Synaptic properties of the lemniscal and paralemniscal pathways to the mouse somatosensory thalamus

Somatosensory information is thought to arrive in thalamus through two glutamatergic routes called the lemniscal and paralemniscal pathways via the ventral posterior medial (VPm) and posterior medial (POm) nuclei. Here we challenge the view that these pathways functionally represent parallel information routes. Using electrical stimulation and an optogenetic approach in brain slices from the mouse, we investigated the synaptic properties of the lemniscal and paralemniscal input to VPm and POm. Stimulation of the lemniscal pathway produced class 1, or driver, responses in VPm relay cells, which is consistent with this being an information-bearing channel. However, stimulation of the paralemniscal pathway produced two distinct types of responses in POm relay cells: class 1 (driver) responses in 29% of the cells, and class 2, or modulator, responses in the rest. Our data suggest that, unlike the lemniscal pathway, the paralemniscal one is not homogenous and that it is primarily modulatory. This finding requires major rethinking regarding the routes of somatosensory information to cortex and suggests that the paralemniscal route is chiefly involved in modulatory functions rather than simply being an information route parallel to the lemniscal channel.