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Located within the brain's interior lies a set of structures collectively referred to as the limbic system. Playing a chief role in emotion and motivation, principal organs include the hypothalamus, the amygdala, and the hippocampus. Parts are arranged in a looping, interconnected network, situated around the top of the brain stem. It communicates with the prefrontal cortex and is also called the "emotional brain."
The structure of the limbic system develops during the period of embryonic growth out of an early forebrain form referred to as the telencephalon. Neural tube maturation establishes the central nervous system, consisting of the brain and spinal cord. The telencephalon becomes the cerebrum where limbic system structures are housed.
The hypothalamus ensures homeostatic balance, necessary for maintaining and regulating body temperature, blood pressure, and metabolism. While the amygdala determines relevance of fear, this structure of the limbic system generates physiological responses such as a racing heart, sweating, and an increased breathing rate. It is a protective reaction to a perceived threat enabling life-saving behaviors.
Associated with anxiety conditions such as panic disorder and phobias, the amygdala is particularly involved with the production of fear. Memories of reactions to frightening events are stored within it, which can cause mental disorder. This small pair of almond-shaped organs located at the bottom of the caudate loop is responsible for the "fight or flight" response.
Important in the learning process, the hippocampus is capable of consolidation — attaching a physical image to memory allowing for its retrieval. Until this occurs, storage of information is brief. Hippocampal structure of the limbic system is primarily involved in the actual learning of a skill or task; it is not the place of permanent storage. Other areas of the brain are responsible for this, such as the cortex.
Psychiatric conditions have been linked to damage of the structure of the limbic system. In schizophrenic patients, for example, several areas including the hippocampus and amygdala are significantly smaller than normal. Schizophrenia is a mental disorder causing problems with thought, perception and cognitive abilities.
Ventricular enlargement indicates a cerebral tissue deficiency which is also linked to dementia, such as in Alzheimer's disease. In Alzheimer's disease, degeneration of brain tissue starts in the hippocampus, affecting memory and cognition. It is thought by researchers that people with schizophrenia may be born with the anomalies or that they have come about sometime during early childhood. Those with Alzheimer's have gradually developed abnormal brain changes over time, typically much later in life.