Limbic structures are the organs in the human brain that make up the limbic system. The limbic system is involved in emotional response, memory, and more rudimentary processes such as motor function. Limbic structures are located in the central part of the brain, at the base of the cerebral cortex. For this reason, a more precise name for the limbic system is the basal ganglia. Ganglia is a term for structures composed of nerve cells, as all brain structures are.
The word limbic comes from a Latin phrase referring to an edge or threshold. Brain scientists of the 20th century observed that the basal ganglia are positioned at a point between the cerebellum, which regulates basic physical processes, and the neocortex, where higher brain functions originate. Thus they referred to these borderline areas as limbic structures, or the limbic system. Scientists such as Dr. Paul MacLean, who coined the term limbic system, believed this area represented an advance in evolutionary development of the brain. Later research has proved this an imprecise theory, but the limbic structures are nonetheless vital to human behavior and survival.
Individual limbic structures include the amygdala and the hippocampus, which regulate emotional response and memory, respectively. The hippocampus is also involved in spatial orientation, as is the nearby parahippocampal gyrus. The hypothalamus and the cingulate gyrus regulate body functions such as sleep, heart rate, and digestion. The thalamus and fornix communicate information to and from these organs to other parts of the brain. The nearby nucleus accumbens, the pleasure center of the brain, is sometimes considered a limbic structure as well.
Despite their high degree of importance in regulating brain and body functions, limbic structures are comparatively tiny. For instance, the hypothalamus in an adult human weighs a tenth of an ounce (4 g). This is a fraction of the brain’s total weight of roughly three pounds (1.3 kg).
Scientists are intrigued by the close connection between the limbic structures, the pleasure center, and the cerebral cortex. This may explain how humans can gain pleasure from unlikely stimuli, such as recalling a fond memory or tasting a fine wine. The function of the limbic structures and other brain organs is a matter of ongoing study. Examination of patients with brain injuries, for example, demonstrates that memory is a function of the brain as a whole and is not dependent solely on the hippocampus. There are indications that the limbic system is an oversimplified idea and that the true relationships between the limbic and other brain structures may be far more complex than once believed.