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J Neurophysiol (September 13, 2006). doi:10.1152/jn.00414.2006
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Submitted on April 19, 2006
Accepted on September 12, 2006

Forelimb movements and muscle responses evoked by microstimulation of cervical spinal cord in sedated monkeys

Chet T Moritz1, Timothy H Lucas2, Steve I. Perlmutter3, and Eberhard E. Fetz3*

1 Department of Physiology and Biophysics, Univ. of Washington, Seattle, Washington, United States
2 Neurosurgery, Univ. of Washington, Seattle, Washington, United States
3 Department of Physiology and Biophysics, University of Washington, Seattle, Washington, United States

* To whom correspondence should be addressed. E-mail: fetz{at}u.washington.edu.

Documenting the forelimb responses evoked by stimulating sites in primate cervical spinal cord is significant for understanding spinal circuitry and for potential neuroprosthetic applications involving hand and arm. We examined the forelimb movements and electromyographic (EMG) muscle responses evoked by intraspinal microstimulation in three M. Nemestrina monkeys sedated with ketamine. Trains of 3 stimulus pulses (10-80 µA) at 300 Hz were delivered at sites in regularly spaced tracks from C6 to T1. Hand and/or arm movements were evoked at 76% of the 745 sites stimulated. Specifically, movements were evoked in digits (76% of effective sites), wrist (15% of sites), elbow (26%) and shoulder (17%). To document the muscle activity evoked by a stimulus current just capable of eliciting consistent joint rotation, stimulus-triggered averages of rectified EMG were calculated at each site where a movement was observed. Typically, many muscles were coactivated at threshold currents needed to evoke movements. Out of the 13-15 muscles recorded per animal, only one muscle was active at 14% of the effective sites, and two to six muscles were coactivated at 47% of sites. Thus, intraspinal stimulation at threshold currents adequate for evoking movement typically coactivated multiple muscles, including antagonists. Histologic reconstruction of stimulation sites indicated that responses were elicited from the dorsal and ventral horn and from fiber tracts in the white matter, with little somatotopic organization for movement or muscle activation. The absence of a clear somatotopic map of output sites is probably due to stimulation of complex mixtures of fibers and cells.







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