Another difference between the two basic neuron classes regards their synaptic connections with the postsynaptic targets. Inhibitory neurons receive input synapses on the dendritic shaft that, from the point of view of signal integration, differs radically from that of spine-bearing dendrites. On the other hand, there is a large variety of non-spiny or smooth dendritic neurons 5, 6 which are inhibitory in nature and contain the neurotransmitter GABA 7. Another characteristic feature of excitatory neurons is that their dendrites bear spines each representing a postsynaptic surface mainly to excitatory inputs arriving either from nearby excitatory neurons or from neurons of other cortical/subcortical areas. The excitatory neuron population shows typically a pyramidal morphology although, in layer 4, spiny stellate and star-pyramidal cell morphology is also common 2, 3, 4. In the visual cortex, excitatory neurons are glutamateergic 1 and represent the dominant cell population in all cortical layers except layer 1. 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. 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%). Nearly half of the postsynaptic dendrites were immunopositive to GABA. Electron microscopy of the boutons revealed that they targeted dendritic spines (78%) and less frequently dendritic shafts (22%). 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. The boutons were uniformly distributed along the axon without forming distinct clusters. 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 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. Here we report the morpho-functional features of a novel type of deep-layer neuron.
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