How plastic is the adult brain?

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SALMA KHANAM KS5BR

HOW PLASTIC IS THE ADULT BRAIN?

For centuries, neuroscientist believed that neurogenesis within the adult brain remained fixed. Another assumption was that the higher vertebrates and humans are born with their full complement of neurons, with the restriction of postnatal neurogenesis taking place. The aim of this essay will target the new findings that will review the dogma that neurogenesis does occur and will be able to give a broader understanding of the complexity of the adult brain and its function.

Previous research into neurogenesis assumed there was no logic that the adult circuitry had the capability of being able to add new components after development. Nevertheless, recent research has provided the key evidence that the adult brain does have the capability for neurogenesis to occur, which will bring this long held dogma down.

 Since the late twentieth century, new research into this dogma has proved to be wrong. It has now been established that there are findings in the brain that suggest that the stem cell do divide, however this occurs in two parts of the brain, the dentate gyrus (Gage et al., 1995) [2] and the subreventicula zone of the adult brain (Reynolds and Weiss, 1992; Richards et al., 1992). [2]They showed that we no longer had to consider that a complex neuron was required to divide for adult neurogenesis to occur. Now we know that these neurons can be generated from primitive cells, similar to what happens in development.

Firstly, the dentate gyrus of the hippocampus, which is the part of the brain associated with our learning and memory. The second part of the brain is the subreventicular zone of the anterior lateral ventricles and its projection through the rostral migratory stream to the olfactory bulb that allows the new neuron to generate within the brain. The discovery of neural stem cells (NSC) and adult neurogenesis provides a new theoretical framework for understanding processes regulating brain plasticity (Gage 2000)[3] which are clearly present but they are suppressed from dividing.

Cameron and McKay 2001 (3) suggested an important role in brain plasticity was the NSC. Thousands of cells, which are repeatedly being formed, of which most of them incorporate as new functional neurons. Gould 1999 a (3) research also supports the previous study and that application of these findings, could be applied to other species. Also environment, genetics, and drugs alter neurogenesis in a manner that is consistent with NSC having an impact on plastic processes such as learning and mood (Duman et al. 2001; Gould 1999 b) (3).

Nottebohm’s study showed a remarkable breakthrough into neurogenesis as far as rodents, primates and humans were concerned. Song singing birds the canaries sing throughout the breeding season and then in the late summer they forget it. It was thought that when a new song repertoire was acquired which may be due the growth of the new dendritic segments and synapse, and the new neurons that were being generated were replacing the dead neurons.

Using (3h) thymidine, which when injected incorporates into the DNA, being manufactured during mitosis, and therefore labels the newly generated cells. The canary birds are capable of producing thousands of new neurons everyday in the ventricle and showing widespread neurogenesis. While the cells were replacing the existing ones in the telecephalon in the adult brain, the neurons based on morphology and physiology, were being recruited into the song singing of the birds.

However, studies such as these led to the questioning that there could be no- new- neurons. The plasticity of the adult brain had to be dependent on them adapting to environmental demands.

Although this phenomenon has not just been witnessed in birds, but has been demonstrated in other mammals using the same technique. After electrolytic lesions combined with intracranial injection of the marker in adult rats, there was an accumulation of reduced silver grains over the nuclei of a few neurons in the neo-cortex and over the granule cells of the hippocampus.

Subsequently, formation of new cells in the granule layers of the dentate gyrus of the hippocampus and olfactory bulb following intraperitoneal or intracentricular injections in normal adult rats and cats were evident.

The willingness to accept adult neurogenesis was further enhanced by the convincing evidence that fetal tissue could be grafted in the adult intact brain. Even more convincing was the evidence that the damaged adult brain and spinal cord allowed these newly grafted cells to survive and differentiate (Bjorklund and Gage, 1985). The grafted cells could also receive and send connections and release transmitter in a behavior- dependent manner in the adult damaged brain (Dunnett and Bjorklund, 1994)

To prove the strength of the production of neurogenesis, research on those who have either suffered from some form of injury to the brain that has restricted them from certain capabilities, such as those who have had a stroke, Parkinson’s disease or Alzheimer patients have shown some remarkable findings as the extent of which the old neurons can be regenerated.

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Injury to the adult central nervous system (CNS) is devastating because of the inability of the central nervous neurons to regenerate correct axonal and dendritic connections. Injury can lead to neuron degeneration and cell death. Raymon y Cajal 1928 and Tello 1911 showed that adult CNS neurons could re-grow if they have access to the permissive environment of a conditioned sciatic nerve.

Further studies were replicated with new methods by Aguayo and colleagues and confirmed that the adult CNS neurons the ability to regenerate.

As the CNS could not regenerate it was not due to intrinsic deficits ...

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