Phylogenetic types of the cortex
The evolutionary development of the central nervous system led to the formation of several types of cortical matter, each with its own morphological features.
The simplest in structure is the paleocortex (paleocortex). Topographically, it is located on the inferomedial surface of the temporal lobe. Its structures include the olfactory tubercle with adjacent areas, as well as the anterior perforated substance zone. Physiologically, this part of the brain primarily serves the olfactory system.
In the course of phylogenesis, the archicortex (archicortex) appears somewhat later. Its important morphological difference from the paleocortex is that it is clearly separated from subcortical centers by a special cortical plate. However, in the adult human, the archicortex still retains a single-layered structure. It includes the hippocampus, which lies deep within the hippocampal sulcus, and the dentate gyrus (gyrus dentatus). Functionally, these zones are closely associated with memory formation processes and the limbic system.
The transitional link between evolutionarily old and new formations is the mesocortex (mesocortex). It is customary to distinguish two key zones within it:
- Periarchicortex — the area of smooth transition from the archicortex to the neocortex.
- Peripaleocortex — the area of transition from the paleocortex to the neocortex.
Topographically, these transitional zones occupy parts of the insular lobe, parahippocampal gyrus, and inferior regions of the limbic area.
The pinnacle of brain development is the neocortex (neocortex). This is the most highly differentiated part, which in humans accounts for about 96% of the total area of the hemispheres. It is characterized by a complex multi-layered cellular organization.
Layered structure of the neocortex
The neocortex is distinguished by a complex multi-layered structure. Based on the morphological characteristics of cells and fibers in the adult neocortex, six laminae are classically distinguished. The arrangement of these layers runs from the surface of the hemisphere deep into the brain matter toward the white matter:
- Molecular layer (lamina molecularis or plexiformis).
- External granular layer (lamina granularis externa).
- External pyramidal layer (lamina pyramidalis externa).
- Internal granular layer (lamina granularis interna).
- Internal pyramidal layer (lamina pyramidalis interna or ganglionaris).
- Multiform layer (lamina multiformis).
Basics of cortical architectonics
The cerebral cortex is not uniform throughout its extent. Its structure exhibits marked differences across various regions of the hemispheres. The structural variability affects:
- the total number of layers;
- the thickness of each individual lamina;
- the shape and size of the constituent neurons;
- the distribution pattern of nerve fibers.
A careful morphofunctional analysis has allowed researchers to identify specific cortical fields. Each such field possesses individual structural features and is responsible for performing specific functions.
The systematization of these data forms the basis of two areas:
- Cytoarchitectonics — the science studying the cellular structure features of various cortical zones.
- Myeloarchitectonics — the discipline investigating the distribution patterns of myelin fibers.
The result of these studies was the creation of detailed maps of the cerebral cortex fields, which made it possible to establish the fundamental laws of its organization.
Briefly: answers to topical questions
What types of neurons form the molecular layer of the neocortex?
The molecular layer of the neocortex is formed predominantly by small neurons — Cajal-Retzius cells. Overall, this layer contains relatively few cells.
- Cajal-Retzius cells — play a major role in the formation of the layered structure of the cerebral cortex. They secrete a specific protein, reelin, which coordinates the movement of migrating neurons.
Some of these cells undergo degeneration immediately after cortical development is complete. Dendrites of Cajal-Retzius cells also run in a tangential direction within this layer.
In which layer of the neocortex are the giant Betz pyramidal cells located?
Giant Betz pyramidal cells are located in the fifth layer of the neocortex — the internal pyramidal layer (lamina pyramidalis interna), also known as the ganglionic layer.
- Betz cells — the largest cortical cells, localized exclusively in the precentral gyrus.
The axons of these giant pyramids are covered with a thick myelin sheath and form the corticonuclear and corticospinal (pyramidal) tracts running to the motor neurons of the spinal cord. The cells have a high conduction velocity and discharge during movement.
Which cells and fibers form the internal granular layer?
The internal granular layer (lamina granularis interna) contains nerve fibers running tangentially and forming the outer band of Baillarger (Baillarger's stripe). In agranular cortex, this layer contains a large number of pyramidal cells.
What Brodmann cytoarchitectonic fields exist and where are they localized?
According to the Brodmann cytoarchitectonic map, there are 52 cortical fields. The following main fields and their localization are described:
- Fields 3, 1, 2 — primary somatosensory cortex (postcentral gyrus, part of the precentral gyrus, and paracentral lobule).
- Field 4 — primary motor cortex (precentral gyrus).
- Fields 6 (6aα, 6aβ), 8 — premotor cortical fields.
- Fields 9, 10, 11, 12, 44, 45, 47 — prefrontal cortical fields (frontal lobe).
- Field 17 — primary visual cortex (occipital lobe).
- Fields 41, 42 — primary auditory cortex.
Topographically, these fields do not strictly correspond to the arrangement of gyri and may partially overlap.
To memorize the 6 neocortex layers: "M-EGl-EPr-IGl-IPr-M" — Molecular, External Granular, External Pyramidal, Internal Granular, Internal Pyramidal, Multiform.
Class and exam questions
What is the morphological difference between the old cortex and the archicortex?
Unlike the paleocortex, the archicortex is separated from the deep subcortex by a special cortical plate, although in the adult human it still remains a single-layered structure.
Which structures belong to the paleocortex?
Structures of the paleocortex (paleocortex) include the olfactory tubercle, the cortex surrounding it, and the area of the anterior perforated substance.
What is the mesocortex and where is it located?
The mesocortex is the intermediate cortex, a transitional zone between the paleocortex/archicortex and the neocortex. It is located in the region of the insular lobe, parahippocampal gyrus, and inferior regions of the limbic area.
What serves as the basis for creating cortical field maps?
The basis for mapping cortical fields is provided by data from cytoarchitectonics (cellular structure) and myeloarchitectonics (distribution of myelin fibers).
What else to study on this topic
- Topography of the olfactory tubercle and anterior perforated substance
- Anatomical structure of the hippocampus and dentate gyrus
- Connection of archicortex structures with the limbic system and memory processes
- Principles of functional mapping of cortical fields
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