The Limbic System

Introduction to Psychology 2 – PY1904 - ICS student 20144782

2.- “Discuss the role of 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 ...

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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).

The hypothalamus has a cone-shaped structure in the brain which is located just below the thalamus and above the pituitary gland. It could probably be said that the hypothalamus is one of the busiest part of the brain. It is a very important part of the limbic system in the mediation of endocrine, autonomic and behavioural functions (5). It is in charge of regulating homeostatic processes in the body, it works as a thermostat , for example if one’s room gets very cold it conveys this information and turns its thermostat on, or vice versa if one’s room is too hot it turns off (4). The hypothalamus is thought to have “the biological clock” which regulates some of the body functions at different times of the day and that differ for a period of time such as the menstrual cycle. The hypothalamus is involved in, concentration of fluids, it controls the release of eight main hormones in the body; storage of nutrients and mediates several aspects of emotion responses and motivation. It is in control of hunger, thirst, levels of pleasure, anger, sexual behaviour, aggression and the autonomous nerves system (ANS). The hypothalamus, as well as having its own receptors, it gets information from a number of external and internal sources and it also controls the pituitary gland secretions. Information about blood pressure from the vagus nerve, input about light and darkness from the optic nerve, and the discharge of hormones from the pituitary are examples of the many stimuli from ’high centres’ performing on the hypothalamus.

Although hypothalamic disorders seem to occur from brain tumours and not being present from birth (3), the hypothalamus seems to be the least well understood area from the hypothalamo-pituitary axis (9).

The thalamus is situated near the central area of the brain, the forebrain. It is known as the relay station for sensory information before it is passed to the cortex; it is involved in the functions of sleep and attention, in coordination with other brain structures besides being responsible for motor control in the body. Experiments with monkeys give the result that the thalamic nucleus are densely interconnected with prefrontal cortex (PFC) which is crucial to new learning and relearning. In addition some have shown that the mediodorsal thalamus is impaired in object recognition and stimulus-reward association. These latest findings prove the importance of the thalamic nucleus in memory processing and during new learning process. Lesioning some of the thalamic nucleus in monkeys provoked a high margin of error in new learning (8). Nuclei groups are directly related to the thalamus, receive pain afferents and have an important role on pain perception, also with extend damage to some nuclei groups movement disorders may occur (11). Based on investigation during surgery in a patient with thalamic stroke, it was identified that thalamic reorganization is possible and may help recovery from such stroke (10).

In conclusion it laid out in is paper some roles of the Limbic System and its interchangeable connectivity. For instance we could see that the amygdala, the hippocampus, the hypothalamus and the thalamus all have in some way to do with emotion and memory, therefore the so called the emotion brain. Such connection is essential to their efficient functions. Even though there still a lot unknown about the Limbic System what has been learnt is definitely encouraged scientists and researchers to carry on looking into it in more detail. As mention above, environmental reasons are one of the elements that have recently been taken into consideration by some investigators. All in all the role of the Limbic System is crucial to the human beings life.

References:

(8) - Depression May Shrink Brain Structure , Sheline YI et al. Depression Duration but not Age Predicts Hippocampal Volume Loss in Medically Healthy Women with Recurrent Major Depression. The Journal of Neuroscience, vol. 19, no. 12, pp. 5034-5043, June 15, 1999. Adapted from materials provided by , material available online from, , retrieved in October 2008.

(4) - Endocrine System, material available online from , retrieved in April 2008.

(9) - FUNCTIONS OF THE HYPOTHALAMO-PITUITARY AXIS, material available online from, , retrieved in October 2008.

(5)-Hypothalamus, material available online from , retrieved in April 2008.

(7) - Indirect Effects of Alcohol on Hippocampal Function, by National Institute of Health, material available online from, , retrieved in October 2008.

(2) - Rathus, S.A. (2007), Psychology, Concepts & Connections, 8th Edition, Wadsworth. USA.

(10) - Reorganization of somatic sensory function in the human thalamus after stroke, Shinji Ohara, MD, PhD, Frederick A. Lenz, MD, PhD, material available online from retrieved in September 2008.

(1) - The Limbic System by V. RajMohan, F. Mohandas, Elite Mission Hospital, Trichur, Kerala, India, material available online from , retrieved in October 2008.

(3) - The Limbic System, material available online from , retrieved in April 2008.

(6) - The hunting of the hippocampus function, by Crusio, Eim E. (1997), material available online from, , retrieved in April 2008.

(11)-Thalamus, material available online from , retrieved in September 2008.

References of other read materials:

- Central Pain in the Absence of Functional Sensory Thalamus, A.G. Parrenta, AM. Lozanoa, O. Dostrovskyb, R.R. Taskera, material available online from , retrieved in September 2008.

- Dissociable Performance on Scene Learning and Strategy Implementation after Lesions to Magnocellular Mediodorsal Thalamic Nucleus by Anna S. Mitchell, Mark G. Baxter, and David Gaffan, Department of Experimental Psychology, Oxford University, Oxford OX1 3UD, United Kingdom, material available online from, retrieved in October 2008.

- Inference: Reply to Greene Michael J Frank*1, Randall C O'Reilly2 and Tim Curran2, material available online from , retrieved in October 2008.

- ICS study guide

- Mediodorsal Thalamic Functions in scene memory in rhesus monkeys by D. Gaffan and A. Parker, Department of experimental Psychology, Oxford University, Oxford UK, material available online from , retrieved in October 2008.

- Rathus, S.A. (2007), Study Guide, Psychology, Concepts & Connections, 8th Edition, Wadsworth. USA.

- The role of the thalamic reticular nucleus in visual processing, by Adam m. Sillito and Helen E. Jones, Dept of Visual Science, UCL Institute of Ophthalmology, London, UK, Thalamus & Related Systems, 2008, 4(1), 1–12. #2007 Cambridge University Press, Printed in the United Kingdom, material available online from, , retrieved in September 2008.

- The Emotional Nervous System, by Dr. C. George Boeree, material available online from , retrieved in October 2008.