Morphometric analysis of feedforward pathways from the primary somatosensory area (S1) of rats

Abstract

In this report, we used biotinylated dextran amine to anterogradely label individual axons projecting from primary somatosensory cortex (S1) to four cortical areas in rats, namely the secondary somatosensory (S2), the parietal ventral (PV), the perirhinal (PR), and the contralateral S1 (S1c). A major goal was to determine whether axon terminals could be classified on the basis of morphological criteria, such as the shape and density of boutons, and the shape and size of individual terminal arbors. Evidence from reconstruction of isolated axon terminal fragments (n=111) supported a degree of morphological heterogeneity. In particular, morphological parameters associated with the complexity of terminal arbors and the proportion of beaded, en passant boutons (Bp) vs. stalked boutons terminaux (Bt) were found to differ significantly. Two broad groups could be established following a discriminant function analysis across axon fragments. Both groups occurred in all four target areas, possibly consistent with a commonality of presynaptic processing of tactile information in these areas. However, more work is needed to investigate synaptic function at the single bouton level and see how this might be associated with emerging properties in the postsynaptic targets.