Segregation of nociceptive and non-nociceptive networks in the squirrel monkey somatosensory thalamus

The somatosensory thalamus (here we examine neurons in the caudal cutaneous portion of ventral posterior lateral nucleus, VPL) is composed of a somato topic arrangement of anteroposteriorly oriented rods. Each rod is a collect ion of neurons with homogeneous properties that relay sensory information t o specific cortical columns. We developed a multi-electrode recording techn ique, using fixed-geometry four-tip electrodes that allow simultaneous reco rdings from small populations of neurons (4-11), in a similar to 150 x 150 x 150 mu m(3) volume of brain tissue (i.e., the approximate diameter of rod s) and study of their spatiotemporal interactions. Due to the fixed geometr y of the four-tip electrodes, the relative locations of these neurons can b e determined, and due to the simultaneity of the recordings, their spike-ti ming coordination can be calculated. With this method, we demonstrate the e xistence of two distinct functional networks: nociceptive and non-nocicepti ve networks. The population dynamics of these two types of networks are dif ferent: cross-correlations in each type of network were different in direct ion and strength, were a function of the distance between neurons, had an o pponent organization for nociceptive networks and a non-opponent organizati on for non-nociceptive networks, and rapidly changed under different stimul us conditions independent of changes in firing rates. A simple neural netwo rk model mimicked these physiological findings, demonstrating the necessity of inhibitory interneurons and different amounts of afferent input synchro nization. Based on these results, we conclude that the somatosensory thalam us is composed of two modules, nociceptive and non-nociceptive rods, and th at the response dynamics differences between these modules are due to spati otemporal differences of their afferent inputs.