Somatic sensory responses in the rostral sector of the posterior group (POm) and in the ventral posterior medial nucleus (VPM) of the rat thalamus: dependence on the barrel field cortex.

The projection from the whiskers of the rat to the S-I (barrel) cortex is segregated into two separate pathways--a lemniscal pathway relayed by the ventral posterior medial nucleus (VPM) to cortical barrels, and a paralemniscal pathway relayed by the rostral sector of the posterior complex (POm) to the matrix between, above, and below barrels. Before investigating how the barrel cortex integrates these sensory pathways, it is important to learn more about the influence of the various inputs to the two thalamic nuclei. Based on the greater density of descending versus ascending projections to POm, it seemed likely that corticofugal inputs play an important role in the sensory activity of POm. To test this, the responses of POm and VPM cells to sensory stimuli were measured before, during, and after suppression of the S-I cortex. S-I was suppressed by application of magnesium or by cooling; the status of the barrel cortex was assessed continuously by an electrocorticogram. All VPM cells (n = 8) responded vigorously to whisker movement even when the barrel cortex was profoundly depressed. In contrast, all POm cells (n = 9) failed to respond to whisker movement once the barrel cortex became depressed, typically about 25 minutes after the start of cortical cooling or magnesium application. POm cells regained responsiveness about 30 minutes after the cessation of cortical cooling or the washoff of magnesium. These findings indicate that the transmission of sensory information through the lemniscal pathway occurs independently of the state of cortex, whereas transmission through the paralemniscal pathway depends upon the state of the cortex itself.