The cerebral cortex It is the outermost layer of the brain. It is composed of a thin sheet of neurons with their interconnections and forms a layer a few millimeters thick that covers the irregular surface of the cerebral hemispheres.
Bark it is the most developed part ofhuman brain and is responsible for thinking, perceiving, producing and understanding language. It performs functions such as language and abstract thinking, along with perceptual, movement and other aspects. Most of the information processing occurs in the cerebral cortex.
- 1 Morphology of the cerebral cortex
- 2 Histological characteristics of the cerebral cortex
- 3 Functional location
- 4 Cortical areas
Morphology of the cerebral cortex
The cerebral cortex is made up of very compact neurons and is the outermost and wrinkled layer that surrounds the brain. He is also responsible for more elaborate thinking processes, including speech and decision making. The cortex is divided into four different lobes, the frontal, the parietal, the temporal and the occipital, which are responsible for the processing of different types of sensory information.
The crust is very folded and only one third is exposed to the surface. The rest is hidden in large and small cracks, more or less deep, called fissures or grooves. The raised surfaces form the brain turns or convolutions.
The most important fissures are the following:
- Central or Rolando
- Side or Silvi
- Longitudinal (separates the two hemispheres: right and left)
The fissures mark boundaries on the cortical surface and divide it into the four large lobes that we have below:
- Frontal (prior to the central fissure)
- Parietal (from the central fissure to the parietooccipital)
- Occipital (after parietooccipital fissure)
- Temporary (ventral to lateral fissure)
Within each lobe there are different convolutions. The most important are the following:
- Precentral or ascending frontal (in the frontal lobe, adjacent to the central fissure)
- Postcentral or ascending parietal gyrus (the parietal lobe, adjacent to the central fissure)
- Cingulate or gyrus of the corpus callosum (in the medial part of the hemispheres, arched around the corpus callosum)
- Hippocampal or hippocampus (in the medial face of the hemispheres, in the temporal lobe)
Histological characteristics of the cerebral cortex
The cerebral cortex does not have the same structure in all places. Almost all that we observe from the outside is of type neocortex. It represents more than 90% in humans and receives this name because it appeared quite late in the evolution of vertebrates.
The rest is formed by paleocortex Y archetype. These names refer to their oldest origins. Paleocortex corresponds to areas of the base of the hemispheres that are responsible for olfactory functions. On the other hand, the archetype corresponds to the hippocampal formation.
On the other hand, if we look at the structure in parallel layers of the cerebral cortex, we see that there are areas that have six layers, called isocortex. Along with areas that have less than six layers is alocortex. The isocortex corresponds to the neocortex, while the allocortex to the oldest phylogenetically cortices.
The two main types of bark cells are the following:
The starry or granular cells
They are small short axon neurons that do not leave the cortex; They are the main cortical interneurons.
The pyramidal cells
They get their name by the pyramidal shape of their soma. They are neurons that vary greatly in size, and giant pyramidal cells can be found in the motor cortex, which are some of the largest in the CNS.
From the upper vertex, there is a long apical dendrite that rises towards the surface of the cortex. Basal dendrites leave the base of the cell that extend horizontally. The axon is usually long and synaptic with other cortical areas or with subcortical structures. They are the main efferent neurons of the cortex.
There are other types of cells in the cortex, as are the following:
- Horizontal (or Cajal) cells. They are interneurons that are in the outermost layer; They usually disappear after birth.
- Fusiform cells. They are spindle-shaped, found in the deepest cortical layer and their axons leave the cortex.
- Martinotti cells. They are interneurons that are found in all layers and send their axons to the surface.
Layers of the bark
The neocortex cells are organized in six layers, which we will call from the surface inwards in the following classification:
- Molecular layer. It is essentially a synaptic layer.
- Outer granular layer. It contains many small cells, both starred and pyramidal; references come from other cortical areas.
- External pyramid layer. It contains pyramidal cells that send their axons to areas of the cortex or subcortical destinations.
- Inner granular layer. It consists mainly of star cells, closely grouped; Many of these neurons receive affectations from the thalamus.
- Internal pyramid layer. It contains pyramidal, starred and Martinotti cells; the axons are of subcortical projection, the trunk and the medulla, mainly.
- Multiform (or polymorphic) layer. Fusiform cells predominate and send information to the cortex, the thalamus, the striated nuclei.
Depending on the numerical balance between granular and pyramidal cells, the isocortex receives various denominations. When the proportion is similar we talk about homotypic isocortex. While when the ratio is broken it is defined as granular heterotypic isocortex (predominance of layers II and IV) or agranular heterotypic isocortex (predominance of layers III and V).
In the motor cortex, we found a predominance of pyramidal cells with long axons that send the motor orders to the muscles (agranular isocortex). On the other hand, in the sensory areas there are small neurons that connect with adjacent areas (granular isocortex).
The white fibers or substance of the cerebral hemispheres are called the medullary center and are formed by the following three types of fibers:
- Projection fibers. They connect the cortex with subcortical structures and can be afferent or efferent. An example is the internal capsule.
- Corticocortical fibers. They connect different areas of the cortex.
- Association Fibers. They connect different cortical areas of the same hemisphere, for example, the arched fascicle.
- Commissural fibers. Both hemispheres interconnect, for example, the anterior commissure and the corpus callosum.
The various cortical areas show quite evident structural differences (for example, granular cortex before granular). Some anatomists tried to map the cortex taking into account these differences and to plot the cortex in different areas.
The system most used as a reference is the one created by Brodmann in 1909. He divided the crust of each hemisphere into fifty-two areas with different structures. He also suggested that they perform different functions.
The neocortex of each hemisphere is formed by the following areas:
Primary sensory areas
The specific thalamic nuclei receive sensory information directly. Each sense (vision, hearing, etc.) has its primary sensory area, which encodes this sensory information. They have a topographic organization. The body surface, the external world or the range of audible frequencies are represented on the surface of the cortex.
There are three main ones, which are the ones we have below:
- Primary somatic or somatic cortex. Receive general sensitivity information (touch, pain, temperature, pressure, etc.) from the posterior ventral nucleus of the thalamus.
- Primary visual cortex. Receive visual information of the lateral geniculate nucleus of the thalamus.
- Primary auditory cortex. Receive auditory information of the medial geniculate nucleus of the thalamus.
Primary motor area
It is located in the frontal lobe. The descending motor tracts originate (for example, pyramidal fascicle). They range from the cortex to the motor neurons of the trunk and medulla to control the execution of body movements.
Most of the human neocortex is of association and is subdivided into the following two types:
- It is also called upper level sensory cortex.
- It is next to a primary area.
- It prepares the information it receives from the thalamus or the primary area at a more complex level.
The areas surrounding the primary visual area (Brodmann's 17). They are the areas of visual association or higher level visual areas (18 and 19).
- They integrate more than one sensory modality and receive information from the different sensory areas of association.
- They plan movements.
- They have higher intellectual functions (language, writing, abstract thinking, integrated perception, etc.).
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