We plan to investigate the neural interactions generating voluntary muscle activity in primates. In monkeys actively tracking force targets we will analyze activity of neurons 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 simultaneously in the target muscles and cross-correlated with the CM cells. We will determine whether CM cells affect all motor units of a muscle or only specific types, and will quantify their interactions. Similarly, we will record activity of single interneurons in cervical spinal cord (C3 - T1) and document their correlational linkages with forelimb muscles. This will provide the first insight into the activity and output effects of spinal interneurons during normal movements. We will also document synaptic input to these premotoneuronal (Pre-M) interneurons from motor cortex (to test disynaptic links from cortex to motoneurons), and from peripheral receptors (to elucidate their role in movement). Synaptic interactions between motor cortex cells will also be investigated with in vivo intracellular recordings; using extracellular spikes recorded simultaneously from neighboring neurons we will compile spike-triggered averages of membrane potentials. this will document unitary post-synaptic potentials (PSP's) produced between neighboring cortical cells, and allow us to examine changes in the amplitudes of unitary PSP's with natural conditioning depolarizations. We will also investigate longer-range coherent oscillatory activity in sensorimotor cortex. The behavioral function of these oscillations will be tested by documenting the extent and timing of coherent activity during performance of relevant sensorimotor tasks. The underlying synaptic mechanisms will be further analyzed with intercellular recording of membrane potentials. Synaptic interactions between neurons will also be investigated with computer simulations. Realistic ionic currents will be incorporated to model the mechanisms underlying transduction of PSP's into changes in firing probability, and the effect of synaptic noise and synchrony. Dynamic neural network models will be used to simulate mechanisms underlying sensorimotor behavior.
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