Histology of Cerebral Cortex.

There are six layers of cerebral cortex:

Molecular (plexiform) layer
External granular layer
External pyramidal layer
Internal granular layer
Internal pyramidal layer
Multiform (fusiform) layer
Molecular (plexiform)
This is the most superficial layer, laying directly under the pia mater. This layer is very poor with cellular component, which is represented by only a few horizontal cells of Cajal-Retzius. The major portion of this layer is actually the processes of the neurons lying within the deeper layers and their synapses.

Most of the dendrites originate from the pyramidal and fusiform cells, whereas the axons are actually the ending fibers of the afferent thalamocortical tract that originates from nonspecific, intralaminar and midline thalamic nuclei.

External granular
This layer consists mostly of stellate cells. The existence of these small cells in this layer gives that “granular” appearance to this layer, hence its name. Other cellular structures are in a form of small pyramidal cells.

Cells of this layer send their dendrites to various layers of the cortex, especially the molecular layer, whereas their axons travel deeper to the cortex synapsing locally. Besides that intracortical synapsing, axons of this layer can be long enough to form the association fibers that travel through the white matter to finally end in the different structures of the CNS.

External pyramidal
The external pyramidal layer consists predominantly of the pyramidal cells. The superficial cells of this layer are smaller than the deeper layer, that is very large. The apical dendrites of these cells extend superficially and reach the molecular layer, whereas the basal processes join the subcortical white matter and then project again to the cortex, so they serve as both association and commissural corticocortical fibers.

Internal granular
This layer is the main input cortical station (meaning that most of the stimuli from the periphery, arrive here), and for that reason, it is specially developed within the sensory areas. It consists mostly of the stellate cells and a smaller portion of the pyramidal cells. The axons of the stellate cells remain in the cortex and synapse locally, whereas the axons of the pyramidal cells synapse deeper within the cortex, or they leave the cortex and join the white matter fibers.

Stellate cells, as the dominant cellular component, contribute to the formation of specific sensory cortical areas. Those areas receive fibers mostly from the thalamus in the following order:

Stellate cells of the primary sensory cortex receive fibers from the ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei of the thalamus
Primary visual cortex receives fibers from the lateral geniculate nucleus
Stellate cells from primary auditory cortex receive projections from the medial geniculate nucleus
When these sensory fibers “penetrate” the cortex, they turn horizontally so they can spread and diffusely synapse with the cells of the internal granular layer. Since these fibers are myelinated and therefore white, they are clearly seen in the environment of the gray matter. This stripe of the white matter is called the outer stripe of Baillarger. Since the stripe is grossly prominent in the primary visual cortex, it is referred to as the stripe of Gennari to make it distinctive.

Internal pyramidal
This layer consists predominantly of the medium-sized and large pyramidal cells. It is the source of the output or corticofugal fibers. For that reason, it is most prominent within the motor cortex from which it sends fibers that mediate motor activity. The primary motor cortex contains a specific form of these cells, called the cells of Betz.

Since we’re speaking about the cortical level of motor activity, these fibers form tracts that synapse with different subcortical motor centers. The names of these corticofugal tracts are very suggestive when you are trying to remember the exact place of their ending:

Corticotectal tract that reaches the midbrain tectum
Corticorubral tract that goes all the way to the red nucleus
The corticoreticular tract which synapses with the reticular formation of the brainstem
Corticopontine tract synapsing with the pons
The corticonuclear tract that synapses with the motor nuclei in the medullary pyramid
The corticospinal tract which goes to the spinal cord
This layer also contains a horizontally oriented stripe of the white matter which is called the outer stripe of Baillarger. It is formed by the axons of the internal pyramidal layer that synapse locally within the layer, and also with the cells of the layers II and III.

Multiform (fusiform)
This is the deepest layer of the cortex that directly overlies the subcortical white matter. It contains mostly fusiform cells with less dominant pyramidal cells and interneurons.