The Limbic System

It seems that there is not a global agreement on the structure components of the Limbic System. Therefore a brief history will be given next with the intention to make more understandable the role of the limbic system. However the term limbic, that comes from the Latin word ‘limbus’, meaning boarder, was given by Paul Pierre Broca in 1878(1), who referred to it as the great Limbic Lobe. It was not until 1937 that it was given the role of emotion when James Papez seminal paper was titled “A proposed mechanism of emotion” then referred as the Papez circuit. Later in 1952, Paul D.

MacLean came up with the term ‘Limbic System’ to describe Broca’s Limbic Lobe. Maclean’s formally suggested, with his “Triune Brain Theory”, that the human brain was three brains in one: the Reptilian Complex, the Limbic System and the Neocortex (1). Even though the concept of the Limbic System has been expanded since by Nauta, Heimer and others; one is going to focus on the role of the components apparently most written about, of the Limbic System; the amygdala, the hippocampus, the hypothalamus and the thalamus. The Limbic System is evolved completely just in mammals and forms a fringe along the inner edge of the cerebrum.

It is involved in emotion, memory and in the drives such as of aggression, sex and hunger (2). The amygdala was identified in the early 19th century by Burdach (1) because of its two almond-shaped look. It is situated on either side of the thalamus at the lower end of the hippocampus and it is responsible for administering the emotions, mostly associated with the emotions of fear, anxiety and aggression; it is also associated with pleasure though in a negative sense. Studies have shown that the amygdala is activated when people see fearful faces (1) and unfamiliar faces (2).

An example is Phelps et. Al. 2000 (2), study mention in Rathus 2004, European American subjects were shown pictures of European Americans and African Americans. In one of the tasks the subjects had to, at the same time, classify the pictures by race and according to the words such as, “good”, “bad”, “evil” and so on shown on the screen (2). The result of European Americans had connected the negative words to the African American photos was not taken as prejudice against, but it was interpreted that the unfamiliarity with the faces and fear of the unknown triggered activity in the amygdala.

Later evidences have proved that the same have not happened when subjects were shown familiar faces such as Michael Jordan and Denzel Washington (2). Other studies have shown that lesioning and electrical stimulation of the amygdala is related with aggressive behaviour in animals. However the opposite was noticed in Heinrich Kluver and Paul Bucy (1939)(2) experiment, as with destruction of the Rhesus monkeys’ amygdala made them docile. The amygdala also reacts in response to unlikable sights sensations or smells.

Besides, poor function of the amygdala can be connected with anxiety, depression, post-traumatic stress disorders and schizophrenia. It is also said that the Amygdala is the brain structure with most variance between female and male animals (3). The hippocampus is not mature before one is two years old; it is involved in memories storage, being responsible for encoding long-term memories and for recalling spatial relationships with one’s world.

Living in a world where everything one experiences fades away is the result of a damaged hippocampus structure. It has an important role not only in the formation of new memories but also with the consolidation of episodic memories which are associated with experience and emotion. Therefore the hippocampus is also involved in ganing, iliminating and recovering of fear to cues and contexts. However, some critics (6) are of the opinion that the Hippocampus has evolved as a consequence of selection influences from animals’ ecological setting.

For that reason researchers should take into account the ecological surroundings to explain clearer the hippocampus functions, because it seems that the hippocampus function might differ depending on the species and their environment subsequently influencing the animal to behave differently. Some studies have showing that other circuits in the brain apart from the hippocampus are involved in memory. Nevertheless studies with alcohol says that such substance is directly implicated in disruption activity in the hippocampus through other routes, so far this impact is poorly understood, still it is seem as a high probability for memory damage.

(7). Another important aspect that affects people’s learning and memory is depression. Investigators have scanned the brain of healthy women and women with a background of different periods of depression and found that those with the longest periods of depression history have smaller hippocampus; they have also scored lower in tests of verbal memory which is directly connected to hippocampus function. This contradicted previous studies that thought reduction of the hippocampus happened with age (8).

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