Cerebral cortex

Cellular composition of the cortex

Histologically, the cortex consists of nerve cells, the glial apparatus, and fibers. The neuronal composition is represented by three main groups:

• Pyramidal neurons (85%). Efferent cells of the cortex. They have a triangular shape: the apex with the apical dendrite is directed toward the cortical surface, and the base with the axon toward the white matter. They form excitatory synapses.
• Stellate (spinous) neurons. These are the analog of the granule cells of the cerebellum. They receive afferent signals and transmit excitation to pyramidal cells. They are concentrated mainly in layers II and IV.
• Inhibitory neurons (interneurons). Present in almost all layers. Cells with an axonal brush inhibit afferent fibers, basket and axo-axonal cells inhibit the cell bodies and axons of pyramids, and double bouquet cells inhibit other interneurons, "disinhibiting" pyramidal cells.

Depending on the number of stellate cells, two types of cortex are distinguished:

1. Granular cortex — high content of stellate cells (layers II and IV are well developed). Typical of sensory centers.
2. Agranular cortex — granular layers are poorly developed. Characteristic of motor centers.

Six-layered structure (cytoarchitectonics)

The arrangement of cells follows strict patterns. There are six main layers, although the boundaries between them are less sharp than in the cerebellum:

1. Layer I (Molecular layer). The most superficial. Few neurons are present (small inhibitory ones); the bulk consists of processes of underlying cells running parallel to the surface (tangentially).
2. Layer II (External granular layer). Has a high density of small pyramidal, stellate, and inhibitory cells, giving it a "granular" appearance.
3. Layer III (Pyramidal layer). Medium pyramids dominate. The overall cell density here is lower than in adjacent layers.
4. Layer IV (Internal granular layer). Consists of small excitatory stellate neurons. Inhibitory and pyramidal neurons are absent here or extremely rare.
5. Layer V (Ganglionic layer). The layer of large pyramids. In motor areas (precentral gyrus), giant Betz cells are located here. Their axons descend into the spinal cord, forming the pyramidal tracts.
6. Layer VI (Polymorphic layer). Borders the white matter. Consists of small pyramidal and inhibitory cells, whose concentration decreases with depth.

Nerve fibers and modular principle

According to their direction, myelinated fibers are divided into association fibers (connections within one hemisphere), commissural fibers (connecting different hemispheres), and projection fibers (two-way connections with the spinal cord and brainstem).

The cerebral cortex operates as an assembly of vertical modules — cylindrical columns passing through the entire thickness of the cortex. The axial part of such a module is formed by afferent fibers (one corticocortical and two thalamocortical fibers from the thalamus). Pyramidal and stellate neurons group around this axis, forming a local information processing network.

Glia and age-related changes

The vital activity of the cortex is supported by astrocytes (forming the blood-brain barrier), microglia (immune defense), and oligodendrocytes. A feature of myelination in the central nervous system is that a single oligodendrocyte insulates several axons at once, and the basal lamina around the fiber is absent.

Ontogenesis of the cortex:

• In newborns myelination is not complete everywhere (sheaths in the frontal and temporal lobes form later).
• In childhood processes and glia actively grow. The neuronal density decreases ("pushed apart" by growing elements), but their absolute number remains the same.
• In old age neuronal loss (apoptosis) occurs. The largest cells (giant Betz pyramids) die most actively due to impaired blood flow and accumulation of damage.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of myelination and lipoprotein composition of the oligodendrocyte membrane
• Diagram of interneuronal connections: formation of functional columns
• Blood-brain barrier and the role of protoplasmic astrocytes
• Molecular mechanisms of neuronal apoptosis in old age

Other topics in the section «Nervous system: brain»

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