CEREBRAL CORTEX

 

The cerebral cortex is the layer of gray matter that covers the white matter. Only a small part of the cerebral cortex is specialized to receive sensory information and projects motor impulses. The greater part of the cortex serves is involved associative functions.

 

The cerebral cortex may be named accordingly to the bones of the cranial vault that cover them in: frontal, parietal, temporal and occipital cortices (see images). However, a more functional approach is to folllow a phylogenetical classification in allocortex[1] (subdivided into paleocortex[2] and archicortex[3]), proisocortex  or transitional cortex, and a neocortex[4] or isocortex.

 

The paleocortex is phylogenetically the oldest cortex.  There is a basal part, separated from the neocortex by the lateral rhinal sulcus, and a septal part. The basal part is composed of the olfactory bulb, the olfactory peduncle, the olfactory tubercle and the piriform lobe. The septal part is composed of the subcallosal area, the diagonal gyrus and the telencephalic septum[5]. The subcallosal area is located ventral to the rostrum and genu of the corpus callosum.

 

The archicortex forms part of the limbic system. It is composed of the hippocampal formation[6] which, itself, is formed by the subiculum, the hippocampus and the dentate gyrus.

 

The proisocortex is a transitional cortex between the isocortex and the allocortex. Is formed by the cingulate gyrus and the parahippocampal gyrus. The cingulate gyrus is a band of associative fibers connecting the septum with the parahippocampal gyrus. The parahippocampal gyrus is located caudally to the hippocampal formation and reaches the piriform lobe. The cortex that covers the parahyppocampal gyrus and the caudal part of the pirifom lobe is the entorhinal cortex.

 

The archicortex refers to the hippocampal formation. Its parts are: the subiculum, the hippocampus proper and the dentate gyrus (see images). In the XVI century, Arantio named the structure located on the floor of the lateral ventricle, hippocampus (after the fish Sygnathus hippocampus or “seahorse”). A century later, it was named pes hippocampus, making reference to Neptune’s horses.  Winslow compared it to the horns of a ram and Hyrtl to the horns of the Egyptian God, Ammon. Over the years, the terminology has become excessive and confusing (Page 430 of Spanish translation of “Functional neuroanatomy” by Afifi, A. K. and Bergman, R. A.). According to Evans (Page 929 of “Miller’s anatomy of the dog,” 3rd Ed.), the hippocampus can be referred to as pes hippocampus or cornu Ammonis.

 

The paleocortex (the basal and septal parts) and the archicortex constitute the olfactory cortex or the rhinencephalon[7]

 

The neocortex is a part of the cerebral cortex which is only found in mammals. The size of the neocortex, which can vary up to one hundred thousand times between different mammals (consider the smallest shrew compared to the size of whales), is related to body size. Other parts of the cerebral cortex do not change in size as much, such as the hippocampus, which can vary only up to eight times. When the neocortex increases its size, the associated white matter grows disproportionally to the gray matter, the white matter growing more than the gray (Page 39 of the Spanish translation of “Evolving brains” by Morgan Allman, J.  Ariel Neurociencia. 2003). Topographically, the neocortex is divided into: the frontal cortex (associated with movement, relationships, associations, and learning), the parietal cortex (associative function), the occipital cortex (vision and association) and the temporal cortex (equilibrium, hearing and association). The topographic structures which form part of the neocortex are named after the bones of the cranium that they are in contact with.  

 

To correlate structure with function, one must refer to the cerebral gyri. In the dog, the primary motor cortex is situated at the level of the postcruciate gyrus and the suprasylvian gyrus.  There is a supplementary motor cortex located at the level of the precruciate gyrus. The somesthetic cortex overlaps the motor cortex and is found at the level of the rostral suprasylvian gyrus and marginal gyrus. The primary visual cortex corresponds to the caudal marginal gyrus and the occipital gyrus. The cortex that is involved with equilibrium is situated at the level of the caudal sylvian gyrus. The auditory cortex is at the level of the rostral sylvian gyrus. Lastly, the prefrontal cortex, which is related to intelligent behavior, is localized around the prorean and frontal gyri.

 

The neurons of the cerebral cortex are disposed in columns with layers. All the neurons in a column respond to the same type of stimulus. Depending on its function, the thickness of each layer varies. On the basis of the thickness, shape and form of the layers, the German anatomist Korbinian Broadman identified 50 divisions in the human cerebral cortex. For the neocortex, sis layers have been indentified (Page 684 and 685 from Howard H. Evans and Alexander de Lahunta in “Miller’s anatomy of the dog”,4th edition. Page 439 from Lennart Heimer in “The human brain and spinal cord”, 2nd edition. Page 82 to 84 from John H. Martin in “Neuroanatomy”, 2nd edition).

 

The molecular layer (I) contains dendrites and very few nuclei (among them neuroglia and the Cajal’s horizontal cells). The outer granular layer (II) has excitatory interneurons that project from the entorhinal cortex to the hippocampus. The outer pyramidal layer (III) projects to other cortical areas as associative and commissural fibers. The inner granular layer (IV) is formed by excitatory interneurons. The inner pyramidal layer (V) projects to subcortical regions as basal nuclei, brain stem and spinal cord. It also receives thalamocortical fibers.

 

The proisortex has four layers that correspond tio layers II, III, V and VI. The archicortex, three layers (molecular layer, pyramidal layer and polymorph layer). In the case of the paleocortex, the piriform cortex is made of three layers, and the olfactory cortex of three layers (glomerular layer, mitral cell layer, and external plexiform layer and mitral cells).

 

[1] From the Greek allos, “other” and from the Latin cortex, “cover”.

[2] From the Greek palios, “old” and from the Latin cortex, “cover”.

[3] From the Greek arche, “beginning” and from the Latin cortex, “cover”.

[4] From the Greek neos, “new” and from the Latin cortex, “cover”.

[5] The terminology septum pellucidum is restricted to the microsmatic mammals  in which the olfactory sense is poorly developed (humans) and the septum has fewer nerve cells.

[6] From the Greek hippokampos “sea horse”.

[7] From the Greek rhinos, “nose”; gkephalon, “encephalon”.