N T R O D U C T I O NThe primate motor system is a highly complex system leading to sophisticated motor activities resulting from the concerted activity of many cortical, sub-cortical and skeletal modules involving multiple feedback loops (Dum and Strick 2005). One of the key components in this system is the primary motor cortex (M1), which plays a major role in voluntary limb movement. Projections from M1 influence muscles through direct synapses onto motorneurons and indirectly through spinal inter-neurons (Kandel et al. 2001). It is now well understood that M1 is highly heterogeneous (Alexander and Crutcher 1990; Ashe 2005;Crutcher and Alexander 1990;Kakei et al. 1999) and that different populations in M1 represent different motor control signals and therefore cannot be uniformly interpreted. However, physiology provides evidence that some M1 neurons are highly correlated with muscle activity (e.g., Fetz and Cheney 1980;Morrow and Miller 2003). We refer to such neurons as muscle-related cells. Our model is aimed at explaining the functional behavior of these muscle related cells in M1 in the context of arm movement.Many statistical models (e.g., Georgopoulos et al. 1982;Paninski et al. 2004;Sanger 1996) describe how neuronal activity of single neurons changes with hand variables and play an important role in neural prosthetic applications (Schwartz 2004). However, these approaches do not provide answers to the following basic questions. How do neural activities, projected from the motor cortex toward the spinal cord, result in hand movement and force? Given the redundancy of the controlled system (Bernstein 1967), how does the brain select specific control signals to achieve a motor task? The model provided here di...