Cerebral Cortex

Cellular Composition of the Cortex

Histologically, the cortex consists of nerve cells, glial cells, and fibers. The neuronal population includes three main groups:

• Pyramidal neurons (85%). The efferent cells of the cortex. They are triangular: the apex with an apical dendrite points toward the cortical surface, and the base with the axon points toward the white matter. They form excitatory synapses.
• Stellate (spiny) neurons. These are the cortical equivalent of cerebellar granule cells. 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. Axon-brush cells inhibit afferent fibers, basket and axo-axonal cells inhibit the cell bodies and axons of pyramids, and double bouquet cells inhibit other interneurons, thereby "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 areas.
2. Agranular cortex — granular layers are poorly developed. Characteristic of motor areas.

Six-Layered Structure (Cytoarchitecture)

Cell arrangement follows strict organizational patterns. There are six main layers, although the boundaries between them are less distinct than in the cerebellum:

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

Nerve Fibers and the Modular Principle

Based on their course, myelinated fibers are divided into association fibers (connecting areas within the same hemisphere), commissural fibers (connecting different hemispheres), and projection fibers (two-way connections with the spinal cord and brainstem).

The cerebral cortex functions as an assembly of vertical modules—cylindrical columns spanning the entire thickness of the cortex. The axis of each module is formed by afferent fibers (one corticocortical and two thalamocortical fibers from the thalamus). Pyramidal and stellate neurons cluster around this axis, forming a local information-processing network.

Glia and Age-Related Changes

Cortical function is supported by astrocytes (forming the blood-brain barrier), microglia (immune defense), and oligodendrocytes. A unique feature of myelination in the CNS is that a single oligodendrocyte insulates multiple axons simultaneously, and there is no basement membrane surrounding the fiber.

Cortical Ontogenesis:

• In newborns, myelination is incomplete (sheaths in the frontal and temporal lobes form later).
• During childhood, cell processes and glia grow actively. Neuronal density decreases as elements are "pushed apart" by growing structures, but the absolute number of neurons remains stable.
• In old age, neuronal loss (apoptosis) occurs. Due to impaired blood flow and accumulation of damage, the largest cells (giant Betz pyramids) are the most vulnerable.