Functional connectivity of the secondary somatosensory cortex of the rat.

The hierarchical relationship of the rat primary somatosensory cortex (S1) and secondary somatosensory cortex (S2) is controversial. The existence of a direct thalamocortical projection from ventral posterolateral thalamic nucleus (VPL) to S2 is a key factor in determining the relative position of S2 in the processing flow. In this study, the inter-connections of forepaw and hindpaw representations in VPL, S1, and S2 were examined by neuroanatomical tracing and electrophysiological approaches. In the tracing experiments, VPL, S1, and S2 were electrophysiologically identified, and then iontophoretically injected with biotinylated dextran amine (BDA, a bi-directional tracer). In the double-labeling experiments, two of the following retrograde tracers-BDA, Rhodamine dextran (RD), and/or Fluoro-Gold (FG)-were injected into homotypical S1 and S2 forepaw representations simultaneously. In the electrophysiological studies, paired somatic evoked multiunit responses in S1 and S2 were compared. Our results revealed that: (1) VPL forepaw and hindpaw neurons projected to corresponding S1 and S2 areas in a parallel and somatotopic manner; (2) very low percentage of double projecting VPL neurons were found, indicating parallel and independent pathways from forepaw VPL to S1 and S2; (3) forepaw S1 and S2 were symmetrically and reciprocally connected; (4) response latencies of the S1 and S2 multiunits to forepaw stimulation were in accordance with a direct and parallel pathway. This study provides further evidence to support the equivalent hierarchy of S1 and S2 in processing sensory information of the rat.