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The telencephalon (pronounced /tɛlɛnˈsɛfəlɒn/), cerebrum, or forebrain is the most anterior or, especially in humans, most dorsal region of the vertebrate central nervous system. "Telencephalon" refers to the embryonic structure, from which the mature "cerebrum" develops. The dorsal telencephalon, or pallium, develops into the cerebral cortex, and the ventral telencephalon, or subpallium, becomes the basal ganglia. The cerebrum is also divided into symmetric left and right cerebral hemispheres.
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During vertebrate embryonic development, the prosencephalon, the most anterior of three vesicles that form from the embryonic neural tube, is further subdivided into the telencephalon and diencephalon. The telencephalon then forms two lateral telencephalic vesicles which develop into the left and right cerebral hemispheres.
The cerebrum is composed of the following sub-regions:
The cerebrum comprises what most people think of as the "brain." It lies in front or on top of the brainstem and in humans is the largest and most well-developed of the five major divisions of the brain. The cerebrum is the newest structure in the phylogenetic sense, with mammals having the largest and most well-developed among all species. In larger mammals, the cerebral cortex is folded into many gyri and sulci, which has allowed the cortex to expand in surface area without taking up much greater volume. See also Cerebral Cortex.
In humans, the cerebrum surrounds older parts of the brain. Limbic, olfactory, and motor systems project fibers from the cerebrum to the brainstem and spinal cord. Cognitive and volitive systems project fibers from the cerebrum to the thalamus and to specific regions of the midbrain. The neural networks of the cerebrum facilitate complex behaviors such as social interactions, learning, working memory, and in humans, speech and language.
Note: As the cerebrum is a gross division with many subdivisions and sub-regions, it is important to state that this section lists the functions that the cerebrum as a whole serves. See main articles on cerebral cortex and basal ganglia for more information.
The cerebrum directs the conscious or volitional motor functions of the body. These functions originate within the primary motor cortex and other frontal lobe motor areas where actions are planned. Upper motor neurons in the primary motor cortex send their axons to the brainstem and spinal cord to synapse on the lower motor neurons, which innervate the muscles. Damage to motor areas of cortex can lead to certain types of motor neuron disease. This kind of damage results in loss of muscular power and precision rather than total paralysis.
The olfactory bulb in most vertebrates is the most anterior portion of the cerebrum, and makes up a relatively large proportion of the telencephalon. However, in humans, this part of the brain is much smaller, and lies underneath the frontal lobe. The olfactory sensory system is unique in the sense that neurons in the olfactory bulb send their axons directly to the olfactory cortex, rather than to the thalamus first. Damage to the olfactory bulb results in a loss of the sense of smell.
Speech and language are mainly attributed to parts of the cerebral cortex. Motor portions of language are attributed to Broca\'s area within the frontal lobe. Speech comprehension is attributed to Wernicke\'s area, at the temporal-parietal lobe junction. These two regions are interconnected by a large white matter tract, the arcuate fasciculus. Damage to the Broca\'s area results in expressive aphasia (non-fluent aphasia) while damage to Wernicke\'s area results in receptive aphasia (also called fluent aphasia).
Memory formation is attributed to the hippocampus and associated regions of the medial temporal lobe. This association was originally described after a patient known as (HM) had both his hippocampuses (left and right) surgically removed to treat severe epilepsy. After surgery, HM had anterograde amnesia, or the inability to form new memories. This condition is also portrayed in the film Memento, in which the protagonist has to take pictures of people he has met in order to be able to remember what to do in the days following his accident.
Programmed Cell Death (PCD) is not uncommon in the telencephalon. It is thought to be one of the processes by which growth and differentiation occurs, and is a universal feature of the embryonic and postnatal central nervous system , and has been noted in the telencephalons of rats and mice. In some animals, such as the monkey, over 50% of neurons in the cerebral cortex are affected by PCD during early stages of life.[citation needed] This is thought to solicit growth of the brain due to increase in the size of the cranium and other parts of the body expected to grow throughout the life cycle of a monkey.
The main reason for PCD is to create space for new cells. If a neuron does not establish correct synaptic connections, it will die. This is seen as a form of "competition" within the space of the telencephalon and is a form of "survival of the fittest" (see Neural Darwinism). However, there are exceptions to the rule; in rats some cells are even programmed to die during proliferation within the ventricular zones of the telencephalon. It is thought that this is at a stage during which axons are not yet formed or synaptically connected.
PCD in the brain affects glial cells and neurons through apoptosis . According to research on rodents, this period is usually during developmental or adolescent stages. During this time, the regeneration process can take place because the "materials" and environment are a perfect breeding ground for cell regeneration.
In a study of the telencephalon conducted in Hokkaido University on African clawed frogs (xenopus laevis), it was discovered that, during larval stages, the telencephalon was able to regenerate around half of the anterior portion (otherwise known as partially truncated), after a reconstruction of a would-be accident, or malformation of features.
The regeneration and active proliferation of cells within the clawed frog is quite remarkable, regenerated cells being almost functionally identical to the ones originally found in the brain after birth, despite the lack of brain matter for a sustained period of time.
This kind of regeneration depends on ependymal layer cells covering the cerebral lateral ventricles, within a short period before, or within the initial stage of wound-healing. This is observed within the stages of healing within larvae of the clawed frog.
The regeneration within the developed stage of the clawed frog is different from that in the larval stage. Because the cells adhere to one another, they are unable to form an entity that can cover the cerebral lateral ventricles. Thus, the telencephalon remains truncated and the loss of function becomes permanent.
After removing over half of the telencephalon in the developed stage of the clawed frog, the lack of functions within the animal was apparent, manifesting with obvious difficulties in movement, nonverbal communication between other species, as well as other difficulties thought to be similar to those seen in humans.
This kind of regeneration is still relatively unknown in regard to regeneration within larval stages, similar to the human fetal stage.
| Brain: telencephalon (cerebrum, cerebral cortex, cerebral hemispheres) | |
|---|---|
| Primary sulci/fissures | Medial longitudinal, Lateral, Central, Parietoöccipital, Calcarine, Cingulate, Callosal Collateral fissure |
| Frontal lobe | Precentral gyrus (Primary motor cortex, 4), Precentral sulcus, Superior frontal gyrus/Frontal eye fields (6, 8, 9), Middle frontal gyrus (46), Inferior frontal gyrus (44-Pars opercularis, 45-Pars triangularis), Orbitofrontal cortex (10, 11, 12, 47) |
| Parietal lobe | Somatosensory cortex (Primary (1, 2, 3, 43), Secondary (5)), Precuneus (7m), Parietal lobules (Arcuate fasciculus/Superior (7l), Inferior (40)), Angular gyrus (39), Intraparietal sulcus, Marginal sulcus |
| Occipital lobe | Primary visual cortex (17), Cuneus, Lingual gyrus, 18, 19 - Lateral occipital sulcus |
| Temporal lobe | Primary auditory cortex (41, 42), Superior temporal gyrus (38, 22), Middle temporal gyrus (21), Inferior temporal gyrus (20), Fusiform gyrus (37) Medial temporal lobe (Amygdala, Hippocampus, Parahippocampal gyrus (27, 28, 34, 35, 36) |
| Cingulate cortex/gyrus | Subgenual area (25), anterior cingulate (24, 32, 33), Posterior cingulate (23, 31), Retrosplenial cortex (26, 29, 30), Supracallosal gyrus |
| white matter tracts | Corpus callosum (Splenium, Genu, Rostrum, Tapetum), Septum pellucidum, Internal capsule, Corona radiata, External capsule, Olfactory tract, Fornix (Commissure of fornix), Anterior commissure, Posterior commissure Terminal stria Superior and Inferior longitudinal fasciculus, uncinate fasciculus, cingulum, Inferior occipitofrontal fasciculus |
| Neurotransmitter systems | Dopamine system (mesocortical pathway, mesolimbic pathway, nigrostriatal pathway, tuberoinfundibular pathway) |
| Basal ganglia | Striatum (Putamen,Caudate nucleus, Nucleus accumbens), Globus pallidus, Claustrum, Subthalamic nucleus, Substantia nigra |
| Other | Insular cortex Olfactory bulb, Anterior olfactory nucleus, Septal nuclei, Basal optic nucleus of Meynert |
| Some categorizations are approximations, and some Brodmann areas span gyri. | |
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