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Located just beneath the cerebrum, the limbic system is a series of structures in the brain that perform several tasks related to memory and emotion. The link between the limbic system and emotion is so well formed that some researchers refer to the system as "the emotional brain." From an evolutionary standpoint, this system is quite old, and is present even in the brains of lower mammals, which is why it is also known as the paleomammalian brain.
Several structures are present in this system, including the amygdala, the hippocampus, the hypothalamus, the fornix, and the septum. The nearby limbic lobe is also an essential component linking the limbic system and emotion. Some of the other structures mentioned here play roles in emotional regulation, as well.
No single structure is a complete one to one causal factor between the limbic system and emotion. These structures help to regulate certain emotional states, but they do not completely cause an emotion. Specific structures do, however, help to regulate certain sets of emotional behavior.
The amygdala is one structure essential for helping emotional memories to form. Activation of the amygdala from a threatening stimulus causes this structure to work with the hippocampus in order to remember it. When the same stimulus is encountered again, this structure causes a fear response. It also seems to be involved in anger, affection, and sexual responses. Animals without an amygdala will not show mothering instincts or signs of rage, or react to threatening or sexually arousing stimuli.
Certain areas of the thalamus and hypothalamus help express emotion, but do not generate it. The anterior thalamic nuclei seem to influence how individuals react to emotional stimuli. On the sides, or lateral areas, of the hypothalamus, there are nuclei involved in the expression of pleasure and anger. Displeasure and aversion to stimuli are mediated by the center, or medial area.
The cortex also helps join the limbic system and emotion. One component of the limbic cortex, the cingulate gyrus, helps to control blood pressure and heart rate. It also causes an emotional reaction to pain. Additionally, it may play a role in expressing rage. Wild animals that have this area of the brain taken out do not show aggression.
So-called "pleasure centers" in the ventral tegmental area show another connection between the limbic system and emotion. Activity in these cells causes them to release the chemical dopamine. Dopamine can cause pleasant feelings that range from a mild rewarding sensation to strong feelings resembling sexual gratification.
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