Electrophysiologic analysis of MI pyramidal cells has demonstrated two ways in which motor commands are encoded. First, the force of muscle contraction is directly related to the firing rate of the appropriate pyramidal cell. High-frequency firing of the upper motor neuron stimulates a high-frequency firing rate of the lower motor neuron, causing summation of intracellular responses in the muscle and increased tension. The direction of the intended movement is encoded in a different way; it involves the combined responses of populations of pyramidal cells. Individual pyramidal cells in MI are directionally selective and fire most vigorously in association with initiation of movement in the preferred direction; however, they are only broadly tuned, meaning that increased firing can occur in association with movements that vary 45° either way from the preferred direction. The firing pattern of any one cell alone is thus a poor predictor of movement direction, but the average output of clusters of broadly tuned, directionally selective cells is highly predictive, providing a combined, or population movement vector that matches well with the direction of muscle movement. These clusters of cells are arranged in the cortex so that population vectors are provided for directions covering a full 360°.
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