Firing properties of spinal interneurons during voluntary movement. II. Interactions between spinal neurons.

The relationship between the activity of pairs of simultaneously recorded spinal interneurons (INs) in the cervical enlargement was studied in five monkeys performing voluntary wrist movements. The tendency for INs to exhibit similar response properties and synchronized firing was tested as a function of physical distance between the cells and their correlational linkages with forearm muscles. Nearby INs tended to have more similar torque and direction turning (signal correlation) and more similar response profiles (e.g., tonic vs phasic firing) than INs that were far apart. This suggests that nearby cells receive common synaptic input. In contrast, the trial-to-trial covariation of rate around the mean rate for all trials (noise correlation) was independent of the distance between the neurons. Furthermore, signal and noise correlation were independent, suggesting different underlying mechanisms. Surprisingly, spike-to-spike correlation between INs was relatively infrequent and weak, as measured by cross-correlation histograms. In contrast, single motor units (SMUs) in forearm muscles fired more synchronously, particularly for SMUs in single extensor muscles. Either common drive to INs is too weak to induce synchronized firing, or there is an active decorrelation mechanism within IN networks.