We plan to investigate the neural interactions generating voluntary muscle activity in primates. In monkeys trained to perform isometric force tasks, we will analyze activity of cells with confirmed linkages to wrist and finger muscles. Corticomotoneuronal (CM) cells will be identified by post-spike facilitation of their target muscles in spike-triggered averages of EMG activity. Single motor units, identified by their twitch tension, recruitment threshold and firing pattern will be recorded in the target muscles and cross-correlated with CM cells. We will determine whether CM cells affect all motor units of a muscle or only specific types, and quantify their interaction. Similarly, we will record activity of single Ia afferent fibers in cervical dorsal root ganglia and document their correlational linkages with muscles. Comparing the response patterns of Ia afferents and CM cells during specific active and passive movements will elucidate the role of central vs peripheral pathways impinging on motoneurons. Interactions between CM cells and neighboring cortical cells will also be investigated by cross-correlating their activity during active and passive wrist movements. This will elucidate the intrinsic cortical mechanisms generating controlled muscle activity. Synaptic interactions between motor cortex cells will be further analyzed by intracellular recordings in anesthetized monkeys. Spike-triggered averages would reveal unitary post-synaptic potentials (PSP's) in pyramidal tract neurons produced by identified neighboring cells. The mechanisms underlying transduction of PSP's into changes in firing probability will also be examined for motor cortex cells, and investigated with a computer model. A threshold-crossing model that has simulated empirical observations in motoneurons will be used to study the influence of synaptic noise and PSP parameters on the transform between PSP and cross-correlograms, for both motoneurons and cortical cells.
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