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It exists several types of neurons that can be classified from their morphology (pyramidal, basket, granular, stellar ...) or their function (excitatory vs inhibitory). Indeed, one can also link morphology and function distinguishing thorny neurons (pyramidal and stellar cells) that are excitatory and smooth neurons that are inhibitory.
Neurons are not set randomly in the neocortex. Instead, they are set in separate layers in such a way that one can distinguish 6 layers (fig 2.). This organization is specified by the horizontal white fibers (afferent thalamo-cortical and cortico-cortical fibers and efferent fibers, i.e. axons).
- Layer I (molecular): the cell density is low, cells are called fusiform and are set horizontally. Axons project within this layer (local connexions).
- Layer II (external granular): this layer is characterized by a high density of small cells: pyramidal cells that project onto deep layers and granular and basket cells. These cells receive input for deeper layers.
- Layer III (external pyramidal): most of pyramidal cells are located within this layer, their sizes vary from middle to big from the external (surface) to the internal (deep) part. Apical dendrites go to the layer I, whereas axons go outside the neocortex and form associative and commissural white fibers.
- Layer IV (internal granular): this layer contains many different cell types although dominated by smooth stellar cells. It receives thalamic fibers that form the external Baillarger stria.
- Layer V (internal pyramidal): it contains middle and big size pyramidal cells that project dendrites onto, respectively, layers IV and I. Axons go to sub-cortical nuclei, branching usually within the same layer.
- Layer VI (multiform): this layer contain mainly fusiform and pyramidal cells which axons project onto the thalamus and/or form associative fibers with neighbouring gyri (arched fascicles).
Figure 2. Organization of the neocortex in 6 layers.