5C and D, right). It will be of great interest

in future studies to examine the functional consequences of the layer-specific projections from S1 to M1. In addition, anterograde tracers injected into M1 (Veinante & Deschênes, 2003) and retrograde tracers injected into S1 indicate that S1 and M1 are reciprocally connected (Fig. 5B). In addition to the prominent axonal projections from S1 to S2 and M1 on the same hemisphere of the brain, a number of reciprocal projections to other cortical regions are seen: bilateral projections to perirhinal cortex (temporal association cortex; Fig. 4A), projections to ipsilateral orbital cortex and weaker projections to the contralateral somatosensory cortex (Petreanu et al., 2007) and contralateral learn more motor cortex. The bilateral projection from S1 to perirhinal cortex extends across a large part of the rostrocaudal axis and connectivity is clearly weaker to the contralateral perirhinal cortex. This projection from S1

to perirhinal cortex could underlie the signalling of sensory information towards brain regions involved in higher level object-oriented coding and might contribute to hippocampal sensory responses (Pereira et al., 2007). Sensory information in S1 arrives via ipsilateral GSK-3 phosphorylation thalamocortical inputs from at least two subdivisions of the thalamus (VPM and POM), which are labelled by injection into the C2 barrel column of FG or AAV6-Cre (Fig. 6A). These ipsilateral thalamic nuclei are also prominently innervated by corticothalamic axonal projections from S1 into VPM and POM (Fig. 6B and C; Chmielowska et al., 1989; Bourassa et al., 1995; Deschenes et al., 1998; Veinante et al., 2000). No S1 projections to contralateral thalamus are observed. Specific labelling of supragranular

vs. infragranular neurons using Lenti-GFP indicates that corticothalamic projections from S1 are mediated by infragranular neurons. Although the axonal density from infragranular S1 is high in both VPM and POM, the fine-scale structure of the boutons is quite different (Fig. 6B). The S1 projection to a barreloid of VPM, originating primarily from layer 6 corticothalamic neurons, has small boutons (Fig. 6B, bottom Fenbendazole left), whereas the S1 projection to POM, originating from layer 5B corticothalamic neurons, has both small and very large boutons (Fig. 6B, bottom right; Hoogland et al., 1991; Groh et al., 2008). The large size boutons in POM derive from layer 5B pyramidal neurons and have been suggested as representing driver synapses (Sherman & Guillery, 1998), providing a strong excitation to the postsynaptic POM neurons (Diamond et al., 1992; Groh et al., 2008). On the other hand, the small size boutons terminating in VPM may have a more modulatory role. The glutamatergic corticothalamic axons therefore directly contribute to depolarizing and exciting thalamic relay neurons, which in turn form excitatory projections back to the cortex.