Distinct layers or a continuum? A morphological and functional analysis of pyramidal cells in the supragranular layers of rat barrel cortex

Pyramidal neurons in supragranular layers II and III of rodent sensory cortices are a main target of ascending sensory information conveyed by columnar projections of layer IV as well as contextual information from neighboring columns or higher cortical areas. However, layer II is not separable from layer III on cytoarchitectonic grounds. We therefore investigated to which extent pyramidal neurons in the supragranular layers differ in their input-output connectivity. We obtained detailed spatial maps of layer-specific intracortical functional input connectivity for electrophysiologically and morphologically identified supragranular pyramidal neurons by combining local photolysis of caged glutamate with whole-cell patch-clamp recordings using biocytin-containing pipettes in rat barrel cortexin vitro. The main source of excitatory inputs onto all supragranular pyramidal cells was layer IV of the same column. This translaminar excitatory source was even more prominent than local and transcolumnar excitatory inputs from within the supragranular layers, both in density and strength. Additionally, many pyramidal neurons received a prominent excitatory layer Va input, often originating from beyond the “home” column. Among those pyramidal neurons we detected a significantly higher fraction of cells located in a putative layer II than in TZ or putative layer III. Our results indicate a strong but differential information transmission from layer IV as well as layer Va, both important cortical entry points for parallel streams of sensory information, toward the supragranular layers. Within supragranular layers, information processing in pyramidal neurons can be "fine tuned" through local and transcolumnar excitatory networks. Finally this integrated information is forwarded with a prominent transcolumnar component by putative layer II pyramidal cells but with an intracolumnar preponderance, including significant layer IV-backprojections, by putative layer III pyramidal neurons