Architectonics and cortical types
The distribution of neurons in the cortex is uneven. Historically and in modern practice, functional zones are determined using Brodmann's cytoarchitectonic map. Topographically, these areas may partially overlap and exhibit individual variability, not always strictly corresponding to the gyri on the brain surface.
Based on cellular composition, two main types of cortex are distinguished:
- Granular cortex: also called koniocortex. In this type, the density of small granular cells significantly exceeds the number of pyramidal neurons (the "granularization" phenomenon). This structural type is characteristic of primary sensory areas: somatosensory (areas 3, 1, 2), auditory (area 41), and visual (area 17).
- Agranular cortex: characterized by the dominance of large pyramidal neurons ("pyramidalization"). These are the primary and secondary motor centers localized in the frontal lobe (areas 4 and 6).
In addition to cells, brain mapping can be performed based on the distribution of myelin fibers (myeloarchitectonics), glial cells (gliaoarchitectonics), and blood vessels (angioarchitectonics). In modern science, immunohistochemistry is actively used to search for specific neurotransmitters, peptides, and calcium-binding proteins.
Microstructure and neuroplasticity
Cortical architectonics is not genetically immutable. Under the influence of environmental factors and afferent impulse activity, the microstructure and synaptic connections can undergo reorganization. This property forms the basis of neuroplasticity.
A striking example of cortical differentiation in response to stimuli is the visual system. Elements of visual information are processed separately:
- Color is analyzed in specific areas called blobs.
- Spatial localization and orientation are processed in ocular dominance columns.
A similar specialization is observed in other systems. For example, in the somatosensory cortex of rodents, there are cortical barrel columns (barrels). These are ring-shaped clusters of cells, each strictly responsible for analyzing information from a single vibrissa (whisker).
White matter tracts
Beneath the cortical layer lies the subcortical white matter, formed by myelinated fibers and glia (predominantly oligodendrocytes). These fibers provide connection between various zones of the nervous system and are divided into three groups:
- Association fibers: connect cortical areas strictly within a single hemisphere. They can be short (arcuate fibers — fibrae arcuatae cerebri) and long (e.g., superior and inferior longitudinal fasciculi, uncinate fasciculus — fasciculus uncinatus). An extensive network of association pathways ensures the integration of functions and serves as a basis for the recovery of lost skills after strokes or trauma.
- Commissural fibers: coordinate the activity of the left and right hemispheres by connecting their symmetrical areas. The largest structure here is the corpus callosum (corpus callosum), whose fibers form the forceps anterior and forceps posterior. The anterior commissure (commissura anterior) also belongs to this group.
- Projection fibers: connect the cortex with underlying structures (basal nuclei, brainstem, spinal cord). Efferent pathways gather into a dense bundle, forming the internal capsule (capsula interna), and continue as the corticospinal, corticonuclear, and corticopontine tracts.
Role of the thalamus in information processing
The vast majority of afferent (ascending) pathways cannot reach the primary sensory areas of the cortex directly. Their final and most important synaptic relay station is the thalamus, often referred to as the "gateway to consciousness."
Fibers from the cerebellum, globus pallidus, and mamillary bodies relay here before heading to the cortex (to areas 3, 1, 2, and 4). A complex topographically organized system of reciprocal (direct and feedback) connections exists between the thalamus and the cortex. These massive myelinated bundles form the anterior, posterior, superior, and inferior stalks of the thalamus, collectively known as the corona radiata (corona radiata).
Briefly: answers to topical questions
What are the functions of the corpus callosum?
The function of the corpus callosum is to connect identical (symmetrical) areas of the cortex of both cerebral hemispheres. It is formed by commissural fibers that connect mirror-positioned (homotypic) zones.
Since the corpus callosum is shorter than the hemispheres, the fibers at its ends curve in a U-shape:
- Forceps minor (forceps minor) — formed by the anterior end (rostrum and genu), connecting the frontal poles.
- Forceps major (forceps major) — formed by the posterior end (splenium), connecting the occipital poles.
Connections between symmetrical areas are absent in the primary visual cortex (area 17) and the somatosensory cortex (in the hand and foot innervation zones).
To avoid confusing cortical types: GRAN-ular cortex is responsible for the GRAN-t (boundary) with the outside world (sensitivity, sensory analysis, many small cells). Agranular is responsible for movement generation (large pyramidal cells).
Class and exam questions
What is the difference between homotypic and heterotypic cortex?
Homotypic cortex has a classic six-layered structure. In heterotypic cortex, not all six layers are identified — some may be absent or poorly expressed.
Which Brodmann areas are characterized by agranular cortex?
Agranular cortex is typical for motor zones, where motor functions predominate. According to Brodmann's map, these are areas 4 (primary motor cortex) and 6.
Which sensory system transmits information to the cortex, bypassing the thalamus?
The only exception to the "gateway of consciousness" rule is the olfactory system. Its fibers reach the cortical analyzers directly, without synapsing in the thalamus.
What else to study on this topic
- Topography of Brodmann areas on the medial and basal surfaces of the hemispheres
- Anatomical structure of the internal capsule (anterior limb, genu, posterior limb)
- Limbic system: fibers of the cingulate gyrus and parahippocampal zone
- Structure and functions of the cortical speech center (area 44)
- Histological structure of homotypic cortical layers from I to VI
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