This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. During the period covered by this report, studies continued on the properties of neurons in the cervical spinal cord. These studies will elucidate the contribution of spinal cord circuits to the control of voluntary movements in normal, behaving primates. Our long-term goal is understand, and overcome, the motor deficits associated with CNS injury or disease. To date, the activity of C3-C4 neurons has been recorded in 3 awake, behaving macaques trained to make wrist, reaching, and isometric head movements. Most neurons whose activity was modulated during the arm behaviors did not respond during isolated head movements. Spike-triggered averages (STAs) of electromyographic activity (EMG) identified functional linkages with muscles. 27% of 316 neurons exhibited significant features in STAs. The majority (64%) of effects straddled or preceded the spike discharge time; their onset latencies were too early to be caused by synaptic connections of the neuron to motoneurons. Most of the spike-related effects were in shoulder and arm muscles. Cross-correlations between smoothed neuronal firing rate and EMG documented the covariation of activity for each task. 61% of the tested neurons had significant peak correlations with EMG, with similar proportions for neck, shoulder, and arm muscles. Our data support a role for C3-C4 neurons in recruiting task-dependent muscle synergies during arm movements. It is also possible that these cells help coordinate the activity of neck and arm muscles during arm movements. The directional tuning of neurons in the C6-T1 segments was determined for multi-dimensional wrist movements before and after the iontophoretic application of GABAergic and glycinergic antagonists. Block of inhibition had little effect on the neurons' directional tuning for the majority of neurons. This suggests that inhibition regulates spinal neuron excitability and gain control, rather than shaping directional tuning.
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