NEOCORTEX: FUNCTION, STRUCTURE AND PATHOLOGY - GEOGRAFÍA - 2025

The neocortex or neocortex is a structure divided into thin layers that covers the brain of mammals, including humans. Its main functions are sensory perception, spatial reasoning, language, learning, memory, among others.

Whereas, in the brain of monkeys and humans, this area has numerous convolutions and surrounds almost the entire brain. On the contrary, in smaller mammals it is less important, occupying less space. Thus, in a mouse brain, the neocortex occupies only the upper part of the brain. In addition, its surface is fine and there are hardly any twists.

The difference in the neocortex between humans and other species is that its thickness is much greater and it has more convolutions. This seems to symbolize the ability to apply much more complex cognitive skills.

It is called "neo," which means "new," because it is the evolutionarily newer part of the cerebral cortex. However, it can also be called "isocortex" or "neopalyo". Some authors use the neocortex and the cerebral cortex (or cortex) interchangeably, although the latter also includes subcortical structures such as the hippocampus and the perirrinal cortex.

Neocortex in humans

Cytoarchitecture of the visual neocortex: at the center numbering of layers one (I) to six (VI). Left column: neuron cell bodies marked by violet Nissl stain. Right column: glutamatergic neurons marked for MR-GEF in blue. Source: Bithell A, Hsu T, Kandanearatchi A, Landau S, Everall IP, Tsuang MT, et al. via Wikimedia Commons

In the human species, the neocortex is the largest part of the brain, and covers both cerebral hemispheres. The rest of the structures are called "allocortex".

The neocortex performs a wide variety of functions. Therefore, when some type of injury occurs in this area, the loss of certain cognitive abilities is common.

Depending on the location of the damage, the ability to socially relate adequately, understand language, control movements… In addition, visual and spatial perception may be altered, among other things.

In times past, they studied cognitive functions and their location in the brain by observing the behavior of injured patients. Thus, if a large group of patients who had damaged the same area of ​​the neocortex had difficulty performing the same tasks, a relationship was established between brain location and abilities.

Thanks to this, it is known that the neocortex is made up of several areas that have different functions. Many areas are present in various species of mammals. While others, such as color vision or the ability to speak, only in specific species.

Structure of the neocortex

Neocortex and its gyri or convolutions. Source: Hagmann P, Cammoun L, Gigandet X, Meuli R, Honey CJ, et al. (2008). via Wikimedia Commons

The neocortex is 2 to 4 millimeters thick, and has about 30 billion neurons. In primates and humans it is full of grooves and ridges (convolutions). These folds arose from the great increase in their size.

All people have a brain with the same pattern of convolutions and sulci, although some details may vary slightly from one individual to another.

In addition, there are neurodevelopmental disorders in which these twists do not develop as they should, leading to various cognitive alterations. Likewise, they can be lost with degenerative diseases such as Alzheimer's.

In humans, the neocortex makes up about 76% of the brain's volume. This structure arises in the embryonic stage of the dorsal telencephalon. Little by little it is divided into the famous lobes: the occipital, the parietal, temporal and frontal.

Frontal lobe (orange), parietal lobe (pink), occipital lobe (purple), temporal lobe (green).

These lobes are distinguished by their functions. Thus, the occiput stands out for the primary visual cortex (to process vision). While the temporal, it has the primary auditory cortex (for sounds). The parietal corresponds to tactile sensations, perception of one's own body, and visuospatial abilities.

In the human species, the frontal lobe has very complex and advanced functions in relation to other species. Such as language processing (Broca's area), socially desirable behavior, and emotional control.

There are two types of cortex in the neocortex according to the architecture of the cell bodies: the neocortex itself and the proisocortex. The latter is found in some parts of the brain such as the cingulate gyrus, the insula, the hippocampal gyrus or the subcallosal area.

The neocortex is the most developed brain tissue, as can be seen in its organization and number of layers.

It is made up of gray matter, that is, unmyelinated nerve cell bodies. It covers a deeper area of ​​white matter, that is, axons (neuronal extensions) filled with myelin.

However, although the neocortex carries out the most complex brain functions, hardly any cellular differences are observed when compared to other parts of the brain.

So what makes the neocortex so specialized? It seems that what differentiates him is his ability to create, modify and control a large number of neural connections. It generates such a dynamic and flexible structure, which allows a great exchange of information between the different neural circuits.

Layers of the neocortex

Nervous system and brain

The neocortex has a practically uniform structure, which is why it is also called “isocortex”. It is made up of 6 horizontal layers of nerve cells numbered I through VI. The first is the newest, while the sixth is the oldest.

They are organized according to a phylogenetic perspective, that is, each one arises from a different moment in evolution. Thus, as the species has advanced, new layers have developed.

These layers contain both excitatory (approximately 80%) and inhibitory (20%) neurons. The former activate other neurons, while the latter block them.

The layers are mainly made up of "type cells" or "dense cells", and connections between them. The layers are differentiated by the predominant types of nerve cells, their arrangement and connections.

Layer IV is smaller, and is found in the primary motor cortex. It is the main receptor of sensory information. Later, it transmits that information to other layers so that it is processed and interpreted.

In this way, this layer receives a large part of the synaptic connections from subcortical structures such as the thalamus. This is because the thalamus is connected with different sensory organs such as the ear or the eyes.

Layers II and III send projections primarily to other parts of the neocortex. While layers V and VI usually transmit information outside the cortex, such as the thalamus, brainstem or spinal cord.

Columns of the neocortex

Visual cortex of the monkey (Macaca rhesus). Representation of a cortical column. On the left the layers of the neocortex, in Roman numerals from III to VI. Pyramidal cells a, b. Stellate cells with spines c, d. Stellate cells of recurrent axon e, f, g. Pyramidal cells of recurrent axon h, i. Sensory afferent fibers in red F. Drawing of a preparation stained by the Golgi method. Source: Mavavf via Wikimedia Commons

In the neocortex there are also vertical structures called columns. They are areas of approximately 0.5 millimeters in diameter, and 2 millimeters deep.

Apparently, each column is associated with the sensory perception of each part of the body. Although there are also some dedicated to perceiving sounds or visual elements.

In human beings there seem to be about 500 thousand columns, each one having about 60 thousand neurons. However, they are difficult to define and there is no clear consensus on their specific anatomy, size, or functions.

Neocortex functions

MRI of the brain

The main functions of the neocortex are:

- Sensory perception: in the neocortex there are areas that process and interpret the information that comes from our senses.

- Generate motor commands: thanks to this brain structure, we can make sequences of movements that we are not even aware of. In this area, all the motor patterns necessary for walking, writing, or playing an instrument, for example, are planned.

- Spatial reasoning: there are regions of the neocortex involved in understanding space and acting in relation to it. It also serves to guide us and place elements.

- Language: this is a uniquely human capacity that distinguishes us from other animals. There are areas of the neocortex that predispose us to learn the sounds of the language from an early age and produce them. As well as associating certain groups of sounds or written symbols with a meaning.

- The so-called executive functions such as reasoning, decision-making, self-control, concentration, self-reflection, problem solving, etc. That is, the ability to know how to behave at all times and carry out a series of behaviors to achieve a goal.

- Learning, memory and sleep: it has been proven that the neocortex is also essential for the storage of knowledge.

In fact, certain parts of the neocortex seem to be the seat of semantic memory, which is that related to general knowledge about the world. For example, what we learn in school, such as that Paris is the capital of France.

The same happens with autobiographical memory, which is the one associated with important events in our personal lives.

Instrumental information is also stored, that is, information that involves automatic behaviors such as driving or riding a bicycle.

On the other hand, certain neurons in the neocortex are also activated during sleep. It seems that the neocortex dialogues with the hippocampus while we sleep, helping to consolidate and fix what has been learned during waking periods.

Neocortex and evolution

For the neocortex to evolve to a larger size, it is necessary that the brain of the species is also larger so that it can support it.

The neocortex is also present in other primates besides Homo sapiens. A larger size of the neocortex compared to the rest of the brain is related to different social variables such as group size, as well as the complexity of social relationships (competition, cooperation, union, etc.).

Evolutionarily increasing the size of the neocortex has implied greater inhibitory control. This may explain the transformation of behaviors and a greater social harmony with respect to our ancestors.

Humans have a large neocortex compared to other mammals. Thus, for example, there is a 30: 1 ratio of neocortical gray matter to marrow size in the brain stem of chimpanzees. In humans that ratio is 60: 1.

Pathologies or lesions in the neocortex

As the neocortex has a great extension in humans, it is easy for any acquired injury to involve this structure. As it can happen after a head injury, stroke or tumors.

In addition, it is important to mention that depending on the area of ​​the neocortex where the damage occurs, the symptoms will vary. It is possible that the patient has difficulties in the use of language, in perceiving objects in space or that he suffers from inhibition problems and performs undesirable behaviors.

The neocortex can also be affected by neurodegenerative diseases. As, for example, in Alzheimer's disease, there is a disruption in the transmission of information from the sensory neocortex to the prefrontal neocortex.

This leads to symptoms such as impaired cognitive abilities, personality changes, and dementia.

If the degeneration encompasses the temporal lobe, semantic dementia may develop. That is, a progressive degeneration of memory associated with semantic facts (things learned from our culture, what they teach us at school, data on the use of language, etc.)

References

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