Neocortical neurogenesis and circuit assembly

The mammalian brain is an extraordinarily complex organ. The outermost part of the brain is the cerebral cortex, which plays a key role in higher-order brain functions, such as perception, language, and decision-making. Since the invention of Golgi staining, which allowed for visualization of individual neurons, defi ning neural circuits underlying various brain functions has been a fi eld of intense study for over a century. In this chapter, we will discuss the formation of neocortical circuits, emphasizing on how individual components are generated and assembled during development and how early developmental processes, including neurogenesis and neuronal migration, may guide precise circuit assembly. The mammalian cerebral cortex is composed of the archicortex (hippocampal region), the paleocortex (olfactory cortex), and the neocortex. As the evolutionarily newest addition, the neocortex is the site of higher brain function. It contains two primary types of neurons: glutamatergic neurons (70-80 %) and GABA (ã-aminobutyric acid)-ergic neurons (20-30 %). Glutamatergic neurons release glutamate as neurotransmitter, which elicits excitation in the postsynaptic neuron, and are the principle neurons in the neocortex responsible for generating circuit output. GABAergic neurons, on the other hand, release GABA as neurotransmitter, which usually triggers inhibition in the postsynaptic neuron, and are crit cal for shaping circuit output.