Cerebral cortex

Macroscopic relief and cortical centers

The cortical surface forms a complex pattern of sulci and gyri. Some of them are relatively constant and serve as boundaries of lobes or house important functions:

• Central sulcus (Rolandic fissure). Separates the frontal lobe from the parietal lobe.
• Precentral gyrus. Located in the frontal lobe, contains the primary motor cortex.
• Postcentral gyrus. Belongs to the parietal lobe, houses the primary somatosensory cortex.
• Parieto-occipital sulcus. Runs on the medial surface, separating the lobes of the same name.
• Calcarine sulcus (sulcus calcarinus). Runs toward the occipital pole. The primary visual cortex is localized in its depths and on the adjacent gyri.

The insula (insula) is distinguished separately, lying deep within the lateral (Sylvian) sulcus. To see it, one must retract the operculum (operculum)—the overhanging fragments of the frontal, parietal, and temporal lobes. The insula has its own central sulcus, as well as short and long gyri. In the hidden part of the superior temporal gyrus lie the transverse temporal gyri of Heschl, which are responsible for the primary auditory cortex.

Basal (inferior) surface of the hemispheres

On the inferior surface of the brain, the topography of the gyri has its own features:

1. Frontal lobe: the hemispheres are separated by the longitudinal fissure. The olfactory sulcus (sulcus olfactorius) runs here. Medial to it is the rectus gyrus (gyrus rectus), and lateral to it are the orbital gyri (gyri orbitales).
2. Temporal and occipital lobes: the relief is formed by the collateral sulcus. The uncus (uncus), parahippocampal, inferior temporal, lingual, and fusiform gyri, as well as the lateral and medial occipitotemporal gyri, are distinguished.

Phylogenetic division of the cortex

Histologically and evolutionarily, the cortex is heterogeneous. An important boundary is the cingulate sulcus (sulcus cinguli), which separates the neocortex from the transitional cortex (mesocortex).

• Isocortex (neocortex) — occupies most of the hemispheres and has a typical six-layered structure (according to K. Brodmann).
• Allocortex (evolutionarily old cortex) — is contrasted with the neocortex. It includes the paleocortex (ancient cortex, which includes the olfactory region) and the archicortex (old cortex). The fasciolar gyrus (gyrus fasciolaris), hippocampus (hippocampus), dentate gyrus, and parahippocampal gyrus belong to the archicortex.

Cytoarchitectonics of the isocortex

When stained with Nissl stain (for neuronal cell bodies), six layers are clearly distinguished in the neocortex, running from the outside inward from the pia mater:

1. Molecular layer. Poor in cells. Contains small Cajal–Retzius cells, which determine the laminar structure of the cortex (some degenerate after development), and distal dendritic branches from the lower layers.
2. External granular layer. Consists of numerous inhibitory GABAergic granule cells and a small number of excitatory pyramidal cells.
3. External pyramidal layer. Formed by small pyramidal neurons. Their axons extend into the white matter, forming association and commissural fibers.
4. Internal granular layer. The main target for afferent information from the thalamus (thalamocortical pathways). Contains many non-pyramidal cells.
5. Internal pyramidal layer. Medium and large pyramidal neurons lie here. Only in the precentral gyrus of this layer are giant Betz cells found. Their thick myelinated axons form the projection motor tracts (corticospinal and corticonuclear).
6. Polymorphic layer. Consists of polymorphic cells (the inner part is denser, the outer contains larger cells).

Cortical neurons and their connections

Basically, cortical neurons are divided into excitatory pyramidal (releasing glutamate) and inhibitory non-pyramidal (granular, interneurons). Interneurons include basket cells, chandelier cells, and double bouquet cells.

An important regulatory mechanism is recurrent inhibition: pyramidal neurons give off local collaterals that activate inhibitory GABAergic interneurons. These, in turn, inhibit the pyramidal cells themselves, forming a local feedback loop.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Anatomic boundaries and topography of the frontal, temporal, and occipital lobes
• Myeloarchitectonics of the cortex: bands of Baillarger and tangential band
• Efferent pathways: corticospinal and corticonuclear tracts
• Structures of the archicortex: hippocampus and dentate gyrus
• Mechanisms of local recurrent inhibition in cortical neural networks

Other topics in the section "DeMyer's The Neurologic Examination" / "Duus' Topical Diagnosis in Neurology"

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