Columnar Organization of Dendrites and Axons of Single and Synaptically Coupled Excitatory Spiny Neurons in Layer 4 of the Rat Barrel Cortex

TLDR

Cortical columns are the functional units of the neocortex, especially prominent in the barrel field of the somatosensory cortex. The study describes the morphology of spiny stellate and star pyramidal neurons in layer 4 of the barrel cortex. The authors characterized the dendritic and axonal architecture of these neurons, focusing on their columnar organization. Within a single barrel, the somata of spiny stellate and star pyramidal cells cluster, their dendrites largely confined to layer 4 (with star pyramidal apical dendrites reaching layer 2/3), while their axons span layers 1–6 but project densely within layers 4 and 2/3, forming >90 % of synapses on excitatory dendritic spines and shafts, indicating these neurons amplify thalamic input and relay excitation vertically within a single cortical column.

Abstract

Cortical columns are the functional units of the neocortex that are particularly prominent in the "barrel" field of the somatosensory cortex. Here we describe the morphology of two classes of synaptically coupled excitatory neurons in layer 4 of the barrel cortex, spiny stellate, and star pyramidal cells, respectively. Within a single barrel, their somata tend to be organized in clusters. The dendritic arbors are largely confined to layer 4, except for the distal part of the apical dendrite of star pyramidal neurons that extends into layer 2/3. In contrast, the axon of both types of neurons spans the cortex from layer 1 to layer 6. The most prominent axonal projections are those to layers 4 and 2/3 where they are largely restricted to a single cortical column. In layers 5 and 6, a small fraction of axon collaterals projects also across cortical columns. Consistent with the dense axonal projection to layers 4 and 2/3, the total number and density of boutons per unit axonal length was also highest there. Electron microscopy combined with GABA postimmunogold labeling revealed that most (>90%) of the synaptic contacts were established on dendritic spines and shafts of excitatory neurons in layers 4 and 2/3. The largely columnar organization of dendrites and axons of both cell types, combined with the preferential and dense projections within cortical layers 4 and 2/3, suggests that spiny stellate and star pyramidal neurons of layer 4 serve to amplify thalamic input and relay excitation vertically within a single cortical column.