Form, synapses and orientation topography of a new cell type in layer 6 of the cat's primary visual cortex

Here we report the morpho-functional features of a novel type of deep-layer neuron. The neuron was selected from a large pool of intracellularly labelled cells based on the large cell body, numerous spine-free dendrites with an overall interneuron morphology. However, the axon gave off long-range axons up to 2.8 mm from the parent soma in layers 5/6 before entering the white matter. The boutons were uniformly distributed along the axon without forming distinct clusters. Dendritic length, surface area and volume values were at least 3 times larger than any known cortical neuron types with the exception of giant pyramidal cells of layer 5. Electron microscopy of the boutons revealed that they targeted dendritic spines (78%) and less frequently dendritic shafts (22%). Nearly half of the postsynaptic dendrites were immunopositive to GABA. Superimposing the axonal field on the orientation map obtained with optical imaging showed a preponderance of boutons to cross-orientations (38%) and an equal representation of iso- and oblique orientations (31%). The results suggest an integrating role for the layer 6 stellate neuron which projects to a functionally broad range of neurons in the deep cortical layers and to other cortical and/or subcortical regions.

Automated summary

This study reports the characteristics of a novel deep-layer neuron, which has distinct morphological and functional features. It has a large cell body and multiple spine-free dendrites, and its axon can project up to 2.8 mm before entering the white matter. The dendritic length, surface area, and volume of this neuron are significantly larger than other known cortical neuron types. The boutons of this neuron mainly target dendritic spines and show immunopositivity to GABA. Optical imaging reveals an uneven distribution of the boutons in different orientations. The layer 6 stellate neuron plays an important integrating role in deep cortical layers and other cortical and/or subcortical regions.