The cerebral cortex Or cerebral cortex is the nerve tissue that covers the surface of the cerebral hemispheres. Said other form, it constitutes the most superior region of the brain.
This brain structure reaches its maximum development in primates, is less developed in other animals and is related to the development of more complex cognitive and intellectual activities.
The cerebral cortex is a basic brain area for the functioning of humans. In this region functions such as perception, imagination, thought, judgment or decision are performed.
Anatomically, it consists of a series of thin layers consisting of gray matter, which are located above a large collection of White matter .
The cerebral cortex adopts a convoluted form, so if extended it would present a very extensive mass. Specifically, research suggests that the total surface of the cerebral cortex could consist of about 2500 square centimeters.
Also, this large mass of brain, is characterized by contain an enormous number of neurons in its interior. In general, it is postulated that in the cerebral cortex that witness about 10 billion neurons, which would perform about 50 trillion synapses.
The main features of the cerebral cortex are explained below. Their layers, their neurons and their functional organization are specified, and the functions carried out in this region of the brain .
Characteristics of the cerebral cortex
The cerebral cortex of mammalian animals is represented by a sheet of gray matter, which covers the two brain hemispheres .
It consists of a highly complex structure in which different sensory organs are represented in specific areas or zones, which are called primary sensory areas.
Each of the five senses that humans possess (sight, touch, smell, taste, and touch) develop in a region of the specific cortex. That is, each sensory modality has a delimited territory within the cerebral cortex.
Apart from the sensory regions, the cerebral cortex also has multiple somatic secondary, association and motor regions. In these areas, afferent cortical and association systems are developed, thus giving rise to learning, memory And behavior.
In this sense, the cerebral cortex is considered as a particularly relevant region when it comes to developing the higher activities of the human brain.
The most advanced and elaborate processes of human beings such as reasoning, planning, organization or association are performed in different areas of the cerebral cortex.
For this reason, the cerebral cortex constitutes a structure that from the human perspective acquires a maximum complexity. The cerebral cortex is the result of a slow evolutionary process that may have begun more than 150 million years ago.
Brain cortex layers
The main feature of the cerebral cortex is that it is made up of different layers of gray matter. These layers conform the structure of the cortex and define its structural and functional organization.
In addition, the layers of the cerebral cortex are characterized not only from a structural point of view, but also from a phylogenetic perspective.
That is, each of the layers of the cerebral cortex corresponds to a different evolutionary moment. At the onset of the human species, the brain was less developed and the cortex had fewer layers.
Through the evolution of the species, these layers have been increasing, a fact that is related to the increase of cognitive and intellectual capacities of humans over time.
1- Molecular layer
The molecular layer, also known as the plexiform layer, is the most superficial region of the cerebral cortex and, therefore, the most recent appearance.
It consists of a dense network of nerve fibers that are tangentially oriented. These fibers are derived from dendrites of Cells Pyramidal and fusiform, axons of the stellate and Martinotti cells.
In the molecular layer afferent fibers can also be found that originate in the thalamus, association and commissural. As the most superficial region of the cortex, in the molecular layer a large amount of synapses between different neurons are established.
2- External granular layer
The outer granular layer is the second most superficial region of the cortex and lies below the molecular layer. It contains a large number of small pyramidal and starry cells.
The dendrites of the cells of the outer granular layer terminate in the molecular layer and the axons enter into deeper layers of the cerebral cortex. For this reason, the outer granular layer is interconnected with the different regions of the cortex.
3- External pyramidal layer
The outer pyramidal layer, as its name implies, is composed of pyramidal cells. It is characterized by irregular shape, that is, the size of the layer increases from the surface boundary to the deepest boundary.
The dendrites of pyramidal layer neurons pass through the molecular layer and axons travel as projection, association, or commissural fibers to the white matter between the layers of the cerebral cortex.
4- Internal granular layer
The internal granular layer is composed of stellate cells that are arranged in very compact form. It has a high concentration of horizontally arranged fibers known as the Baillarger outer band.
5- Ganglionar layer
The lymph node layer or internal pyramidal layer contains very large pyramidal cells of medium size. They also contain a large number of horizontally arranged fibers that form the inner band of Baillarger.
6- Multi-layer cover
Finally, the multiform layer, also known as the polymorphic cell layer, basically contains spindle cells. It also contains modified pyramidal cells containing a triangular or ovoid cell body.
Many of the nerve fibers of the multiform layer enter the underlying white substance and connect the layer to the intermediate regions.
Functional organization of the cerebral cortex
Nervous system and brain
The cerebral cortex can also be organized according to the activities carried out in each region. In this sense, certain areas of the cerebral cortex process specific signals of sensory, motor and association nature.
1- Sensitive areas
Sensitive areas are regions of the cerebral cortex that receive information of a sensitive nature and are closely related to perception.
The information accesses the cerebral cortex mainly through the posterior half of both cerebral hemispheres. Primary areas contain the most direct connections to peripheral sensing receptors.
On the other hand, the secondary sensory and association areas are usually adjacent to the primary areas. In general, they receive information from both the primary and lower Encephalon .
The main task of the areas of association and the secondary areas is to integrate the sensory experiences to generate patterns of recognition and behavior. The main sensitive regions of the cerebral cortex are:
- The primary somatosensory area (areas 1, 2 and 3).
- The primary visual area (area 17).
- The primary auditory area (area 41 and 42).
- The primary gustatory area (area 43).
- The primary olfactory area (area 28).
2- Motor Areas
The motor areas are located in the anterior part of the hemispheres. They are responsible for initiating the brain processes related to the movement and give rise to such activities.
The most important motor areas are:
- The primary motor area (area 4).
- The drill language area (area 44 and 45).
3- Areas of association
The areas of association of the cerebral cortex correlate with the more complex integration functions. These regions perform activities such as processes of memory and cognition, the management of emotions, and the development of reasoning, will, or judgment.
Likewise, the areas of association play an especially important role in the development of the personality traits and characteristics of the people. Likewise, it is a brain region essential in the determination of intelligence.
The areas of association comprise both certain motor areas and specific sensitive regions.
Nerve cells of the cerebral cortex
The cerebral cortex presents a great variety of cells in its interior. Specifically, five distinct types of neurons have been specified in this region of the brain.
1- Pyramidal cells
Pyramidal cells are Neurons Which are characterized by a pyramid shape. Most of these cells contain a diameter of between 10 and 50 millimeters.
However, there are also large pyramidal cells. These are known as Betz cells and can have a diameter of up to 120 millimeters.
Both small pyramidal cells and large pyramidal cells are found in motor preconcentral circonvolution and mainly perform movement-related activities.
2- Starred cells
Stellate cells, also known as granulosa cells, are small neurons. They usually have a diameter of about 8 millimeters and have a polygonal shape.
3 - Spindle cells
Spindle cells are neurons that have their vertical longitudinal axis at the surface. They are mainly concentrated in the deepest cortical layers of the brain.
The axon of these neurons originates in the inferior part of the cellular body and is directed towards the white substance like fiber of projection, association or commissural.
4- Horizontal cells of cajal
The horizontal cells of cajal are small fusiform cells that are oriented horizontally. They are found in the most superficial layers of the cerebral cortex and play a critical role in the development of this region of the brain.
These types of neurons were discovered and described by Ramón y Cajal at the end of the 19th century, and subsequent research showed that they are indispensable cells to coordinate neuronal activity.
To reach their position in the cerebral cortex, the horizontal cells of cajal must migrate in a coordinated way during embryogenesis of the brain. That is, these neurons travel from their place of birth to the surface of the cerebral cortex.
Regarding the molecular pattern of these neurons, Victor Borrell and Óscar Marín of the Institute of Neuroscience of Alicante, showed that the horizontal cells of cajal present an orientation of neuronal layers of the cortex during the embryonic development.
In fact, the dispersion of these cells originates during the early stages of embryonic development. The cells are born in different regions of the brain and migrate to the surface of the brain to cover it completely.
Finally, it has recently been shown that meningeal membranes have other functions apart from the protective ones that were supposed at first. The meninges serve as substrate or path of the horizontal cells of cajal for their tangential migration through the surface of the cortex.
5 - Martinotti cells
The last neurons that constitute the neuronal activity of the cerebral cortex are the well-known Martinotti cells. They consist of small multiform neurons present at all levels of the cerebral cortex.
These neurons are named after Carlo Martinotti, a student investigator of Camilo Golgi who discovered the existence of these cells of the cerebral cortex.
Martinotti cells are characterized by being multipolar neurons with short arborescent dendrites. They are disseminated through several layers of the cerebral cortex and send their axons to the molecular layer, where the axonal tree plantations are formed.
Recent research on these neurons has shown that Martinotti cells participate in the inhibitory mechanism of the brain.
Specifically, when a pyramidal neuron (which is the most common type of neuron in the cerebral cortex) begins to overexcite, Martinotti cells begin to transmit inhibitory signals to the nerve cells in their surroundings.
In this sense, it follows that epilepsy could be strongly associated with a deficiency of Martinotti cells or a deficiency in the activity of these neurons. At that time, the nervous transmission of the brain ceases to be regulated by these cells, a fact that causes an imbalance in the functioning of the cortex.
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