CEREBRAL CORTEX: LAYERS, FUNCTIONS, NEURONS - NEUROPSYCHOLOGY - 2025

The cerebral cortex or cerebral cortex is the nervous tissue that covers the surface of the cerebral hemispheres. It is the highest 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 human beings. In this region, functions such as perception, imagination, thought, judgment or decision are carried out.

Anatomically it has a series of thin layers made up of gray matter, which are found above a wide collection of white matter pathways.

The cerebral cortex adopts a convoluted shape, so if it were extended it would present a very extensive mass. Specifically, research indicates that the total surface area of ​​the cerebral cortex could consist of about 2,500 square centimeters.

Likewise, this great mass of brain is characterized by containing a huge number of neurons inside. In general, it is estimated that in the cerebral cortex there are about 10 billion neurons, which would make about 50 trillion synapses.

Characteristics of the cerebral cortex

The cerebral cortex of humans is represented by a sheet of gray matter, which covers the two cerebral hemispheres. It has 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 human beings possess (sight, touch, smell, taste and touch) develop in a specific region of the cortex. That is, each sensory modality has a defined territory within the cerebral cortex.

Apart from the sensory regions, the cerebral cortex also has multiple secondary somatic, association, and motor regions. In these areas, the cortical afferent and association systems are elaborated, giving rise to learning, memory and behavior.

Cerebrovascular system. Source: Bruce Blaus via Wikimedia Commons

In this sense, the cerebral cortex is considered an especially relevant region when 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 carried out in different areas of the cerebral cortex.

For this reason, the cerebral cortex constitutes a structure that, from the human perspective, acquires maximum complexity. The cerebral cortex is the result of a slow evolutionary process that may have started more than 150 million years ago.

Layers

The main characteristic of the cerebral cortex is that it is made up of different layers of gray matter. These layers make up the structure of the cortex and define its structural and functional organization.

The layers of the cerebral cortex are not only characterized by being defined from a structural point of view, but also from a phylogenetic perspective. In other words, each of the layers of the cerebral cortex corresponds to a different evolutionary moment. At the beginning of the human species, the brain was less developed and the cortex had fewer layers.

Development of the human cerebral cortex. Source: Van Essen Lab (Washington University in St. Louis), in collaboration with Terrie Inder, Jeff Neil, and Jason Hill, among others. GNU Free Documentation License, via Wikimedia Commons

Through the evolution of the species, these layers have been increasing, a fact that is related to the increase in cognitive and intellectual capacities of human beings over time.

Molecular layer

The molecular layer, also known as the plexiform layer, is the most superficial region of the cerebral cortex and, therefore, the newest one.

It has a dense network of nerve fibers that are oriented tangentially. These fibers are derived from pyramidal and spindle cell dendrites, the axons of stellate and Martinotti cells.

Afferent fibers originating from the thalamus, association and commissural fibers can also be found in the molecular layer. Being the most superficial region of the cortex, a large number of synapses are established between different neurons in the molecular layer.

Outer 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 stellate cells.

The dendrites of the cells of the outer granular layer end in the molecular layer and the axons enter deeper layers of the cerebral cortex. For this reason, the outer granular layer is interconnected with the different regions of the cortex.

Outer pyramidal layer

The outer pyramidal layer, as its name suggests, is made up of pyramidal cells. It is characterized by presenting an irregular shape, that is, the size of the layer increases from the superficial limit to the deepest limit.

The dendrites of the neurons of the pyramidal layer pass to the molecular layer and the axons travel as projection, association or commissural fibers to the white matter located between the layers of the cerebral cortex.

Inner granular layer

The inner granular layer is composed of stellate cells that are arranged very compactly. It has a high concentration of horizontally arranged fibers known as Baillarger's outer band.

Ganglion layer

The ganglionic layer or inner pyramidal layer contains very large and medium-sized pyramidal cells. Likewise, they include a high number of horizontally arranged fibers that form the inner Baillarger band.

Multiform layer

Finally, the multiform layer, also known as the polymorphic cell layer, basically contains spindle cells. Likewise, they include modified pyramidal cells with a triangular or ovoid cell body.

Many of the nerve fibers of the multiform layer enter the underlying white matter and connect the layer with the intermediate regions.

Functional organization

Nervous system and brain

The cerebral cortex can also be organized according to the activities that are carried out in each region. In this sense, certain areas of the cerebral cortex process specific signals of a sensory, motor and association nature.

Sensitive areas

Sensory areas are regions of the cerebral cortex that receive information of a sensitive nature and are closely related to perception.

Information accesses the cerebral cortex mainly through the posterior half of both cerebral hemispheres. The primary areas contain the most direct connections with peripheral sensory receptors.

On the other hand, the secondary sensory and association areas are usually adjacent to the primary areas. In general, these receive information both from the primary association areas themselves and from the lower regions of the brain.

The main task of the association areas and the secondary areas is to integrate the sensitive experiences to generate patterns of recognition and behavior. The main sensitive regions of the cerebral cortex are:

1. The primary somatosensory area (areas 1, 2 and 3).
2. The primary visual area (area 17).
3. The primary auditory area (area 41 and 42).
4. The primary taste area (area 43).
5. The primary olfactory area (area 28).

Motor areas

Main convolutions and sulci of the cerebral cortex. Source: Lorenzo Bandieri via Wikimedia Commons

The motor areas are found in the anterior part of the hemispheres. They are responsible for starting brain processes related to movement and giving rise to such activities.

The most important motor areas are:

1. The primary motor area (area 4).
2. Drill's language area (area 44 and 45).

Association areas

The association areas of the cerebral cortex correlate with the more complex integration functions. These regions carry out activities such as memory and cognition processes, the management of emotions, and the development of reasoning, will or judgment.

Association areas play an especially important role in the development of personality and character traits in people. Likewise, it is an essential brain region in the determination of intelligence.

The association areas comprise both certain motor areas and specific sensory regions.

Nerve cells

The cerebral cortex has a great variety of cells inside. Specifically, five different types of neurons have been specified in this region of the brain.

Pyramidal cells

Human pyramidal neuron observed by the Golgi method. Source: Bob Jacobs, Laboratory of Quantitative Neuromorphology Department of Psychology Colorado College via Wikimedia Commons

Pyramidal cells are neurons that are characterized by having a pyramid shape. Most of these cells contain a diameter of between 10 and 50 microns.

However, there are also large pyramidal cells. These are known as Betz cells and can have a diameter of up to 120 microns.

Both the small pyramidal cells and the large pyramidal cells are found in the precentral motor gyrus and perform primarily movement-related activities.

Stellate cells

Stellate cells, also known as granule cells, are small neurons. They usually have a diameter of about 8 micrometers and have a polygonal shape.

Spindle cells

Spindle cells are neurons that have their vertical longitudinal axis on the surface. They are mainly concentrated in the deeper cortical layers of the brain.

The axon of these neurons originates in the lower part of the cell body and is directed towards the white matter as a projection, association or commissural fiber.

Horizontal cells of Cajal

Cajal horizontal cells are small spindle cells that are horizontally oriented. 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 essential cells for coordinating neuronal activity.

To reach their position in the cerebral cortex, horizontal Cajal cells must migrate in a coordinated manner during brain embryogenesis. 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 from the Institute of Neuroscience of Alicante, demonstrated that the horizontal cells of Cajal present an orientation of the neuronal layers of the cortex during embryonic development.

In fact, the dispersal of these cells originates during the initial stages of embryonic development. The cells are born in different regions of the brain and migrate towards the surface of the brain until they cover it completely.

Finally, it has recently been shown that the meningeal membranes have other functions apart from the protective ones that were originally supposed. The meninges serve as a substrate or pathway for the horizontal cells of Cajal for their tangential migration through the surface of the cortex.

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 researcher of Camilo Golgi who discovered the existence of these cells in the cerebral cortex.

Martinotti cells are characterized by being multipolar neurons with short arborescent dendrites. They are disseminated through various layers of the cerebral cortex and send their axons to the molecular layer, where axonal arborizations are formed.

Recent research on these neurons has shown that Martinotti cells participate in the brain's inhibitory mechanism.

Specifically, when a pyramidal neuron (which is the most common type of neuron in the cerebral cortex) begins to become overexcited, the Martinotti cells begin to transmit inhibitory signals to surrounding nerve cells.

In this sense, it follows that epilepsy could be strongly associated with a deficit of Martinotti cells or a deficiency in the activity of these neurons. At those times, the brain's nerve transmission is no longer regulated by these cells, a fact that causes an imbalance in the functioning of the cortex.

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