The limbic system is not just one part of the brain but is a group of several brain regions with similar functions. Playing a critical role in emotions and long-term memory, the limbic system is referred to as the “old brain” because it evolved in mammals prior to the neocortex. This evolutionary progression affects the way that the brain responds to threatening stimuli and is responsible for managing the fight-or-flight response. The limbic system also serves as the emotional core of the human brain. Emotions in this context are best described as “a group of interrelated superior cerebral functions, resulting from states of reward and punishment” (Roxo et al., 2011). Mechanisms of motivation, learned behavior, and classical conditioning are all rooted in this part of the brain. However, there is some debate as to which particular structures are included in the limbic system.
Generally, parts of the diencephalon, the cerebral cortex, and the rhinencephalon (the part of the brain used for smell) are commonly included in the limbic system. Connecting these structures is a fiber tract called the fornix. The diencephalon comprises the thalamus, the epithalamus, and the hypothalamus. Though not directly involved in emotional processes, the thalamus is the relay station for information while the epithalamus functions to regulate sleep. Fear, anger, and pleasure are all processed by basal nuclei in the hypothalamus. Connecting the hypothalamus to the pituitary gland is the infundibulum. The cingulate gyrus and the parahippocampal gyri are part of the cerebral cortex, and the closely associated hippocampus indexes information for future retrieval. The rhinencephalon’s original purpose was olfactory sensation and processing; however, in humans, only the olfactory bulbs and tracts are still used for this purpose. The remaining structures factor into the emotion and memory functioning of the limbic system. The strong link between olfaction and memories is due to this evolutionary pathway. In conjunction with the olfactory bulbs and tracts, the fornix and uncus make up the rhinencephalon. Though anatomically separate, the orbitofrontal cortex is also considered a part of the system.
The first true pioneer of the limbic system was Paul Pierre Broca, a renowned anatomist who identified the area of speech known as Broca’s area. Although he did not determine the function of the limbic system, he laid its foundation. The discovery of Broca’s area in 1861 was an important event in the history of neuroscience because it was the first time that a specific area of the brain was proven to have a specific function. Broca’s second major contribution was in 1878 when he described the cortical aspect of the limbic system based on its morphological features. Broca’s limbic lobe comprised the parahippocampal and cingulate gyri encircling the corpus callosum. The parahippocampal gyrus surrounds the hippocampus and has a role in memory, specifically in scene recognition and understanding social cues. Both of these structures make up the limbic association area of the cortex and enable the emotional influence of decision making.
Almost a century later, in 1952, John D. MacLean coined the modern term “the limbic system” and included Broca’s limbic lobe and its associated subcortical nuclei in his definition. MacLean is also credited with the triune brain hypothesis, which divides the brain into three phylogenetic metastructures; the most primitive is the reptilian complex, followed by the limbic system, and finally the neocortex. It is important to note that the term “neocortex” is not interchangeable with “cerebral cortex,” as the former specifically refers to this triune concept. The anatomy of the triune layers is consistent with the evolutionary trend of cephalization in which the neurons of the central nervous system gradually develop toward the rostral end of the organism. The reptilian complex is responsible for the most basic survival instincts, notably the fight-or-flight response; and the neocortex processes the more advanced, logical, and conscious thoughts. MacLean proposed that the limbic system initially evolved in mammals to manage the fight-or-flight response by learning how to react to environmental stimuli. Emotional memories are created from truly threatening stimuli, and when the stimuli are encountered again, the fight-or-flight response from the reptilian complex is activated. This mechanism allows humans to better learn and adapt to their environments by quickly responding to threats and not wasting metabolic resources on stimuli that are startling but harmless.
Under nonstressful conditions the limbic system is moderated by the neocortex. However, distress causing the fight-or-flight response bypasses the neocortex and decisions are made by the limbic system. According to John Bowlby’s theory of attachment, infants who fail to develop secure attachments to their primary caregivers do not learn how to process social situations in healthy ways. Children’s styles of attachment develop internal working models that create frameworks through which they view the world. Children who have developed an attachment disorder either learn to distrust other people completely, or simply do not learn how to deal with the fear and anxiety of interacting with strangers. They often find social situations to be unnecessarily stressful, triggering a fight-or-flight response. This accounts for the impulsive and aggressive behaviors and poor social skills endemic to the insecurely attached. These symptoms are readily identifiable as the product of the limbic system.
Even when not embroiled in the fight-or-flight response, the limbic system plays an important role in decision making. It is intimately connected to the neocortex via the cingulated gyrus. Thus, many psychologists argue that to bring about fundamental change in a person’s beliefs or behavior, such as eating habits, the limbic system must be involved. This means that to influence behavior, one must make an emotional appeal because the logic being processed by the neocortex is insufficient to permanently influence decision making. This is evident in the emotional appeals made in advertisements and political campaigns. Logical factual data do matter, but emotions have the potential for more impact.
See also: Autonomic Nervous System; Brain Anatomy; Central Nervous System; Olfactory System; Thalamus
Chudler, Eric H. (2015). Milestones in neuroscience research. Retrieved from http://faculty.washington.edu/chudler/hist.html
Dubuc, Bruno. (2002). The brain from top to bottom. Retrieved from http://thebrain.mcgill.ca
Roxo, M., et al. (2011). The limbic system conception and its historical evolution. Scientific World Journal, 11, 2428—2441.