The prosencephalon is part of the central nervous system and consists of the cerebrum (telencephalon) and diencephalon. In the three vesicle stage of early embryonic development, the prosencephalon represents one of the three primary cerebral vesicles.
What is the prosencephalon?
The prosencephalon (forebrain) includes two major anatomical subunits: the cerebrum (telencephalon) and the diencephalon (diencephalon). Together they represent a significant portion of the brain mass. The use of the term “prosencephalon” is particularly common in the context of embryonic development, when the individual brain areas are not yet differentiated. At the beginning of development, the precursor of the brain consists of the anterior neural tube, which divides into prosencephalon, mesencephalon and rhombencephalon by the fourth week of gestation. This condition is also known in medicine as the three vesicle stage, with the three structures mentioned representing the primary brain vesicles. The prosencephalon subsequently divides into telencephalon and diencephalon, while the mesencephalon remains as such but later forms additional structures such as tectum and tegmentum. The rhombencephalon further differentiates into hindbrain (metencephalon) and afterbrain (myelencephalon). Only rarely does neurophysiology equate the terms “prosencephalon” and “telencephalon” without including the diencephalon.
Anatomy and structure
The telencephalon and diencephalon together form the prosencephalon. The diencephalon is also part of the brain stem and is composed of the thalamus, epithalamus, hypothalamus, metathalamus, and subthalamus. In its gross structure, the telencephalon consists of four areas or lobes, which are the anterior frontal lobe, middle parietal lobe, lateral temporal lobe, and posterior occipital lobe. In addition, the gray and white matter can be distinguished: The latter consists of medullary nerve fibers, while the gray matter mainly contains the cell bodies of the neurons. The cerebral cortex includes numerous areas that serve higher cognitive functions. Embedded in the tissue are demarcated core areas: the basal ganglia. The phylogenetically youngest area of the cerebral cortex is embodied by the neocortex, which consists of six layers of neurons, each performing different functions. Archicortex and paleocortex are older than the neocortex from an evolutionary perspective. Alternatively, the cerebral cortex can be divided into isocortex and allocortex, with the isocortex corresponding to the neocortex. Even finer is the subdivision of the cerebral cortex into its individual convolutions (gyri) and furrows (sulci). This highly differentiated distinction is particularly useful in the context of detailed functional studies.
Function and tasks
The diencephalon plays an important role in the processing of sensory information because functional centers are located in it that bring together corresponding stimuli. Hearing, smell, and vision rely on the diencephalon; it is also important for the generation of emotions. In addition, the diencephalon includes sensory processing centers devoted to both surface sensitivity and depth sensitivity. The neocortex of the telencephalon contains the motor cortex, which is responsible for controlling voluntary movements. Pyramidal and some non-pyramidal cells are located in different layers of the neocortex. Like the diencephalon, the neocortex contains sensory areas responsible for processing sensory stimuli. The association center links emotions and behavior to perception (for example, environmental stimuli), and processing is most likely experience-guided. As part of the limbic system, the archicortex deals with emotions, learning, memory processes, drive, as well as some autonomic nervous system tasks. The hippocampus, located within the archicortex, participates primarily in memory formation, with the fimbria hippocampi and dentate gyrus also involved in other processes. In the paleocortex, the brain processes olfactory stimuli, which is why neurology sometimes refers to it as the olfactory brain. Crucial processing centers of olfactory perception are the bulbus olfactorius, the pedunculus olfactorius, the tractus olfactorii lateralis et medialis, and the trigonum olfactorium.
Diseases
Because the prosencephalon constitutes a large part of the brain, there are innumerable possibilities for disorders to manifest themselves. Neurodegenerative diseases are based on the loss of nerve cells and in this way trigger a loss of function of the affected area. Among these diseases is Alzheimer’s dementia, which symptomatically usually begins with problems affecting short-term memory. The progressive disease can also lead to agnosia, apraxia, speech and language disorders, apathy and motor disorders. Its exact causes are still unknown. Multiple sclerosis is also a neurodegenerative disease. It is characterized by multiple foci of inflammation in the brain and leads to demyelination (demarking) of neurons. As a result, the neurons lack their electrical insulation, and information processing suffers. Ischemic stroke belongs to a different category of neuronal diseases: It results from a circulatory disorder that leads to undersupply of the brain. Depending on which artery is affected and to what extent, different regions of the brain may suffer the effects. Typical symptoms of a stroke include, but are not limited to, visual disturbances, impaired coordination or balance, orientation/comprehension/vocabulary/speech problems, general confusion, neglect, dizziness, nausea, vomiting, difficulty swallowing, headaches, paralysis and numbness. Rapid action is required in the event of a stroke, as the brain is progressively damaged. However, permanent lesions of varying degrees are common. Even during embryonic development, the prosencephalon can be damaged: For example, cocaine use during pregnancy is associated with malformations of the prosencephalon, primarily affecting the median level of the forebrain. Neural tube defects at earlier stages of development can result in severe developmental abnormalities, with some incomplete development of the nervous system.
