The cortical neurons are of four basic types: (i) pyramidal cells of Betz, (ii) stellate (granule) cells, (iii) cells of Martinotti and (iv) horizontal cells of Cajal. Of these, only pyramidal cells are projection cells while the other three are confined to the cortical layers.
Cytoarchitecturally, 6 layers are present in the cerebral cortex.
These laminae, that are numbered I to VI outside in, are as follows:
(I) Molecular (plexiform) layer consists of horizontal nerve fibers with interspersed horizontal cells of Cajal.
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(II) Outer granular layer contains stellate cells and small pyramidal neurons.
(III) Outer pyramidal layer contains medium-sized pyramidal cells and a few stellate cells.
(IV) Inner granular layer is densely packed with stellate neurons. The inner zone of this layer is traversed by the tangential fibers of the external band of Baillarger are derived from association fibers.
(V) Inner pyramidal (ganglionic) layer contains large pyramidal cells. Some of the largest pyramidal cells are seen in the precentral gyrus where they are called the giant pyramidal cells of Betz. Tangential fibers of the internal band of Baillarger traverse this lamina.
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(VI) Polymorphous (pleomorphic) layer contains the cells of Martinotti and multipolar neurons which are probably modified pyramidal neurons.