The isocortex is the largest area in the cerebral cortex. As such, it is a part of the human brain and is classified as part of the central nervous system.
What is the isocortex?
The isocortex is also known as the neocortex. It occupies almost the entire portion of the cerebral cortex. The isocortex can be divided into three areas based on its various functions. The processing of sensory afferents takes place in its primary fields. They are also considered the site of origin of the motor pathways. The secondary fields are upstream of the primary fields and interpret the sensory impulses. The association fields usually have no direct tasks, but are devoted to long-range processing of all fields. The isocortex is histologically divided into six layers. The neocortical functions are separated into different areas. These are called lobes. They include the frontal lobe, which contains the pyramidal tract or motor speech center. The parietal lobe processes stimuli and allows them to become conscious. The occipital lobe processes visual stimuli and the temporal lobe contains the auditory pathway. The isocortex consists of pyramidal and non-pyramidal cells. The pyramidal cells are neurons that carry information from the sensory organ to the processing site in the brain. About 85% of the neurons in the human brain are pyramidal cells. The non-pyramidal cells have an inhibitory effect in the transmission of information. They are subdivided into further cells.
Anatomy and structure
The isocortex has a total of six layers. It occupies almost the entire hemispheric surface. It is approximately 3-4 mm thick and contains over 10 million neurons. Furthermore, it contains about 10 times as many glial cells. The isocortex is about 2,000 cm² and occupies about 90% of the cerebral cortex. In it, there are pyramidal cells and non-pyramidal cells. The pyramidal cells are projection neurons and act through the transmitter glutamate. The non-pyramidal cells are interneurons and act via the transmitter GABA. The six layers have different characteristics. They include, in ascending order, the molecular layer or lamina molecularis, the outer granular layer or lamina granularis externa, and the outer pyramidal layer or lamina pyramidales externa. The fourth layer is the inner granular layer or lamina granularis interna. The penultimate layer is called the inner pyramidal layer or lamina pyramidales interna. The sixth and last layer is called the multiform layer or lamina multiformis. They are distinguished by the cells they contain, their tasks, and cell density.
Function and tasks
Most of the tasks and functions of the human organism are coordinated in the isocortex. Information received from the sensory organs, such as the eyes and ears, is processed here. Motor activity is initiated and initiated in the isocortex. Functionally, the isocortex can be divided into primary, secondary and association fields. Primary fields are sensory centers. These receive their sensory afferents directly from the thalamus and serve to transport the received stimulus information into consciousness without further interpretation. Thus, it is possible for visual and certain tactile stimuli to be classified immediately. They include, for example, the visual and auditory pathways or the perception of pain and temperature. Some vital information is classified in the isocortex. The precentral gyrus belongs to the primary motor field. It regulates movement initiations. The primary field is the site of origin for descending motor pathways. The secondary fields include the regions in the brain that perform interpretation of the received stimuli in terms of mapping. The received stimuli are recognized, interpreted accordingly, and action consequences are prepared. The association fields are connected with the primary and secondary fields. So-called circumscribing functions are assigned to them. This includes, for example, the motor speech center. In addition to stimulus interpretation and motor execution, some regions of the isocortex are co-responsible for learning. Together with the hippocampus, it is involved in memory formation. The human brain is plastic. Throughout life, changes take place that are modulated in the isocortex.
Diseases
The third layer of the isocortex has an important function in the symptoms of Alzheimer’s disease. The degneration of cerebral neurons that occurs in this disease affects the lamina pyramidales externa at a very early stage. Moreover, the cell loss of the outer pyramidal layer is particularly affected overall. For this reason, connections to other areas of the cortex are damaged or fail at a very early stage. This is considered to be particularly severe. The fifth layer of the isocortex becomes involved at a later stage in this disease. The inner pyramidal layer is particularly spread out in the motor cortex. It is damaged less or later. As a result, the descending pathways remain unimpaired and perform their tasks in the early stages of Alzheimer’s disease. In everyday life, therefore, Alzheimer’s patients first experience memory loss and less movement disorders. At a later stage, however, paralysis and gait unsteadiness can be expected. The Wernicke and Broca areas are located in the isocortex. Both are language centers. When the Wernicke center fails, there is an inability to understand speech. The patient is no longer able to correctly understand what is spoken or read. As soon as a language is no longer understood, the patient also loses the ability to use the language. Sounds and words can be repeated, but meaningful word formations or sentences can no longer be produced. A failure of Broca’s speech center results in a failure of speech production. At the same time, speech comprehension is preserved.