Opposite adaptive processing of stimulus intensity in two major nuclei of the somatosensory brainstem.

Tactile information ascends from the brainstem to the somatosensory cortex via two major parallel pathways, lemniscal and paralemniscal. In both pathways, and throughout all processing stations, adaptation effects are evident. Although parallel processing of sensory information is not unique to this system, the distinct information carried by these adaptive pathways remains unclear. Using in vivo intracellular recordings at their divergence point (brainstem trigeminal complex) in rats, we found opposite adaptation effects in the corresponding nuclei of these two pathways. Increasing the intensity of vibrissa stimulation entailed more adaption in paralemniscal neurons, whereas it caused less adaptation in lemniscal cells. Furthermore, increasing the intensity sharpens lemniscal receptive field profile as adaptation progresses. We hypothesize that these pathways evolved to operate optimally at different dynamic ranges of sustained sensory stimulation. Accordingly, the two pathways are likely to serve different functional roles in the transmission of weak and strong inputs. Hence, our results suggest that due to the disparity in the adaptation properties of two major parallel pathways in this system, high and reliable throughput of information can be achieved at a wider range of stimulation intensities than by each pathway alone.