Neuronal composition and morphology in layer IV of two vibrissal barrel subfields of rat cortex

The technique of intracellular injection in fixed, flattened slices was used to study neuronal composition and morphology in the postero-medial barrel subfield (PMBSF) and the antero-lateral barrel subfield (ALBSF) in layer IV of rat cortex. The PMBSF and the ALBSF contain the cortical representation of the mystacial and rostral snout vibrissae respectively. Neuronal composition differed between the PMBSF and the ALBSF. Modified pyramidal cells were the most numerous neuronal type in the PMBSF (73.1%), whereas spiny multipolar (stellate) neurons were the most numerous type in the ALBSF (40.9%). Tangential dendritic field areas of modified pyramidal cells and spiny multipolar cells in the barrels of the two barrel subfields were compared. Dendritic field areas of spiny multipolar neurons located in the barrels of the PMBSF and the ALBSF were similar (mean ± SD; 2.44 ± 1.83 x 104 and 2.88 ± 1.47 x 104 μm2 respectively). Likewise, there was no significant difference in 'basal' dendritic field area of modified pyramidal neurons located in the barrels of the two different barrel subfields (4.63 ± 1.96 x 104 and 4.45 ± 1.81 x 104 μm2 for PMBSF and ALBSF respectively). The mean cross-sectional area of PMBSF barrels (20.5 ± 5.69 x 104 μm2) in which neurons were injected was approximately seven times larger than that of the ALBSF (2.94 ± 1.46 x 104 μm2). Thus, on average, the dendritic territories of these two neuronal classes sample a larger proportion of the cross-sectional area of the barrels in the ALBSF than in the PMBSF. We conclude that the close relationship between basal dendritic field area of supragranular pyramidal neurons and module size, reported in studies of other sensory areas, is not evident in all barrel subfields of the rat.