Cerebrum

The telencephalon (pronounced /tɛlɛnˈsɛfəlɒn/) is the name for the forebrain, a large region within the brain to which many functions are attributed. Many people refer to it as the cerebrum; however, it is technically referred to as the telencephalon.

As a more technical definition, the telencephalon refers to the cerebral hemispheres and other, smaller structures within the brain, although the telencephalon is one of the larger divisions (in terms of number). It is the anterior-most embryological division of the brain that develops from the prosencephalon.

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Structure

The telencephalon is composed of the following sub-regions;

• Limbic system
• Cerebral cortex or cortices of the cerebral hemispheres.
• Basal ganglia
• Olfactory bulb

Composition

The telencephalon comprises what most people think of as the "brain." It lies on top of the brainstem and is the largest and most well-developed of the five major divisions of the brain. The telencephalon is the newest structure in the phylogenetic sense, with mammals having the largest and most well-developed among all species. It emerges from the prosencephalon, the first of three vesicles that form from the embryonic neural tube.

The traditional division first sectioned the telencephalon into four parts. More recent research describes further sub-divisions.

In humans, the telencephalon surrounds older parts of the brain. Limbic, olfactory, and motor systems project fibers from subcortical (deeper) areas of the cerebrum to parts of the brainstem. Cognitive and volitive systems project fibers from cortical areas of the cerebrum to thalamus and to other regions of the brainstem. The neural networks of the telencephalon facilitate complex learned behaviors, such as language, and contains white matter and grey matter. Grey matter is highly folded; with respect to function, this is thought to allow a greater number of cells in the same volume due to the increase in its surface area. The telencephalon includes regions of archipalliar, paleopalliar, and neopalliar origin. Profound development of the neopallium, which comprises the cerebral cortex, is unique among humans and primates.

Functions

Note: As the telencephalon is a gross division with many subdivisions and sub-regions, it is important to state that this section lists the functions that the telencephalon as a whole serves.

Language and communication

Main article: Language

Speech and language are mainly attributed to parts of the cerebral cortex, which is one portion of the telencephalon. 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).

Movement

The telencephalon attributes motor function to the body. These functions originate within the primary motor cortex and other frontal lobe motor areas. In many cases, when this part of the brain is damaged, the brain is unable to send signals to nerves that innervate muscles' motorneurons, and can lead to diseases such as Motor Neurone Disease. This kind of damage results in loss of muscular power and precision rather than total paralysis.

Olfaction

Main article: Olfaction

The olfactory bulb is on the underside of the anterior-most portion of the brain. This is a rather large portion of the telencephalon in most mammals. However, in humans, this part of the brain is relatively smaller. Damage to the olfactory bulb results in a loss of the sense of smell.

Memory

Main article: Memory

Memory formation is associated with the hippocampus. This association was originally described after a patient (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 problem is also addressed slightly 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

Purpose

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 ^[1], 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. 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.

Effects

PCD in the brain affects glial cells and neurons through apoptosis (4). 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.

Stages

During the stages of apoptosis, which seems to constitute most PCD in the brain, various morphological changes occur, such as:

1. Cell enlargement
2. Membrane blebbage, or inconsistency within the structure of the membrane
3. Pyknosis, or a condensation of chromatin within the nucleus
4. Karyorrhexis, or fragmentation of the nuclei
5. Lack of inflammation
6. Removal by microphages or adjacent glial cells, as organelles and plasma membrane remain intact throughout the process.

Of course, there are some differences to these stages, but they are relatively similar in practice. The apoptosis in the telencephalon can also be characterised distinctly by its DNA pattern, which becomes fragmented into oligonucleosomal fragments of around 180-200 pairs. These give a typical "ladder" pattern when viewed on or in agarose gel electrophoresis.

Cell regeneration

Xenopus laevis

Larval stage

In a study of the telencephalon conducted in Hokkaido University on African clawed frogs (xenopus laevis)^[2], 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.

Developed stage

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.

Effects of abnormality

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.

References

1. ^  Levi-Montalcini, R. (1949) Proliferation, differentiation and degeneration in the spinal ganglia of the chick embryo under normal and experimental conditions. Pages 450 - 502
2. ^  Yoshino J, Tochinai S. Successful reconstitution of the non-regenerating adult telencephalon by cell transplantation in Xenopus laevis. Dev Growth Differ. 2004;46(6):523–34. PMID 15610142
3. ^  Yaginuma, H., Tomita, M., Takashita, N., McKay, S., Cardwell, C., Yin, Q.- Aminobuytric acid immunoreactivity within the human cerebral cortex. Pages 481 - 500
4. ^  Haydar, T. F, Kuan, C., Y., Flavell, R. A. & Rakic, P. (1999) The role of cell death in regulating the size and shape of the mammalian forebrain. Pages 621 - 626

See also

• List of regions in the human brain