The Axon of Excitatory Neurons in the Neocortex: Projection Patterns and Target Specificity

Abstract

The neocortex of higher mammals consists of six layers and three major populations of neurons, the majority of which are excitatory pyramidal cells in layers 2/3, 5 and 6, so-called spiny stellate and star pyramidal neurons in layer 4 and a very heterogeneous population of GABAergic interneurons found throughout all cortical layers. These neurons form individual, highly specific microcircuits with each other thereby forming networks like the cortical column. This chapter will summarize and discuss the axonal arborization of principal excitatory neurons and it’s implication for neocortical connectivity. It was long thought that pyramidal cells represent a rather stereotyped class of neurons both with respect to their dendritic configuration and their axonal arborization. There is, however, growing evidence that pyramidal cells show profound structural and functional differences, not only between cortical layers but also within a given layer. Principal neurons except spiny stellate neurons and star pyramidal cells in layer 4 generally have two axonal domains: a vertically oriented domain that projects throughout the cortical column into the white matter and from there either to the contralateral hemisphere or to various subcortical brain regions. The second domain forms a discontinuous system of long-range horizontal projections either within a given cortical or between cortical areas within a given sensory or even between different sensory systems. In summary, principal excitatory neurons in the neocortex vary substantially in their axonal projection patterns and the cellular as well as subcellular input and target specificity of their axons thereby contributing to the enormous computational capacity of the neocortex.