Movement induced modulation of afferent transmission to single neurons in the ventroposterior thalamus and somatosensory cortex in rat.

Single neurons were simultaneously recorded in the forepaw areas of the primary somatosensory (SI) cortex and ventroposterolateral (VPL) thalamus of awake rats during rest and running behaviors. Movement dependent changes in somatic sensory transmission were tested by generating post-stimulus histograms of these neurons' responses to stimulation through electrodes chronically implanted under the skin of the forepaw, while the animal ran on a timed treadmill. As viewed in post-paw-stimulus histograms, the evoked unit responses (EURs) could be differentiated into short (4.5 +/- 0.1-10.9 +/- 0.2 ms) and longer (12.9 +/- 0.4-31.3 +/- 0.9 ms) latency components ("SEURs" and "LEURs", respectively). The magnitudes of firing during these responses were measured and normalized as percent increases over background firing. By comparison with resting behavior, treadmill movement suppressed both SEURs and LEURs in the thalamus, as well as the cortex. The SEURs, however, were much more strongly suppressed in the SI cortex (-48.3 +/- 2.7%) than in the VPL thalamus (-28.1 +/- 6.7%). By contrast, similar magnitudes of suppression of LEURs were found in the SI (-25.8 +/- 8.6%) and VPL (-26.5 +/- 11.1%). These results suggest that the suppression of LEURs observed in the SI cortex may result from modulatory actions on subcortical circuits. Major suppression of SEURs, on the other hand, may occur intracortically, with a minor component occurring subcortically. Thus, VPL thalamus and SI cortex in the rat appear to be differentially subject to movement related modulation of sensory transmission.